Re Slope

Program version : 1.0 User Guide issue : 1 Date of issue : April 2002

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CADS ReSlope

Version 1.0 User Guide

Computer and Design Services Ltd. CADS ReSlope User Guide

Issue 1

CADS ReSlope COPYRIGHT 2002 Computer and Design Services Limited, Arrowsmith Court, Broadstone, Dorset, BH18 8AX No part of this manual may be reproduced, transmitted, stored in a retrieval system, or translated into any language in any form or by any means without the written permission of CADS. Whilst the description of CADS programs contained in this user manual is as accurate and up-to-date as possible, it is our policy to continuously improve and expand the facilities which the programs offer. We therefore reserve the right to change the program specifications at any time without prior notice. AutoCAD, AutoLISP and ADS are registered trademarks of Autodesk Inc. All other trademarks acknowledged.

Important Notice

Please read this –

All due care has been taken to ensure that the data produced by this program is accurate. However, it remains the responsibility of the user to verify that any design based upon this data meets all applicable standards. Please refer to the CADS Software Maintenance and Licence Agreement for detailed information.

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2 Issue 1


Issue 1 1

Contents

1. Introduction ....................................................................................1.1 1.1. Program Description.............................................................................. 1.1 1.2. Program Installation............................................................................... 1.1

1.2.1. Installing the Application ............................................................... 1.2 1.2.2. Permissions for the program and protection directories ............... 1.2 1.2.3. Installation procedure ................................................................... 1.2 1.2.4. Authorising the Application ........................................................... 1.3 1.2.5. Modifying an Existing Authorisation .............................................. 1.6 1.2.6. Moving the Application.................................................................. 1.6

1.3. Result Verification.................................................................................. 1.6 1.4. Starting CADS ReSlope ......................................................................... 1.6

1.4.1. Default Values .............................................................................. 1.6 1.5. Using examples ...................................................................................... 1.7

1.5.1. Entering data ................................................................................ 1.7 1.6. Graphical data entry............................................................................... 1.7 1.7. Spreadsheet data entry.......................................................................... 1.8 1.8. The toolbar.............................................................................................. 1.8 1.9. View options ........................................................................................... 1.8

1.9.1. Content ......................................................................................... 1.8 1.9.2. Limits ............................................................................................ 1.9 1.9.3. Grid ............................................................................................... 1.9 1.9.4. Pop-up menu ................................................................................ 1.9

2. Entering Data ..................................................................................2.1 2.1. Sequence for data entry......................................................................... 2.1 2.2. Partial factors ......................................................................................... 2.1

2.2.1. Ramifications of failure (fn) ........................................................... 2.1 2.2.2. Soil self-weight (ffs) ...................................................................... 2.2 2.2.3. Imposed loads (fq) ........................................................................ 2.2 2.2.4. Soil tan(phi) values (fms) .............................................................. 2.2 2.2.5. Soil cohesion values (fms)............................................................ 2.2 2.2.6. Reinforcing material strength (fm) ................................................ 2.2 2.2.7. Sliding on reinforcement (fs)......................................................... 2.3 2.2.8. Reinforcement pull-out (fp) ........................................................... 2.3

2.3. Soil properties ........................................................................................ 2.3 2.3.1. Description.................................................................................... 2.4 2.3.2. Density.......................................................................................... 2.4 2.3.3. Phi................................................................................................. 2.4


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2.3.4. Cohesion....................................................................................... 2.4 2.3.5. Ru ................................................................................................. 2.4 2.3.6. Suction.......................................................................................... 2.5

2.4. Geometry................................................................................................. 2.5 2.4.1. Graphical geometry point entry..................................................... 2.5 2.4.2. Spreadsheet geometry point entry................................................ 2.5

2.5. Loading ................................................................................................... 2.6 2.5.1. Load type ...................................................................................... 2.6 2.5.2. Magnitude ..................................................................................... 2.6 2.5.3. X min. and X max.......................................................................... 2.6 2.5.4. Y ................................................................................................... 2.6

2.6. Water density.......................................................................................... 2.7 2.7. Piezometric grid (optional feature) ....................................................... 2.7 2.8. Seismic acceleration (optional feature)................................................ 2.8 2.9. Reinforcement (optional feature) .......................................................... 2.8

2.9.1. Description.................................................................................... 2.9 2.9.2. Strength ........................................................................................ 2.9 2.9.3. Length........................................................................................... 2.9 2.9.4. Friction coefficient......................................................................... 2.9 2.9.5. Vertical spacing ............................................................................ 2.9 2.9.6. Height of block .............................................................................. 2.10 2.9.7. Base of block ................................................................................ 2.10

3. Analysis and viewing results.........................................................3.1 3.1. Determining pore water pressure ......................................................... 3.1 3.2. Determining vertical pressure............................................................... 3.1 3.3. Slip circle analysis ................................................................................. 3.2

3.3.1. Analysis method ........................................................................... 3.2 3.3.2. Analysis options............................................................................ 3.4 3.3.3. Grid of circle centres..................................................................... 3.4 3.3.4. Circle radii..................................................................................... 3.4 3.3.5. Results grid................................................................................... 3.4 3.3.6. Critical circle summary.................................................................. 3.5 3.3.7. Showing the circles graphically..................................................... 3.5

3.4. User defined surface analysis............................................................... 3.5 3.4.1. Points on defined surface ............................................................. 3.5 3.4.2. Results.......................................................................................... 3.5 3.4.3. Showing the defined surface graphically ...................................... 3.6

3.5. Tie back wedge analysis........................................................................ 3.6 3.5.1. Wedge specification ..................................................................... 3.6 3.5.2. Results grid................................................................................... 3.6 3.5.3. Critical wedge summary................................................................ 3.7


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3.5.4. Showing the wedges graphically................................................... 3.7 3.6. Two part wedge analysis ....................................................................... 3.7

3.6.1. Wedge specification ..................................................................... 3.7 3.6.2. Results grid................................................................................... 3.7 3.6.3. Critical wedge details.................................................................... 3.8 3.6.4. Showing the wedges graphically................................................... 3.8

4. Printed Output ................................................................................4.1 4.1. Page set-up ............................................................................................. 4.1 4.2. Project details......................................................................................... 4.1 4.3. Printer selection ..................................................................................... 4.1 4.4. Print selection......................................................................................... 4.2

5. Worked Example ............................................................................5.1

6. Reinforced Soil – Background Knowledge...................................6.1 6.1. Glossary .................................................................................................. 6.1 6.2. BS8006 Design concepts / philosophy................................................. 6.2 6.3. Reinforced soil geometry....................................................................... 6.2

6.3.1. Wall facing .................................................................................... 6.2 6.3.2. Geometry of reinforcement ........................................................... 6.3 6.3.3. Vertical spacing of reinforcement ................................................. 6.3 6.3.4. Walls - External Stability ............................................................... 6.3

7. Supporting Theory .........................................................................7.1 7.1. Basic concepts ....................................................................................... 7.1 7.2. Slip circle analysis ................................................................................. 7.2 7.3. Steep slip surfaces................................................................................. 7.7 7.4. Two-part wedge analysis ....................................................................... 7.7 7.5. Bibliography............................................................................................ 7.8

8. Validation ........................................................................................8.1 8.1. Introduction ............................................................................................ 8.1 8.2. Validation exercise SC1......................................................................... 8.2 8.3. Validation exercise SC2......................................................................... 8.3 8.4. Validation exercise SC3......................................................................... 8.4 8.5. Validation exercise SC4......................................................................... 8.5 8.6. Validation exercise SC5......................................................................... 8.6 8.7. Validation exercise SC6......................................................................... 8.7 8.8. Validation exercise SC7......................................................................... 8.8 8.9. Validation exercise SC8......................................................................... 8.9


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8.10. Validation exercise UD1....................................................................... 8.10 8.11. Validation exercise UD2....................................................................... 8.11 8.12. Validation exercise TB1 ....................................................................... 8.12 8.13. Validation exercise TB2 ....................................................................... 8.13 8.14. Validation exercise TP1 ....................................................................... 8.14

Authorisation Fax Sheets

Customer Enquiry Fax Sheet


Issue 1 1.1

1. Introduction Chapter Objectives

This chapter provides an overview of the program and gives information on installation.

1.1. Program Description ReSlope is a general slope stability software package. The basic software module deals with unreinforced slope analysis using Bishops simplified method and circular slip surfaces. An additional optional module offers further methods of analysis; piezometric grid definition and the ability to analyse user defined failure surfaces. A further optional module adds the ability to analyse reinforced soil including the use of tie back wedge analysis and two part wedge analysis to the requirements of BS8006: 1995 (and the DMRB implementation BD70/97) and HA68/94. ReSlope is designed to be as easy to use as possible. Standard Windows interface conventions are followed where possible. It is possible to specify the geometrical slope data by drawing on the screen with the mouse although more conventional methods are also available. The derivation of all of the methods employed in the software is covered in detail in supporting technical sections of this manual. The software and this manual are written assuming the use of limit state analysis procedures but can equally be used to determine conventional global factors of safety. Validation of the software results through a series of examples is also covered in this manual. In 1995 the publication of BS8006 provided a sound and logical basis for the design of reinforced soil structures using limit state design concepts. Before the publication of BS8006 materials suppliers dominated the design of reinforced soil structures which led to a confusing mix of methods employed in the design and checking of reinforced soil structures. The fact that reinforced soil structures can provide significant cost savings compared to conventional structures makes them attractive for clients, designers and contractors. BS8006 was preceded by the DMRB document HA68/94 covering the design of reinforced soil slopes. HA68/94 uses the concept of critical soil strengths (rather than partial load factors) and covers analysis using two-part wedge theory from a rather limited viewpoint. The use of BS8006 for highway structures has since been implemented by the DMRB document BD70/97 (which replaces BE 3/78), the main requirement of BD70/97 above that of BS8006 is the use of materials that have a current BBA certificate.

1.2. Program Installation The software is a 32-bit program that requires Windows 95, 98, ME or NT version 4.0 or later to run. The software is supplied with an installation routine that will install a standalone or network license of the software.

Network installation Installation on a network is identical to the single user installation, except that you must specify the appropriate drive letter.


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Installation directory You are advised to set up one parent directory for CADS Windows applications, since they use common resources and will need to be in the same parent directory to fully communicate with each other. This directory is also used to keep the security information but it only needs write permission at install and authorisation time.

1.2.1. Installing the Application Four types of installation are possible, depending upon the system you have purchased. These are Standalone, Network, Workstation and Network Protection.

Standalone A stand-alone installation is for a single workstation not connected to a network. Select Standalone when the Installation Options dialog is displayed.

Network A network installation allows an authorised number of workstations to operate the program which is fully installed only on the server. The server installation must be completed before installation on the workstations connected to it is started. Select Server when the Installation Options dialog is displayed. The authorisation procedure (see below) should be completed with details of the number of workstations.

Workstation The server installation must be completed before starting the workstation installation. At the workstation, install the program, selecting Workstation when the Installation Options dialog is displayed. Only the icons which point to the server will be installed - not the program files.

Network Protection In this type of installation, the program is installed on the workstation and the protection on the server. The workstation accesses the server for authorisation each time the application is started. The Help button gives access to summary information on the installation procedure which is described in detail in the following section.

1.2.2. Permissions for the program and protection directories Please note the following directory access requirements:

During installation During installation, full permissions for the program directory and the directory in which the protection files are installed will be required by the person installing the program.

During program operation During program operation, full permissions for the protection directory will be required by the user.

1.2.3. Installation procedure Installation is effectively the same for each type of installation, with the exception of the selection at the Installation Options dialog, as described above. The program should be installed in the CADS parent directory. The procedure is as follows: 1. Insert Program CD into drive d: (or other as appropriate).


Issue 1 1.3

2. Select the Run option from the Start menu and type d:setup.exe in the Command Line input field of the Run dialog. Click on OK.

3. A message warning you that the installation is about to start will be

displayed. Click on OK to continue. 4. The Welcome dialog then appears. Click on Next. 5. The Installation Options dialog (Figure 1.1) showing a brief description

of each type of installation beside its selection button will be displayed.

Figure 1.1 Installation Options Dialog

Select the type of installation you require and then click on Next. 6. The Select Application Directory dialog showing the default installation

directory is then displayed. If you wish the program to be installed in this default directory, click on the Next button, otherwise click on Browse. A Select Destination Directory dialog is then shown to allow you to choose an alternative directory. Click on OK after you have selected the directory. Click on Next in the Select Application Directory dialog and the installation will continue.

7. A Select ProgMan Group dialog then allows you to choose a group for

the program icons. Select the group and then click on Next. 8. A Release Information dialog containing details of Windows Support

installation requirements and authorisation is displayed. Click on Next and a further dialog advising you that installation is ready to be started is shown.

9. Click on Next to start the installation and insert Program disk 2 if

requested by the installation. A message confirming successful installation appears on completion. Click on Finish and a Window with the CADS applications, including the application’s icon, is displayed.

After you have installed the application, it will have to be authorised before you can use it.

1.2.4. Authorising the Application The method of protection for all types of installation other than standalone systems and for standalone systems without a dongle ties the program to the


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system upon which it is installed. Changing the authorised system, moving the program or re-formatting the drive upon which it is installed will require a new authorisation code. To authorise an installation when this method is used, start the application by double clicking on its icon in its group window. A message appears with the information that the RESLOPE.INI file is not available. Click on OK. The Licensing Information dialog (Figure 1.5) will then open.

Figure 1.2 Licensing Information Dialog

Fields are provided to allow you to enter the User Name, the Company Name, the Address (three fields) and the Phone number (two fields). The minimum entries the program will accept are User Name and Company Name. Enter at least these fields and then click on OK to proceed. You are then asked to confirm the information. Click on the Yes button. The “Modify protection” dialog shown in Figure 1.3 is then displayed.

Figure 1.3 Modify Protection Dialog

The options accessed by the buttons in this dialog are as follows:


Issue 1 1.5

Authorise Single User Version – Click on this button to start the authorisation process for use of the program on a single computer not running the program on a network. Authorise Network Version - Use this button to start the authorisation process for use of the program on a network. Authorise Timed Demo – The program may be authorised as a demonstration to run for a limited period using this button. The period and authorisation obtained from CADS must be entered in the following dialog. To extend the period, click on the Extend Demo Period button. Authorisation will again be required. Authorise Limited Run Demo - The program may be authorised as a demonstration to run for a limited number of runs using this button. The number of runs and authorisation obtained from CADS must be entered in the following dialog. To extend the number of runs, click on the Extend Number of Demo Runs button. Authorisation will again be required. Set Number of Network Licences – If after an initial network installation you wish to change the number of workstations, use this button and enter the required change. Authorisation will again be required. De-authorise Application – This allows the program to be de-authorised. The program must be de-authorised if, for example, it is to be moved to another machine. Make a note of the following information: • The program name • The Code entry number • The Computer ID displayed • Which modules you have purchased • How many concurrent users are to be allowed, if you are installing on a

network. Fax this information to CADS using the Authorisation Code Fax Sheet which appears at the end of this user guide. You can enter several installations at once on the Fax sheet if you are installing more than one program or a single program several times. We will then return authorisation codes to enter into the ‘Authorisation code’, ‘Network code’ and ‘Module code’ fields provided. The ‘Network code’ and ‘No. of network licences’ entries are only required if the program is installed on a network. Note that these codes are all numeric and they must be entered exactly as stated. Select the modules you have purchased from the ‘Available modules’ list. If you have entered the codes incorrectly, the dialog shown in Figure 1.4 is displayed and you will have to restart the process.

Figure 1.4 Authorisation Failure Dialog


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When you have completed this, click on OK and a message confirming successful authorisation will be displayed. If you are authorising a network installation, a dialog showing the number of licences authorised is displayed. Click on OK. The CADSPPP dialog listing the authorised modules is then displayed. Click on OK. The program will then start.

1.2.5. Modifying an Existing Authorisation If you later wish to modify the authorisation, e.g. to add more users, select File > Modify Protection. The “Modify protection” dialog will be displayed. Click on this button for the required change and the dialog shown in Figure 1.3 will be displayed. Authorisation must be obtained for the change.

1.2.6. Moving the Application For a standalone system with dongle authorisation, you may simply move the dongle to the new system but for other types of installation, if you wish to move the application to another workstation, then the existing installation must first be de-authorised. To do this, select File>Modify Protection. The “Modify protection” dialog will be displayed. Click on the De-authorise Application button and the dialog shown in Figure 1.5 appears.

Figure 1.5 Remove Authorisation Dialog

Contact CADS for the De-authorisation code and enter it. Click on OK. Install the application on the new workstation and then follow the authorisation process as for a new installation. Please include a page in your fax on your company headed paper formally requesting the change.

1.3. Result Verification The validation section of this manual demonstrates that the software produces correct results for a range of input data. It is not possible to envisage all of the data that a user may input into the software. It is the responsibility of the user to verify that the software is producing correct results in any use outside the scope of these validation exercises.

1.4. Starting CADS ReSlope In Windows you may start the program by selecting Start > Programs > CADS ReSlope

1.4.1. Default Values In line with most windows software there is no specified order in which to enter the data. Because of this, default data for a soil is provided which allows the software to operate when it is first started. This default data is simply overwritten by the user with data for the particular soil to be analysed.


Issue 1 1.7

1.5. Using examples ReSlope is based on what is known as a Multiple Document Interface (or MDI). Common examples of this type of software interface are Microsoft Word and Excel. In ReSlope you cannot open more than one file at a time but you can open multiple windows within the software containing different types of data. This means for instance that you can edit both the loading and soils properties in two different windows while observing the results immediately in another window. The screen shot below demonstrates this type of multiple window operation.

Figure 1.6

ReSlope keeps track of all open windows and will automatically update all windows (including any necessary analysis calculations) as new data is entered. The constant need to update all open windows can slow the software down and in most cases it is advisable to close results windows if major changes to input data are required. Some windows are modal (when a modal window is opened the user cannot activate or edit other open windows). Examples of modal windows are the print and view option windows.

1.5.1. Entering data The data should be entered such that the failure mechanisms investigated cause the soil to move towards the left.

1.6. Graphical data entry One of the greatest benefits of the ReSlope user interface is the ability to enter most of the data using the windows graphical user interface. To enter a point on a soil strata top surface:- Select the relevant strata from the drop down list on the toolbar Right click on the drawing window to reveal the pop-up menu Select the menu option “Add soil surface point” Click at the position on the drawing window where you want the new point Carry on adding as many new points as you like Right click on the drawing to cancel adding further points Using simple mouse sequences it is possible to add water surface points and to move or delete previously defined points. At any stage during graphical data entry pressing the F4 function key will display a spreadsheet type interface allowing point positions to be added edited or deleted with total


1.8 Issue 1

precision. This graphical method of entry will be second nature to users who are familiar with CAD systems and is easily learnt by all users.

1.7. Spreadsheet data entry Many of the data entry windows utilise a spreadsheet style of data entry. The details of specific data entry are covered in the next section of the manual. The spreadsheet is activated by clicking with the mouse on any cell or using the tab key to cycle through each item on the form. Then once the spreadsheet is activated the current cell is shown with a dotted black outline around the cell. To move to a different cell either use the arrow keys or click on the new cell with the mouse. To start edit mode on the current cell either press the enter key or double click the mouse. When edit mode is started the contents of the cell are highlighted and can be changed in the normal way using the keyboard. To cancel edit mode and save the changes to the cell press the Enter key again. To cancel edit mode and return to the original cell contents press the escape key in the top left corner of the keyboard or click on a new cell.

1.8. The toolbar There is a toolbar under the menu bar at the top of the main software window. This can be seen in the previous graphic. The toolbar duplicates many of the menu features providing shortcuts to commonly used features.

1.9. View options The first window displayed within the software is the drawing window. The drawing window is central to the operation of the software. The view options window that is displayed when View/Options is selected in the menu system controls the information displayed in the drawing window. The view options window appears as below.

Figure 1.7

1.9.1. Content The content section is a series of tick boxes that control the visible content of the drawing window. Each item can be ticked to display the item or cleared to remove the item from the display.


Issue 1 1.9

1.9.2. Limits The limits define the extent of the view displayed when View/Zoom/Limits is selected from the menu system. The limits also define the extent of the graphics in the printed output. The limits are automatically set when the software is started or File/New is selected. The drawing window can be zoomed out past these limits and the limits are automatically extended if data is defined outside the existing limits. The data input is validated and must lie within the range –1000 to 1000 and maximum values must exceed minimum values by 10. Input limits are also checked to ensure they lie outside all previously defined geometric and loading data points.

1.9.3. Grid The grid is similar to that used by all CAD systems. The grid increment can be set and is validated. The grid increments must lie between the limits 0.1 and 10. If point coordinates are required with a second decimal place then they should be entered in spreadsheet mode rather than graphically. The tick boxes relate to the way the grid is shown and control the snap to grid function. When snap to grid is enabled and a drawing function is selected then a second green cursor is displayed that shows the grid point that will be selected if the left mouse button is pressed. The “Show grid” and “Snap to Grid” options are repeated on the pop-up menu that is activated by a right click with the mouse on the drawing window.

1.9.4. Pop-up menu The pop-up menu is displayed by a right click with the mouse on the drawing window. If a drawing function has already been selected the first right click will end the function and a second right click is required to display the pop-up menu.

Figure 1.8

The pop-up menu provides quick access to commonly used menu items and can speed up the use of the software by reducing the need to select items from the menu system. This system is commonly used in CAD software.


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Issue 1 2.1

2. Entering Data

Chapter Objectives This chapter describes the normal sequence for entering the data for the soil layers, the soil properties and the geometry of the slope to be analysed.

2.1. Sequence for data entry It is recommended that data is entered in the following order (this is the same order that items appear on the edit menu).

2.2. Partial factors The partial factors form is displayed after selecting Edit/Partial Factors from the software menu system. The form will display as shown below.

Figure 2.1

This form shows the current values of all of the partial factors used by ReSlope. The two buttons at the base of the list allow the user to reset default values appropriate to ultimate limit state and serviceability limit state analysis to BS8006. The following sections of text describe each of these factors in detail.

2.2.1. Ramifications of failure (fn) Reference BS8006: 1995 Clause 5.3.2 Default value for ultimate limit state = 1.10 Default value for serviceability limit state = 1.00 This factor deals with the economic ramifications of failure. Essentially the default value of 1.10 for the ultimate limit state assumes the structure is listed in category 3 of BS8006: 1995, Table 3. This list is “Abutments, structures directly supporting motorway, trunk or principle roads or railways or inhabited


2.2 Issue 1

buildings, dams, sea walls and slopes, river training walls and slopes”. In figure 13 of BS8006 “directly supporting” is clarified as where the toe of the wall or slope lies within a 45 degree zone of influence from the foundation of the road, railway or structure. For structures outside this list (i.e. category 1 or 2 structures) a value for fn of 1.0 is appropriate. The application of this factor is not clearly identified within BS8006. ReSlope applies this factor to soil shear strengths (Tan Phi and Cohesion) and also to reinforcement strengths. This concept is essentially equivalent to directly factoring the restoring forces in the stability calculations.

2.2.2. Soil self-weight (ffs) Reference BS8006: 1995 Clause 5.3.6 Default value for ultimate limit state = 1.50 Default value for serviceability limit state = 1.00 The soil self-weight factor is applied in all calculations where self-weight of soil is included. It should be noted that when considering the stability of walls or steep slopes face angles greater than 70 degrees) that consideration should be given to checking stability with this factor set to unity as this may produce lower stability factors.

2.2.3. Imposed loads (fq) Reference BS8006: 1995 Clauses 5.3.6, 6.2.2 and 7.2.2 Default value for ultimate limit state = 1.50 Default value for serviceability limit state = 1.00 The value of 1.5 is a high value generally appropriate for loading on walls or abutments (BS8110 Clause 6.2.2). For the design of general slopes a value of 1.3 may be more appropriate (BS8110 Clause 7.2.2). Where different loads require different load factors the user can change this factor to unity and include the load factors with the magnitudes of the loads input in the loading data window.

2.2.4. Soil tan(phi) values (fms) Reference BS8006: 1995 Clauses 5.3.4, 6.2.3 and 7.2.3 Default value for ultimate limit state = 1.00 Default value for serviceability limit state = 1.00 Soil parameters used within BS8110 designs should be characteristic values based on peak soil shear strengths (not critical state or worst credible). The fms partial factor is applied to generate worst credible peak soil shear strengths. Where possible the appropriate value for fms should be “assessed directly” (see BS8110 clause 5.3.4). In cases where the factor cannot be assessed then the default values used within ReSlope are taken from BS8110 tables 16 and 26.

2.2.5. Soil cohesion values (fms) Reference BS8006: 1995 Clauses 5.3.4, 6.2.3 and 7.2.3 Default value for ultimate limit state = 1.60 Default value for serviceability limit state = 1.00 See comments above in section 4.2.4

2.2.6. Reinforcing material strength (fm) Reference BS8006: 1995 Annex A Default value for ultimate limit state = 1.25 Default value for serviceability limit state = 1.00 The default values used within ReSlope and the following text are based on the use of polymeric (geogrid type) reinforcement. The derivation of reinforcement material factors depends on the type of grids being used, the type of fill and the construction method. This text is intended to give a guide on the range of values involved.


Issue 1 2.3

TD = TB / fm where TD = Design tensile strength of the grid in kN/m TB = Base tensile strength of the grid (see section 1.2) fm = Partial factor on material strength TD is usually seen in BS8006 equations further factored by fn. fm = fm1 x fm2 = fm11 x fm12 x fm21 x fm22

where fm11 = Consistency of manufacturing fm12 = Extrapolation of test data fm21 = Installation damage fm22 = Environmental damage For geogrids from established manufacturers the fm11 and fm12 factors can be taken as unity. fm22 can be taken as unity unless the grid is exposed to direct sunlight or severe contamination in which case the advice of the grid manufacturer should be sought. fm21 depends on the grid/fill combination. In most cases it will be necessary to consult individual manufacturers literature. For preliminary design a value of 1.25 is recommended and this is set as a default value.

2.2.7. Sliding on reinforcement (fs) Reference BS8006: 1995 Clauses 6.2.4 and 7.2.4 Default value for ultimate limit state = 1.30 Default value for serviceability limit state = 1.00 This factor reduces the shear resistance available at the base of a wedge where that base is parallel to a block of reinforcement. The factor is not applied to slip circle analysis calculations.

2.2.8. Reinforcement pull-out (fp) Reference BS8006: 1995 Clauses 6.2.4 and 7.2.4 Default value for ultimate limit state = 1.30 Default value for serviceability limit state = 1.00 Where reinforcement passes through any soil failure surface the available resistance from that reinforcement is checked against the design strength of the reinforcement (i.e. possible tensile failure) and against anchorage failure. Anchorage depends on friction and effective stress normal to the reinforcement. The calculated anchorage resistance is divided by this factor.

2.3. Soil properties The soil properties form is displayed after selecting Edit/Soil properties from the software menu system. The form will display as shown below.


2.4 Issue 1

Figure 2.2

Initially the software will add a dummy soil named Soil1 as default data when a new file is created. Use of the spreadsheet style interface is covered in section 1.7 above. The dummy soil data should be overwritten with your project specific soil data. Further soil strata can be added using the Add Soil button that will add a new line to the bottom of the spreadsheet. The order of the data is not important as the relative levels are defined by the soil strata surface points that are subsequently defined. The Insert soil button does however allow a new soil to be inserted at the current cell position if required. The data should be entered as follows.

2.3.1. Description This column is used for identification and labelling only. Changing this data does not affect the results. The data entered is not validated.

2.3.2. Density This value is entered as kN/m2 and provides the software with the density required determining total vertical overburden at any point below ground surface level. The value entered is validated and must lie between the limits 5kN/m3 to 28kN/m3.

2.3.3. Phi This value is entered as degrees and provides the software with a basis for determining the shear strength of the soil at any point. For analysis to BS8006 the Phi values entered should be characteristic values based on peak soil shear strengths (not critical state or worst credible). The fms partial factor is applied to Tan(Phi) to generate worst credible peak soil shear strengths. Available shear strength due to internal friction being the product of Tan(Phi) and the effective force normal to the failure surface being considered. The Phi value entered is validated and must lie between zero and 50 degrees.

2.3.4. Cohesion This value is entered as kN/m2 and provides the software with a basis for determining the shear strength of the soil at any point. Available shear strength due to cohesion is simply cohesion being the product of Tan(Phi) and the length of the failure surface being considered (the analysis is always in terms or unit length along the slope). The value entered is validated and must lie between zero and 500 kN/m2.

2.3.5. Ru This value relates pore water pressure to total overburden pressure and is expressed as a ratio of water pressure over overburden pressure. Its use in slope stability problems really dates back to times where more empirical


Issue 1 2.5

solutions were favoured as the use of this factor simplifies some of the equations used to solve slope stability problems. In this software the option is provided mainly to allow comparison of software and published results. This value can only be defined if the suction value is zero. The entry is validated and must lie between the limits 0 to 0.5. To understand how pore water pressures are determined during calculation refer to section 3.1 below.

2.3.6. Suction Some soils exhibit soil suction which is negative pore pressure in a granular material. This parameter is effectively a cap on the negative pore water pressure that can occur above the water table. This value is used mainly in the analysis of temporary slopes. The value should be entered in m head (entered as a positive figure). The value is validated and must lie between the limits 0 and 100. This value can only be defined if the Ru value is zero. To understand how pore water pressures are determined during calculation refer to section 3.1 below.

2.4. Geometry Each soil strata is defined by a series of points that lie on the top surface of the strata. The water table is also defined by a series of points. These points can be entered in two ways as described in the following two sections. It is worth noting that if two soil layers have coincident points then the upper soil strata has zero width. This can be used to define soil lenses and other complex geotechnical features.

2.4.1. Graphical geometry point entry The use of graphical data entry for soil strata is described in section 1.6 above. Generally this mode of data entry is started by selecting options from the draw menu or the pop-up menu activated by a right click with the mouse on the drawing window. The water table can also be entered in the same way. Pressing the F4 function key during graphical data entry will bring up the spreadsheet point definition window described below.

2.4.2. Spreadsheet geometry point entry The points definition window appears when Edit/Points is selected from the menu system or when the F4 function key is pressed during graphical data entry. The window will display as shown below.

Figure 2.3

Tabs along the top of the form allow you to select any currently defined soil or the water table (there will always be at least two tabs with one soil and the water tab). The selected tab when the form is displayed will be the current soil


2.6 Issue 1

as shown on the toolbar. This spreadsheet behaves slightly differently than others. Double clicking a cell does invoke editing mode but the edit is carried out in a separate modal window rather than within the cell. The same editing mode can be activated by selecting the “Edit point” button. This form of editing is required to prevent possible errors occurring where editing is carried out both graphically and through the points at the same time. The modal nature of the edit window effectively freezes the software until the edit has been completed and validated. The coordinates entered must lie between the limits –1000 to 1000.

2.5. Loading The loading form is displayed after selecting Edit/Loading from the software menu system. The form will display as shown below.

Figure 2.4

The required loading data should be entered into the spreadsheet. The data required on each column is described below.

2.5.1. Load type When entering edit mode you will be presented with a drop down list from which you need to select the load type. The options are as follows Surcharge – A uniform vertical distributed load specified in kN/m2 (analysis uses a unit width). The load is applied between the X min and X max limits Line – An isolated vertical load (positive downwards) specified in kN/m (analysis uses a unit length). The load is applied at the Xmin. position. Lateral - An isolated lateral load (positive acting towards the left of the screen) specified in kN/m (analysis uses a unit length). The load is applied at the Xmin. position.

2.5.2. Magnitude Enter the magnitude of the load in the specified units. The data is validated and the magnitude must lie between the limits 0 and 10000.

2.5.3. X min. and X max. The X min. and X max. values define the extent or position of the load – refer to the section on load type above. The data is validated and the value must lie between the limits -1000 to 1000.

2.5.4. Y The Y value allows the load to be applied at any level with the soil. This is useful when modelling foundations. The data is validated and the value must lie between the limits -1000 to 1000. A special value may also be entered by


Issue 1 2.7

typing the word “surface” this will place the load on the ground surface and in the case of the surcharge it will contour itself to the ground profile.

2.6. Water density The water density form is displayed after selecting Edit/Water density from the software menu system. The form will display as shown below.

Figure 2.5

Water density is in general a constant value. There may be local variations such as saline water around sea walls where the water density could rise to approximately 10kN/m3.

2.7. Piezometric grid (optional feature) The piezometric grid form is displayed after selecting Edit/Piezometric grid from the software menu system. The form will display as shown below.

Figure 2.6

To set up a piezometric grid you should first enter the grid extent and increment data into the six text entry boxes. The data is validated and the extent data must lie between the limits –1000 to 1000 with increment data


2.8 Issue 1

between the limits 0.5 and 1000. The reset grid button is then used to set up the spreadsheet grid and reset all data. Note that if the piezometric grid is redefined all elevation data is lost. The head of water at each grid point is then defined for each point on the grid. The elevation represents the top level of water that would be found if a piezometer was installed at that point on the grid. All grid positions must have a defined elevation that must lie between –1000 and 1000. The use piezometric grid tick box must be ticked if this grid is to be used in the analysis. The defined water pressures only affect points within the extent of the grid. The software interpolates values for water pressures at intermediate points with the grid. If you close the window without completing the data entry you will be warned about possible problems and the “Use piezometric grid” option will be cleared.

2.8. Seismic acceleration (optional feature) The seismic acceleration form is displayed after selecting Edit/Seismic acceleration from the software menu system. The form will display as shown below.

Figure 2.7

The seismic acceleration factors are applied to the soil self weight. The value of g taken in the software is 9.81 m/s2. The default factors of zero represent the normal situation without earthquake factors. The vertical factor is positive downwards and the horizontal factor is positive towards the left. The data is validated, the vertical factor must lie between –10 and 10 and the horizontal factor must lie between 0 and 10. These factors are applied to soil self weight and act through the centroid of the slice. The user will also need to consider what effect the seismic condition will have on external loads and soil pore water pressure.

2.9. Reinforcement (optional feature) The reinforcement form is displayed after selecting Edit/Reinforcement from the software menu system. The form will display as shown below.


Issue 1 2.9

Figure 2.8

At present the soil nailing option has not been activated within the software. This feature will be added to a future version of the software. The reinforcement is at present restricted to the geogrid or steel strip type used in filled reinforced earth structures. The geogrids are not entered as single layers but are defined as blocks or reinforcement that the software then automatically converts to individual layers. Another feature of the software is that it will automatically detect the intersection between the ground level and the reinforcement layers so that all that is needed is a length of grid. All of these features save time and encourage buildable solutions. The following data must be entered for each block of reinforcing layers.

2.9.1. Description The description is a text field that does not affect the results. It is included for identification purposes only and there is no validation of this data.

2.9.2. Strength Entered as kN/m width. This value should be the creep limited long term value quoted by the manufacturers of the reinforcement. The user needs to take care that the figure supplied is correct. In geogrid reinforcement the design figure is usually very much smaller than the ultimate tensile loads that are sometimes quoted in manufacturers literature. There is more guidance on this subject within BS8006. The value entered is validated and must lie between 0 and 10000 kN/m.

2.9.3. Length This is the length of the individual grids behind ground surface. The value entered is validated and must lie between 1 and 50 metres.

2.9.4. Friction coefficient The friction coefficient is the interaction coefficient between the geogrid and the soil. This coefficient depends on both the reinforcement and the soil type/grading. The default value given by the software is 0.7 but values can typically range between 0.6 and 0.9. The advice of the reinforcement manufacturer should be sought. These values are commonly determined from shear box type tests. The data is validated and must lie between 0.2 and 1.0.

2.9.5. Vertical spacing This is the vertical spacing of the geogrids. The spacing should be fairly constant in the structure and sufficient spacing should be allowed to make the


2.10 Issue 1

structure buildable. Spacing of around 0.2m is generally the smallest desirable. The data is validated and must lie between 0.1m and 3.0m.

2.9.6. Height of block This is the height of the reinforcement block. The number of grids provided is obviously a whole number and as such the last grid may be lower than this height of block. Beware defining contiguous blocks of reinforcement where the top layer of one block is coincident with the bottom layer of the next block. The data is validated and must lie between 0.2m and 100m.

2.9.7. Base of block This is the elevation of the lowest layer of reinforcement. The data is validated and must lie between –1000 and 1000.


Issue 1 3.1

3. Analysis and viewing results

Chapter Objectives This chapter describes the procedure used for the slope analysis and provides guidance on interpreting the various results.

3.1. Determining pore water pressure Pore water pressure can be derived from the input data in a number of ways. For a particular point within a soil mass the pore water pressure used in the analysis is the first valid value derived from the three possible methods of calculation below. Interpolation of head within a defined and active piezometric grid (x water density) Ru ratio x total overburden pressure Depth relative to defined water level (x water density) If the point under consideration lies within the horizontal and vertical limits of the piezometric grid and the grid is enabled then the appropriate head of water is determined. The piezometric head is interpolated from the nearest points and the head relative to the current point is multiplied by the density of water. If this method is used the other methods described below are not considered. If a piezometric grid value is not used the software checks if the soil has a non-zero Ru value. If a Ru value exists the ratio is multiplied by the vertical total stress at that point to derive the pore water pressure. If neither of the previous methods produces an answer the head relative to the defined water table (if it exists) is calculated this head may be positive or negative and is multiplied by the density of water. If the water pressure value derived from these methods is negative then this is checked against the permitted suction defined for the soil type. For instance if the suction value is zero (the default value) then negative pore water pressures are not permitted. If the suction value is two metres then the maximum permitted negative pore water pressure is taken as 2xdensity of water. Any derived negative pressures above the suction cut-off are truncated to the cut-off value.

3.2. Determining vertical pressure. The derivation of vertical total and effective stresses is fundamental to any slope stability program so it is worth explaining the methods employed within the software. To determine overburden pressure at any point the ground surface vertically above that point is found and the density of all of the different soils layers are multiplied by the relevant strata thickness at the particular X coordinate of the point. To this overburden pressure the software adds the effects of loads by allowing a 2V:1H load spread between the level of application of the load and the point under consideration (subject to this load spread not extending through the ground surface). From these two calculations the vertical total stress can be determined. The vertical effective stress is this total stress minus the pore water pressure determined using the methods outlined above.


3.2 Issue 1

3.3. Slip circle analysis This section of the manual deals with the operation of the software. An in depth presentation of the underlying theory and its limitations can be found in a later section of this document. The slip circle analysis window is displayed when Calculation/Slip Circles is selected from the software menu system or when slip circle results are displayed within the drawing window. The slip circle analysis window appears as shown below.

Figure 3.1

3.3.1. Analysis method There are a number of analysis methods all based on the principle of analysing the slope as a series of slices. ReSlope derives the forces in each slice from first principles and applies the simplifications from the original method derivations as a second stage. This allows the methods to be extended to include influences not covered by the original papers. Each method and its application will be presented in turn. Fellenius (Swedish method of slices). This is a simple method of analysis in which all interslice forces are ignored and the factor of safety is based on rotational equilibrium (Force equilibrium is not satisfied). This method has no benefits for modern computer analysis and is included in this software mainly to assist the user to compare results to hand calculations. The factor of safety produced is always conservative. Bishop simplified (Moment equilibrium) This is a method presented by Bishop as a general slope stability solution. It is the default method used by the software and is suitable for most slope stability calculations. The method does include horizontal interslice forces but does not include interslice shear. This simplification does not greatly affect the results other than in ill-conditioned circles where the failure surface gradients at one or both of the circle ends are high. The method satisfies moment equilibrium but not force equilibrium. BS8006 Method (Bishops method with χ=1.25) This method is presented in BS8006 as a slip circle method to be used in designing reinforced soil slopes and walls. It is essentially Bishops simplified method but contains an additional factor χ described as a moment correction factor. CADS have been unable to find the reason for use of this factor but it has the effect of reducing the soil shear resistance by 20% as soon as you add reinforcement to the slope. This means that marginally stable slopes


Issue 1 3.3

need considerable quantities of reinforcement. This method should not be used for unreinforced slopes. At this time there are no further recommendations that can be given about the use of this method of analysis. Janbu simplified (Force equilibrium) Generally this method makes the same assumptions and simplifications as Bishops method with regard to forces on a slice. The difference is that this method solves for circle stability using horizontal force equilibrium rather than moment equilibrium. The result is a stability factor that needs correction depending on the geometry of the slope and circle. Further details regarding the correction factor are given in the supporting theory section. ReSlope does not apply the correction factor within the software. The principle benefit of this method is that it can be applied to non-circular failure surfaces, as moment equilibrium is not required. Rigorous solution It is possible to satisfy both moment and force equilibrium if certain assumptions are made regarding the distribution of forces between slices. Assume that the shear force and normal force at each interslice boundary are linked by a function. The basis of the function is that the shear/normal force is a ratio Lambda (i.e. the resultant force is at an angle). This basic angle is modified along the length of the slip by a function f(x). If f(x)=1 then the angle is constant and this is equivalent to the solution presented for parallel interslice forces by Spencer. Another common form is f(x)=Sin(x) which has been investigated and found to be a valid distribution in some slopes. ReSlope allows the definition of the function f(x) in numerical form by using the “Define f(x)” button that loads the window shown below.

Figure 3.2

The default factor values are all unity and can be reset using the “Set default values for parallel interslice forces” button. A second button allows values to be set for f(x)=Sin(x). The user can in fact enter any numerical values he wishes to define f(x). The values entered in the factor boxes are not validated. The software performs iterative calculations varying the basic ratio Lambda


3.4 Issue 1

until the factors of safety for moment equilibrium and force equilibrium are equal. This method may not yield convergence in certain circumstances (warnings are given where appropriate) and the method is included more for research than a tool for the practising designer. In any case the use of this method with reinforced soil solutions is not recommended, as the complex equations involved do not always converge in such problems.

3.3.2. Analysis options The minimum number of slices is set using the up/down arrow keys next to the display box. The lowest possible value is six and the highest is 50. Slice boundaries are automatically set at changes of ground profile and any intersection between the circle and soil or water surfaces. Additional slice boundaries are then added so that no slice width exceeds the total width of the slice divided by the minimum number of slices. Changing this value does not trigger an analysis, as the influence of this parameter is generally not great. If required an analysis must be forced by changing analysis methods or some other part of the input data. The tension crack depth is set using the text box. The data is validated and the value must lie between 0 and 10 metres. The right hand side of all circular failure surfaces terminate at tension crack depth below ground level. An additional lateral force is applied in the analysis equivalent to this tension crack being full of water. This does not affect the pore water pressure used in the analysis.

3.3.3. Grid of circle centres The circle centres are defined as a grid. The grid origin is at the coordinates X minimum, Y minimum. The grid increments in X and Y directions can be specified. Generally stability factors are more sensitive to changes in X value than changes in Y value. The extent of the grid is limited to the X maximum, Y maximum figures and no centres outside these limits are checked. The data is validated, coordinates must be between –1000m and 1000m, increments must be between 0.1m and 10m. The “Automatically extend grid” tick box allows the use of a grid that will extend in each direction until a minimum stability factor is found.

3.3.4. Circle radii The circle radii can be specified in four ways. The most common way is to specify a common point through which all circles must pass. In this option the X and Y coordinates of the common point are specified. The second option is to make all circles tangential to a line that is specified by a point and a gradient. This option is useful when constraining circles to pass close to a feature (perhaps a rockhead) in the ground. The next option is to have all circles of constant radius. The last option is to specify circles of varying radii specifying the minimum radius, maximum radius and increment between values applied. The data is validated; coordinates must be between –1000 and 1000, the gradient between –1 and 1, radius between 1 and 100.

3.3.5. Results grid The results grid shows one row of results for each circle analysed and has the following columns. X – The X coordinate of the centre of the circle in metres Y – The Y coordinate of the centre of the circle in metres Radius – The radius of the circle in metres Disturbing – The disturbing moment for the circle in kNm Soil – The restoring moment due to soil shear strength in kNm Reinforce – The restoring moment due to reinforcement in kNm Factor – The stability factor (or safety factor if partial factors are unity).


Issue 1 3.5

In the case of a Janbu type analysis horizontal forces in kN replace the moments. All moments and forces are for a unit length of slope (i.e. the analysis is for a 1m wide slice through the slope).

3.3.6. Critical circle summary The critical circle summary shows the same results for the circle with the smallest stability factor. If a rigorous analysis has been performed this will also show the value of the interslice force factor Lambda that resulted in equilibrium.

3.3.7. Showing the circles graphically To show the circles graphically select View/Show surfaces/Slip circles. The circles and their centres will display on the drawing window shown in grey except for the critical wedge shown in red. To display the grid of circles with the relevant stability factor press the button “Draw grid of circle centres” at the bottom of the slip circle dialog. A separate graphics window will be opened with the grid of circles shown, the critical circle will again be shown in red.

3.4. User defined surface analysis The user defined surface analysis window is displayed when Calculation/Defined slip surface is selected from the software menu system or when user defined results are displayed within the drawing window. The defined slip surface window appears as shown below.

Figure 3.3

The analysis uses the Janbu simplified method based on horizontal force equilibrium. This method requires a correction factor to be applied that is described in the supporting theory section of this document.

3.4.1. Points on defined surface The failure surface must be defined as a series of points. A valid surface must have at least three points. The extreme left and right points must lie above ground level. All intermediate points must lie below ground level. The points can be entered manually in the same way as points are defined for a soil or water surface. The points can also be picked on the drawing form, to activate this method press the “Pick points” button a right click with the mouse or pressing the F4 function key ends this graphical selection mode.

3.4.2. Results The results of the Janbu analysis are presented in a similar form to that described previously for the slip circle analysis form.


3.6 Issue 1

3.4.3. Showing the defined surface graphically To show the surface graphically select View/Show surfaces/Defined slip surface. The surface will display in the drawing window in red.

3.5. Tie back wedge analysis The tie back wedge analysis window is displayed when Calculation/Tie back wedges is selected from the software menu system or when tie back wedge results are displayed within the drawing window. The tie back wedge analysis window appears as shown below.

Figure 3.4

The tie back wedge analysis is a simple analysis. For a wedge of reinforced soil the stability factor is calculated by determining the restoring forces due to reinforcement and dividing by the total net disturbing forces. The method is described in BS8006 6.6.4.

3.5.1. Wedge specification The software will automatically select the toe of the wedges to be analysed. The levels are based on having the toe of the wedges just above each reinforcement layer. An additional wedge toe is positioned one vertical reinforcement spacing below the bottom reinforcement. The maximum and minimum wedge angles and the increment between the angles are set by the software to appropriate values for the specified soils. The default data can be changed only if you remove the tick from the box at the base of the window.

3.5.2. Results grid The results grid shows the results of the analysis with each line representing a wedge that has been analysed. The columns contain the following information. Level – The toe level of the wedge Angle – The angle of the rear of the wedge relative to horizontal Restore – The horizontal restoring force due to the reinforcement Disturb – The net horizontal disturbing force on the wedge


Issue 1 3.7

Factor – The stability factor obtained as the ratio Restoring Forces/Disturbing Forces

3.5.3. Critical wedge summary The critical wedge summary contains the same information for the wedge with the lowest stability factor.

3.5.4. Showing the wedges graphically To show the wedges graphically select View/Show surfaces/Tie back wedge. The wedge boundaries will display on the drawing window with the failure surfaces shown in grey except for the critical wedge shown in red.

3.6. Two part wedge analysis The two part wedge analysis window is displayed when Calculation/Two part wedges is selected from the software menu system or when two part wedge results are displayed within the drawing window. The two part wedge analysis window appears as shown below.

Figure 3.5

The two part wedge analysis is an important tool in designing reinforced soil structures. The method is more complex than tie back wedges and is dealt with in detail in the section on supporting theory in this document.

3.6.1. Wedge specification The software will automatically select the toe of the wedges to be analysed. The levels are based on having the toe of the wedges just above each reinforcement layer. An additional wedge toe is positioned one vertical reinforcement spacing below the bottom reinforcement. All other data is set automatically by the software to appropriate values for the specified soils. The default data can be changed only if you remove the tick from the box at the base of the window.

3.6.2. Results grid The results grid shows the results of the analysis with each line representing a wedge that has been analysed. The columns contain the following information. Toe Level – The toe level of the wedge in metres Base Angle – The angle of the base of wedge part 1 relative to horizontal in degrees Base Width – The horizontal width of wedge part 1 in metres Rear Angle – The angle of the rear of wedge part 2 relative to horizontal in degrees Rforce Restore – The restoring force due to the reinforcement in kN Soil Restore – The restoring force generated by shear on the base of the wedges in kN


3.8 Issue 1

Soil Disturb – The horizontal disturbing force from the wedge in kN Stability Factor – The stability factor as the ratio Reinforce Restoring/Net Disturbing

3.6.3. Critical wedge details The critical wedge summary contains the same information for the wedge with the lowest stability factor.

3.6.4. Showing the wedges graphically To show the wedges graphically select View/Show surfaces/Two part wedge. The wedge boundaries will display on the drawing window with the boundaries shown in grey except for the critical wedge shown in red.


Issue 1 4.1

4. Printed Output Chapter Objectives

Details of the facilities provided in the program for printing program data can be found in this chapter.

4.1. Page set-up The best output from this software is achieved when printing to A4 size paper in normal portrait mode. The software will however output to other sizes of paper and will adjust font size and number of text lines per page automatically to fit the size of paper available. Some options can be changed. Selecting File/Default settings/User Details allows the two lines of user details printed at the top of each page to be changed. Selecting File/Default settings/Printout Font allows changes to be made to the output font. Selecting File/Default settings/Printout margins allows some control of the margins used in the printout.

4.2. Project details The project details are defined in a window that appears when you select File/Project details. The window is shown below

Figure 4.1

The fields are all optional and the text as entered will appear on every page of the printed output.

4.3. Printer selection By default the output will be sent to the default Windows printer (and this is sufficient for most users). Where a specific printer needs to be selected this can be carried out on the printer selection dialog that is displayed when you select “Select printer” from the file menu. The dialog shown below is then displayed.


4.2 Issue 1

Figure 4.2

The window contains a list of all of the printers currently available to Windows. The desired printer should be highlighted and the OK button pressed. This action does not affect the default printer in Windows but remains in effect until you exit the software.

4.4. Print selection The print selection window is displayed by selecting File/Print from the software menu system. The window appears as shown below.

Figure 4.3

This print selection form allows the user to accurately define which parts of the input data and results he wishes to print at any time. Complete sections of the printout can be selected or removed by clicking on the tick boxes with bold captions. Individual sub sections can then be selected or removed in the selected sections. The Preview button loads a print preview window that will show the pages currently selected for printing. The use of print preview on large printouts on slow computers is not recommended. The print button will send the selected pages to the printer previously selected in the printer selection window (or otherwise the default Windows printer).


Issue 1 5.1

5. Worked Example Chapter Objectives

This chapter contains a worked example showing the data entry, calculation and results printing for validation exercise 1.

The exercise is a simple slip circle analysis of a slope measuring 6m horizontally and 4m vertically in a granular soil with Phi=32 degrees and Density = 17 kN/m3. A surcharge of 10kN/m2 is applied to the upper level. A single circle passing through the toe of the slope is analysed with its centre at coordinates measured from the toe of the slope of (1.59m, 6.82m). You can work through the following procedure to input the necessary data.

Step1: Start the software in the normal way or select File/New from the menu system to start a new file.

Step 2: Selecting partial factors Select Edit/Partial factors from the menu system. The default factors are set for ultimate Limit State but we wish to determine the conventional factor of safety of the slope. Click on the “Set defaults for serviceability limit state”. All of the values should change to 1.00. If all the values are correctly set click on the OK button to close the partial factor window.

Step 3: Entering soil properties Select Edit/Soil properties from the software menu system. The Soils window will appear with a dummy soil already entered. Double click on the description cell for the soil that reads “Soil 1”. The cell should now be in edit mode (Text will highlight in blue and a text cursor will appear in the cell). The text can be edited using the keyboard as you would normally. While the text is highlighted you can simply type the description for the soil “Granular soil”. When you are happy with the cell contents press the enter key and the cell will change back to normal mode. You can in fact just start typing into a current cell and it will automatically enter edit mode and change the data. Select the density cell either with the mouse or the arrow keys and change the value from 20 to 17 kN/m3. Select the Phi cell and change the value from 30 to 32 degrees. The soils window should now appear as below.


5.2 Issue 1

Figure 5.1

When you are happy with the data click on the “Close” button to close the Soils window.

Step 4. Entering the soil surface points We could select Edit/Soil and water surface points from the menu system and a spreadsheet type points window would be displayed. However for this example we will use the graphical data entry system. In the tool bar at the top of the main ReSlope window a drop down list should show that the current soil is “Granular Soil”. Select Draw/Add a soil surface point from the menu system. If you move the cursor to the drawing window it should change to a cross hair with a second green cross hair behind it. The second green cross hair shows where the cursor will snap to and the current coordinates are shown in the bottom left corner of the screen (Also note that hints on what to do next appear on the bottom bar). Move the cursor to the point (4,6) and click the left mouse button. A line should have appeared representing the ground surface together with a point (shown as a cross at the point (4,6). Move the cursor to point (10,10) and left click with the mouse. The soil surface will now show our slope. Right click on the drawing window (or press escape to end the graphical input mode).

Step 5. Entering the surcharge load Select Edit/Loading from the software menu system. The applied loads window will appear. Press the “Add load” button and a line will appear on the spreadsheet. The load type and magnitude are what we need so we can leave these cells. Change the Xmin value from 0 to 10 in the same way as you changed the data in the Soils window. Change the Xmax value from 1 to 20. Again the Y value is acceptable. The applied loads window should now look as shown below.

Figure 5.2

Press the close button to close the applied loads window. The drawing window should now appear as shown below.


Issue 1 5.3

Figure 5.3

Step 6. Entering the project details Select File/Project details from the menu system. Select the main title box by clicking on it with the mouse. Enter the text “Worked example”. Select the Engineer box by clicking on it. Enter your name or initials. Press the OK button to close the project details window.

Step 7. Saving the data Select File/Save from the menu system. Enter the required file destination folder and file name in the normal way and click the “Save” button.

Step 8. Analysing the slip circle Select Calculation/Slip Circles from the menu system. The slip circle analysis window will appear. You need to enter the coordinates of the centre of the slip circle (5.59,12.82). Click on the X minimum box and enter 5.59. Click on the Y minimum box (note the X maximum box automatically changes value), enter 12.82. Now move to the Circle Radii area, click on the X box and enter 4. Click on the Y box and enter 6. To force a calculation you must click on another item (perhaps back on the X box) so that the data item you have just changed is registered. The slip circle form should now appear as below.

Figure 5.4 Note the analysis is already complete and the factor of safety has been determined as 1.40


5.4 Issue 1

Step 9. Displaying the circles in the drawing window. To display the circle in the drawing window select View/Results/Slip Circles from the menu system.

Step 10. Save the data Select File/Save from the menu system. The data will save automatically to the same file as you previously specified.

Step 11. Printing the results. Select File/Print from the menu system. The print selection window will appear. Click on the “Slip circle analysis” tick box to enable printing of the slip circle results. You can change the other ticked options if you wish. Press the “Print” button to print or the “Preview” button to preview the printout.


Issue 1 6.1

6. Reinforced Soil – Background Knowledge

Chapter Objectives This chapter contains a glossary of terms and their definition as used in the context of the CADS Reslope Application.

6.1. Glossary Reinforced Soil For the purpose of this manual the term “reinforced soil” is taken to mean a fill material reinforced with layers of geogrid or steel straps. The reference “reinforced earth” is generally avoided due to the copyright of the Reinforced Earth Company. Geogrid A geogrid is a sheet comprising a polymer material that has been formed either by punching and stretching or extruding into a grid pattern. Geogrids may be either uniaxial (strong in one direction only) or biaxial (equal strength in two perpendicular directions). Reinforcement Base Strength A grid is usually specified by a minimum tensile strength (in kN/m width). Grid suppliers quote this value as the creep limited tensile or rupture strength. To assess the tensile strength of a material the supplier should assess the design life of the structure together with in-service temperatures and upper limits on creep. In practice suppliers frequently quote tensile strengths on 120 year design life in a typical UK environment. This base value has partial material strength factors applied to it as part of the design process. The factored strength is referred to as the Design Tensile Strength. Wall A structure where the retained face is between 70 and 90 degrees to the horizontal. Slope A steep slope is where the retained face is between 45 and 70 degrees to the horizontal. A shallow slope has a retained face angle of less than 45 degrees. All steep slopes need external stability checks. Shallow slopes do not require these checks. Internal stability This refers to checks where the failure surface is contained within the reinforced soil block Rupture (tensile failure) and tie back wedges are examples of internal stability checks. External stability This refers to checks where the failure surface is outside the reinforced soil mass. Such checks include sliding, overturning, bearing and slip circles. Combined stability This refers to checks where the failure plane is partly inside and partly outside the soil block. In practice this includes two part wedges and slip circles. In fact many of the tie back wedge checks are combined stability checks as the wedges may intersect the rear face of the reinforced soil block.


6.2 Issue 1

6.2. BS8006 Design concepts / philosophy A designer needs to understand the basic concepts of both soil mechanics and limit state design. The recommendations of BS8006 are implemented as stated in section 2 of the code, provided the designer understands the concept of limit state design most of section 2 is intuitive. The remainder of this page is devoted to listing some basic points to be considered when designing to BS8006. BS8006 is a limit state design code. Structures should be checked at both ultimate limit state and serviceability limit state. It is normal to design a reinforced soil structure for the Ultimate Limit State and check for the Serviceability Limit State (basically settlement of the foundation). Soil parameters used in BS8006 designs should be moderately conservative peak values. These values are generally taken as peak strengths derived from shear box tests and quoted in site investigation reports. Residual or critical strength soil parameters are only applicable to undisturbed soils with existing shear surfaces. Reinforcing material strengths are factored with partial material factors (fm) and a partial factor based on economic consequences of failure (fn). Soil strengths are factored by partial material factors applied to cohesion and Tan(Phi) and fn. Loads are factored by partial load factors. For all reinforced soil structures external stability checks are required. External stability checks comprise checks for: - Bearing capacity of foundation soil (using Meyerhof pressure distribution). Overturning Sliding Overall slip circles (or other external failure type) Items 1 to 3 above are not required for shallow slopes (Slopes less than 45 degrees) All walls and slopes require internal stability checks using tie back wedges (for walls and steep slopes) or two part wedges (for steep and shallow slopes). The use of compound stability (part internal part external) slip circles should also be considered (BS8006 Clause 7.4.5). The geometry of reinforcement in walls should generally follow the guidelines laid down in BS8006 (Pages 67-69). No guidelines are given for slopes but in steep slopes it is recommended that reinforcement should not be less than 0.7 x height of the slope.

6.3. Reinforced soil geometry The overall geometry of a reinforced soil wall and a preliminary reinforcement layout must be entered. This data is subsequently checked using the methods described earlier. The following section is intended to guide the designer to a practical wall design. For practical reasons there should ideally be no more than two strengths of grid in a wall.

6.3.1. Wall facing Consider what facing type is to be used for the wall. The facing may have some influence on the required vertical geogrid spacing. Walls will usually have either a wraparound facing (plus cladding for permanent works) or a precast concrete panel facing. In either case a basic compacted layer height should be decided (recommend as 200mm for granular materials) and spacings should all be a multiple of this value. Also a maximum vertical


Issue 1 6.3

spacing should be decided (recommend 600mm for wraparound and possibly 1000mm max for precast concrete).

6.3.2. Geometry of reinforcement Decide on geometry of the wall reinforcement. See the following in BS8006 Table 19, Page 46 Table 20, Page 47 Figure 19, Page 67 Figure 20, Page 68

6.3.3. Vertical spacing of reinforcement The actual vertical spacing and grid strength should be proposed based on the principles of BS8006 Clause 6.6.4.2.1 and 6.6.4.2.2 part a. This clause calculates the force Tj in a geogrid layer based on its depth in the structure and the vertical spacing of reinforcement. For a granular fill and uniform surcharge loading Tj = Tpj. In all cases TD >= Tj (that is to say the strength of the reinforcement should be greater than the applied force). The adherence criteria of clause 6.6.4.2.2 part b can be ignored for preliminary layout as this can be checked within ReSlope using tie back wedges to check internal stability.

6.3.4. Walls - External Stability Following the steps outlined below ensures the external stability reinforced earth of walls. Check bearing. A bearing capacity check should be carried out using a Meyerhof distribution. See BS8006 clause 6.5.2, Page 47. Check overturning. BS8006 makes no specific mention of overturning failure in external stability. The bearing check is described as a bearing and tilt check, this implies that a structure is deemed not to overturn if bearing is acceptable. Overturning can therefore be assumed to be satisfactory if the wall geometry complies with references in 7.3.2 above. Check sliding at both soil to soil and soil to reinforcement interfaces. For cohesive soils checks should be made for both short term and long term soil properties. See BS8006 clause 6.5.3, Page 47. Check settlement. See BS8006 clause 6.5.4, Page 47.


6.4 Issue 1


Issue 1 7.1

7. Supporting Theory Chapter Objectives

This chapter describes the theory on which the slope analysis is based.

7.1. Basic concepts ReSlope uses theory derived from basic soil mechanics principles to allow the analysis of stability of a wide variety of slopes including slopes reinforced with geogrids and soil nails. The failure surfaces take one of the following forms Slip circle – The classic form of slope failure mechanism Defined surface – A failure surface defined by a series of nodes Tie back wedge – A simple wedge failure used in reinforced soil checks Two part wedge – A wedge mechanism used in reinforced earth Typical uses of the different forms of failure surfaces are illustrated in Fig 7.1

Figure 7.1 – Typical failure surfaces

Rock Head


7.2 Issue 1

Scope and terminology of this document The theory behind all of the failure mechanisms is based on soil mechanics principles that are described fully in the following text. The theory presented assumes the reader has a basic knowledge of soil mechanics. Much of the theory is also covered in papers listed in the Bibliography. The theory has been updated from original published papers to cover the principle of limit state design and partial load factors and the inclusion of soil reinforcement. The terminology and symbols are derived from BS8006:1995 “Code of practice for strengthened/ reinforced soils and other fills”. At the time of writing no current British Standard adequately covers the stability analysis of un-reinforced slopes. Definition of the factor of safety ReSlope uses the theory of limit equilibrium of forces and moments to compute the factor of safety against failure. The factor of safety is traditionally defined as that factor by which the shear strength of the soil must be reduced in order to bring the mass of soil into a state of limiting equilibrium along a selected slip surface. This definition however does not fit well with the inclusion of reinforcing elements within the soil mass. The definition of factor of safety used within ReSlope is therefore F = Available Restoring Forces ….Equation 1 Total Disturbing Forces Restoring forces include soil shear strength and reinforcing element strengths. Disturbing forces include soil self weight, water pressure, imposed loads and earthquake forces. The factor of safety is based on either moment equilibrium or horizontal force equilibrium. All of the elements of this equation are subject to partial load and material strength factors. The following sections do not show partial factors within the equations. For example, where cohesion is shown in a formula this value is the factored cohesion after the partial material strength factor has been applied. Moment correction factor The definition of factor of safety above implies that the same factor of safety exists on both soil shear strength and reinforcement capacity. This assumption is not really valid but provides a convenient basis to allow the application of conventional slope stability solutions to reinforced soil. In BS8006 clause 7.4.4.3 defines a simple method for a slip circle solution (though no derivation is given). The method includes an additional factor called the moment correction factor (χ) applied to MRS (the soil shear strength component). The moment correction factor has the value 1.25 for ultimate Limit State and 1.00 for serviceability Limit State. If the user wishes to model the application of this additional factor in other methods of analysis the soil strength partial material factors can be increased to give the same overall result.

7.2. Slip circle analysis The following section shows the derivation of the formula used in the software to derive stability factors for slip circles. The theory relating to horizontal force equilibrium methods are equally applicable to user defined surface analysis. Analysis involves defining a slip surface through the earth mass and dividing the inscribed portion into vertical slices. The slip surface may be circular or consist of any shape defined by a set of nodes. Figure 2 shows all the forces acting on a typical slice on a circular slip surface. The same parameters apply for a defined slip surface except that no centre of rotation exists and therefore moment equilibrium cannot be calculated.


Issue 1 7.3

α

Figure 7.2 – Typical slice definition on a circular failure surface.

The variables are defined as follows: W = total weight of a slice of width w and height h N = total normal force on the base of the slice S = shear force mobilised on the base of each slice.

E = horizontal interslice normal forces. (Subscripts designate the side of the slice). X = vertical interslice shear forces. (Subscripts designate the side of the slice). kW = horizontal seismic load applied through the centroid of each slice.

Fv = vertical external load. Fh = horizontal external load.

H = resultant external water forces. (Subscripts designate the side of the slice). R = radius for a circular slip surface.

x = horizontal distance from the centroid of the slice to the slip centre e = vertical distance from the centroid of the slice to the slip centre. d = perpendicular distance from an external load or water force to the slip centre.

α = angle between the base of the slice and the horizontal. Summation of external forces ReSlope deals with some of the forces applied externally to the slope on a global basis rather than slice by slice. It is only possible to deal with the external horizontal loads in this way as the vertical loads affect the internal forces of each slice. For moment equilibrium solutions Mext = HR x dR – HL x dL + Fh x dh Where Mext = Sum of disturbing moments due to external horizontal loads For horizontal force equilibrium Hext = HR –HL + Fh Where Hext = Sum of disturbing horizontal loads Summation of reinforcement effects ReSlope considers each level of reinforcement (either geogrids or nails) in turn. The position where the failure surface crosses the reinforcement is


7.4 Issue 1

determined. The limiting strength for the level will then be determined by maximum strength or anchorage considerations (whichever is the lower capacity). The restoring effect of the grids are added together to produce either. For moment equilibrium: MRR = Sum of restoring moments due to reinforcement For horizontal equilibrium: HRR = Sum of restoring forces due to reinforcement Soil shear strength For an effective stress analysis, the shear strength is defined as: s = c′ + ( σn - u ) tan φ′ .Equation 2 Where: s = shear strength c' = effective cohesion φ′ = effective angle of internal friction σn = total normal stress u = pore-water pressure Mobilised shear force at the base From equation 2, the limiting shear force for equilibrium is derived as, S = s B = B ( c′ + ( σ n - u ) tan φ′ ) F F Or S = c′B + (N – uB) tan φ′ .Equation 3 F where: S = shear force mobilised at the base of the slice s = shear stress mobilised at the base of each slice

B = the base length of each slice F = the factor of safety σn = average normal stress at the base of each slice

Slice normal force at the base (N) The normal force at the base of a slice is derived from the summation of the forces in a vertical direction on each slice. -W + ( XL – XR ) + N cosα + S sinα - FV = 0 .Equation 4 Substituting equation 3 into equation 4 and solving for N gives N = W + ( XR – XL ) – (c′B sin α + uB sin α tan φ′ ) + FV .Equation 5 F cos α + sin α tan φ′ F There are several things to note about this equation 1. The value for N is dependent on the factor of safety 2. The equation cannot be directly solved because XL, XR and F are unknown 3. The denominator could under some circumstances become zero All of the above points are addressed in later parts of this document Calculating factors of safety The factor of safety for the slope as a whole should be determined by the summation of the forces acting on the individual slices. Unfortunately statics and failure conditions alone do not provide sufficient equations to obtain values for all the defined forces (the problem is indeterminate). More information must be known about the interslice force distribution. It is at this


Issue 1 7.5

point that the various papers written on slope stability solutions begin to diverge. To obtain the overall factor of safety it is necessary to consider either moment or horizontal force equilibrium. Ideally both of these conditions would be satisfied but that is not possible in most of the commonly used solutions. Equation for moment equilibrium From figure 7.2

ΣWx + ΣkWe + Mext - Σ(S/F)R - MRR/F = 0

or F = ΣSR + MRR .Equation 6

ΣWx + ΣkWe + Mext substituting equation 3 into equation 6 gives

F = Σ (c′BR + (N – uB) R tan φ′) + MRR .Equation 7

ΣWx + ΣkWe + Mext This equation is non-linear as N is itself a function of F (see equation 5). Equation for horizontal equilibrium From figure 2

Σ( EL – ER ) - Σ( N sinα ) - Σ((S/F) cosα ) - ΣkW + Hext - HRR/F= 0

the term Σ( EL – ER ) must be zero over the entire sliding mass.

Hence F = ΣS cosα + HRR .Equation 8

ΣWx + ΣkW + Hext substituting equation 3 into equation 8 gives

F = Σ (c′BR + (N – uB) R tan φ′) + MRR .Equation 9

ΣWx + ΣkWe + Mext Again this equation is non-linear as N is itself a function of F. Solving equations of equilibrium The assumptions used to solve these equations that have been used in the past are as follows. Fellenius (1936) or the Swedish method of slices Uses moment equilibrium as the basis of the solution. Ignores both normal and shear interslice forces (EL = ER = XL = XR = 0). Equation 5 becomes N = W Cos α - kW Sin α + Fv Cos α This equation can then be used to solve equation 7 directly without iteration as F no longer appears. Equation 7 is solved numerically within ReSlope.


7.6 Issue 1

This solution has the attraction of producing a linear equation that can be directly evaluated without iteration. The factor of safety obtained is a lower bound solution and is generally only used to compare with solutions calculated manually. Reslope uses this method internally to provide seed values of factor of safety for use in iterative methods. Bishops simplified method This method assumes that the interslice shear forces are zero (XL = XR = 0). Equation 5 now becomes N = W – (c′B sin α + uB sin α tan φ′ ) + FV ….Equation 5 F cos α + sin α tan φ′ F Again this equation can be used to solve equation 7 but this time it is an iterative process. ReSlope again solves equation 7 numerically and generally requires only 2 or 3 iterations starting with the seed value of F determined from the Fellenius solution. BS8006 method The basis of this equation is not described in the British Standard but it is essentially a rewrite of Bishops equation with F=1 (i.e. at Ultimate Limit State) and an additional factor called a moment correction factor. Janbu’s simplified method This method again assumes that the interslice shear forces are zero (XL = XR = 0). However the method then used horizontal force equilibrium rather than moment equilibrium to provide the solution for F. Equation 5 has the same form as shown above for Bishops method. This time equation 5 is used to solve equation 9 and again the solution is iterative. Rigorous method This method is the only method used within ReSlope that satisfies both moment and horizontal force equilibrium. Vertical force equilibrium is automatically satisfied in all methods as this is the basis for determining forces within each slice. Assume (Interslice shear force)/(Interslice normal force) = λ. f(x) Where λ is effectively Tan(α) where α is the angle of the resultant interslice force with the horizontal. f(x) is a function defined at the start of an analysis. Common functions are either f(x)=1 which implies all interslice forces are parallel and f(x)=Sin(x) which has been investigated in several papers on slip circle stability. To determine the factor of safety for the rigorous solution ReSlope will use the following stages Stage 1 Determine a seed value for F using Bishops simplified method. A seed value for Lambda is set at 0.2. Stage 2 Work from left to right across the slope determining all of the slice forces (EL,ER,XL,XR,N) for the current value of Lambda. For each slice this is an


Issue 1 7.7

iterative calculation. EL for slice 1 must be 0. ER can be determined by resolving horizontally the forces for the slice, this leaves an equation for ER in terms of EL and N. Vertical equilibrium gives N so the final value of ER for a slice is determined by iteration until the slice is in equilibrium. ER for this slice is always EL for the next slice. If ER for the last slice is not zero then the value of Lambda is incorrect and the process is repeated with a new prediction of Lambda (the value is predicted based on the trend of the calculations to that time). If convergence cannot be found for equilibrium of any slice within 100 iterations a warning is given to the user. If convergence for total horizontal forces cannot be found within 100 iterations a warning is given to the user. Only one warning is given for each circle analysed. The stability factor results for such circles are still given but should be treated with caution. Stage 3 Given the slice force distribution now determined in stage 2 the factors of safety for moment equilibrium and horizontal force equilibrium can be determined using equations 7 and 9. If these factors of safety are not the same then a new trial value for the factor of safety is predicted based on the results obtained up to that time and we loop back to the beginning of stage 2. After several iterations of stages 2 and 3 (including the iterative calculation within stage 3) the factors of safety for moment and horizontal force equilibrium will usually converge. The software allows a total of 50 loops of these stages before posting a warning message and abandoning the analysis.

7.3. Steep slip surfaces The use of ReSlope allows the user to analyse a large number of slip circles very quickly. However the user must ensure that these surfaces comply with the assumptions of the underlying soil mechanics. The most common problem relates to slip surfaces that enter (at the top of the slope) or exit (at the bottom of the slope) at very steep angles. Slip Circles should not enter a slope at an angle significantly steeper than an active wedge i.e. (45 + Phi)/2. Slip Circles should not exit a slope at an angle significantly steeper than a passive wedge i.e. (45-Phi)/2. One way of dealing with such slopes is to specify a tension crack depth that will effectively reduce the entry slope angle and model such failure surfaces more appropriately.

7.4. Two-part wedge analysis Two part wedges are intended primarily for use in internal or compound stability checks. BS8006 also allows two-part wedges to be used for external stability checks, but the designer may still need slip circle analysis to search for deep-seated slips in real world situations where there are several soil strata. The two-part wedge analysis method presented in BS8006 clause 7.4.4.2 does not explain fully the mechanics or mathematics of a two-part wedge. A two part wedge can have friction between the two parts but this is not included in the BS8006 method. To ensure ReSlope complies with code requirements friction is ignored. This simplification means that the distribution of forces between the parts of the wedge is no longer important. ReSlope further splits the wedges into vertical slices with boundaries at each soil strata or water definition point and any intersection between the slip surfaces and the soil strata or water surface. This ensures that the wedge self-weights and soil shear capacity can be accurately determined using the same routines as those used for the slip circle theory described above. The total restoring force due to all the grids intersected is determined including checks for anchorage of the grids beyond the failure surface. The


7.8 Issue 1

disturbing and restoring horizontal forces due to self weight and applied loads are determined using statics on each slice in turn. The stability factor is taken as the ratio Reinforcement Restoring / Net Soil Disturbing. ReSlope can be used to model slopes in accordance with HA68/94 and this is demonstrated in the validation exercises.

7.5. Bibliography The following documents provided the necessary background to develop the theory required for the code production of the ReSlope software. 1. The use of the slip circle in the stability analysis of earth slopes. A.W. Bishop, 1955, Geotechnique Vol.5 Pages 7 to 17 2. A method of analysis of the stability of embankments ensuring parallel interslice forces. E. Spencer, 1967, Geotechnique Vol 17 Pages11 to 26 3. Use of computers for slope stability analysis. R.V. Whitman and W.A. Bailey, 1967, Proc. ASCE, Vol 93 SM4 Pages 475 to 498 4. BS8006:1995 Code of practice for Strengthened/Reinforced soil and other fills. Published 1995, British Standards Institution, UK. 5. The department of transport standard HA68/94 Design Methods for the Reinforcement of Highway Slopes by Reinforced Soil and Soil Nailing Techniques. Published 1994, HMSO, UK. 6. Ciria Special Publication 123 Soil reinforcement with geotextiles. Published 1996, Ciria/Thomas Telford UK, ISBN 0 7277 2502 5 7. The design of reinforced structures using Tensar Geogrids Revised 1997, Netlon Ltd, UK.


Issue 1 8.1

8. Validation

Chapter Objectives This chapter documents the procedure by which the results of the slope analysis have been validated.

8.1. Introduction

This section of the manual records the validation work undertaken on ReSlope by the software authors. The document provides a guide for each validation exercise defining the purpose of the run and the results obtained. The files listed in each exercise are distributed with the software. The following validation exercises are described in this manual SC1 Slip circle Basic slope with surcharge using all analysis methods SC2 Slip circle Water pressures including submerged slopes SC3 Slip circle Multiple soil strata with cohesion and vertical face SC4 Slip circle Vertical and lateral loads SC5 Slip circle Piezometric grid SC6 Slip circle Seismic forces SC7 Slip circle Partial factors SC8 Slip circle Reinforcement UD1 User defined surface Basic slope from SC1 UD2 User defined surface User defined surface with reinforcement TB1 Tie back wedge Basic wedge in vertical face with surcharge TB2 Tie back wedge Example from Tensar Design Guide TP1 Two part wedge Example from Tensar Design Guide


8.2 Issue 1

8.2. Validation exercise SC1

Computer files ReSlope file reference - ValidSC1.rsp Excel file reference - ValidSC1.xls Object To show that the slip circle analysis gives correct results for traditional lumped factor of safety analysis (for all method of analysis). The exercise analyses a simple slope with no water and includes comparison with both a spreadsheet and other commercially available software. Description A slope measuring 6m horizontally and 4m vertically in a granular soil with Phi = 32 degrees and Density = 17 kN/m3. A surcharge of 10kN/m2 is applied to the upper level. A single circle passing through the toe of the slope is analysed with its centre at coordinates measured from the toe of the slope of (1.59m, 6.82m). Results The factor of safety results (all rounded to two decimal places) are as follows. Fellenius method Other software 1.24 Spreadsheet 1.24 ReSlope 1.24 Bishops simplified Other software 1.40 Spreadsheet 1.40 ReSlope 1.40 BS8006 Other software Not available Spreadsheet 1.05 ReSlope 1.05 Janbu simplified Other software 1.26 Spreadsheet 1.23 ReSlope 1.23 Rigorous Other software 1.40 Spreadsheet 1.40 ReSlope 1.40 Comments The largest difference is in Janbu’s simplified method that shows a discrepancy of less than 3% that could easily be generated by slightly different slice boundary positions.


Issue 1 8.3


Computer files ReSlope file reference – ValidSC2.rsp Excel file reference – ValidSC2.xls Object To show that the software accurately calculates the lumped factor of safety for slopes including pore water pressures and submerged slopes. Description A slope representing data for an earth fill dam with a water proofed surface still maintains a single soil type. Results The calculated factors of safety for Bishops Simplified Method are as follows. ReSlope Stability factor = 5.42 Spreadsheet Stability factor = 5.42 Comments The spreadsheet confirms that the software is accurate for this exercise.


8.4 Issue 1


Computer files ReSlope file reference – ValidSC3.rsp Excel file reference – ValidSC3.xls Object To show that the software deals with multiple soil strata including the analysis of cohesive soils Description A vertical face representing data for an embedded retaining wall. Two soil types both intersected by the circular slip surface. One of the soil types has cohesion Results The calculated factors of safety for Bishops Simplified Method are as follows. ReSlope Stability factor = 1.55 Spreadsheet Stability factor = 1.55 Comments The spreadsheet confirms that the software is accurate for this exercise. The software is also selecting a slice boundary at the intersection between the soil strata surface and the slip failure surface.


Issue 1 8.5


Computer files ReSlope file reference – ValidSC4.rsp Object Validate the use of vertical and lateral applied loads. Description Similar to exercise SC4 except for the change in loading. The applied loads are a 100kN/m line load with a 30kN/m lateral load. The exercise was checked using another commercially available software package. Results The calculated factors of safety for Bishops Simplified Method are as follows. ReSlope Stability factor = 1.20 Other software Stability factor = 1.20 Comments The two pieces of software yield the same result. The applied loads appear to be working properly.


8.6 Issue 1


Computer files ReSlope file reference – ValidSC5.rsp Excel file reference – ValidSC5.xls Object To show that the software accurately calculates the lumped factor of safety for slopes including a piezometric grid. Description The slope geometry is similar to that used in previous exercises. The piezometric grid is not selected to model any particular situation. The grid does however produce pore water pressures that are significantly higher than the base data with a defined ground water table. Results The results have been compared to a spreadsheet. In the spreadsheet the water pressures were visually interpolated from the grid so results are not expected to be identical. The spreadsheet and software pore water pressures have been compared to expected values. The calculated factors of safety for Bishops Simplified Method are as follows. ReSlope Stability factor = 0.65 Spreadsheet Stability factor = 0.66 Comments The results are considered to be validation for the piezometric grid facility in the software.


Issue 1 8.7


Computer files ReSlope file reference – ValidSC6.rsp Excel file reference – ValidSC6.xls Object To show that the software accurately models the application of seismic acceleration factors. Description The slope geometry is similar to that used in previous exercises. The defined seismic acceleration factors are –0.2 vertical (i.e. acceleration upward) and 0.3 lateral (i.e. towards the failure direction). Negative horizontal accelerations are not permitted in the software data entry. Results The ReSlope calculated factor of safety of 1.08 compares well with the value of 1.09 calculated in the spreadsheet. Comments The results are considered to be validation for the seismic acceleration facility in the software. It should be noted that these factors are not applied to external loads and to water pressures. In real world situations the user would need to consider these in more detail.


8.8 Issue 1


Computer files ReSlope file reference – ValidSC7.rsp Excel file reference – ValidSC7.xls Object To demonstrate that ReSlope applies partial factors correctly for a limit state analysis of a slip circle in accordance with BS8006. Description The slope data is that used previously in validation exercise SC3. The partial factors used are the defaults for ultimate limit state analysis to BS8006 (although no reinforcement is used in this exercise). Results The results from the spreadsheet calculation agree closely with the ReSlope figures. The calculated stability factor being 0.62 in both cases. Comments The analysis is considered to be validation of the use of partial factors applied to slip circle analysis. It is interesting to note that while the lumped factor of safety produced in exercise SC3 might have been considered acceptable in a temporary situation the results from exercise SC7 would normally be rejected. It appears that BS8006 is more conservative in this situation.


Issue 1 8.9


Computer files ReSlope file reference – ValidSC8.rsp Excel file reference – ValidSC8.xls Object To demonstrate that ReSlope correctly incorporates reinforcement strength within a slip circle calculation. Description The data describes a steep reinforced soil slope eight metres high. The slope is reinforced with eight layers of geogrid. The example slip circle passes entirely through the reinforced earth block and does not represent the critical slip surface. The reinforcing strength has been set to give a realistic strength for this slope by checking internal stability with the tie back wedge method. Results The slope has been analysed using Bishops simplified method both with ReSlope and using a spreadsheet calculation. The overall factor of safety of 1.36 is the same in both analysis types. There is a small difference in the calculated reinforcement capacities. This difference stems from the more rigorous treatment of surcharge loads within the ReSlope software. Comments These results are considered to validate the use of ReSlope for the analysis of circular slip surfaces with horizontal reinforcement.


8.10 Issue 1

8.10. Validation exercise UD1

Computer files ReSlope file reference – ValidUD1.rsp Object To demonstrate that the failure surface defined in the validation exercise SC1 can be input as a user defined surface and yield the same results. Description The slope data is taken from exercise SC1 Results The stability factor obtained is 1.23. This value is the same as that determined using the Janbu simplified method in validation exercise SC1 Comments This result shows that the results obtained using user defined surfaces are the same as those obtained using the Janbu method on circular slips. In fact the software routines for calculation of all the slice forces are common to both methods and therefore this is expected.


Issue 1 8.11

8.11. Validation exercise UD2

Computer files ReSlope file reference – ValidUD2.rsp Object To demonstrate that ReSlope accurately calculates the stability factor for a user defined surface with reinforcement and partial factors. Description The slope and failure surface data are taken from the previous validation example SC8. Results The stability factor obtained from the user defined surface is 1.31. The same ReSlope data file also contains data for the circular slip analysed using the Janbu simplified method that yields the same result. The total available reinforcement capacity can be verified by referring back to the spreadsheet file ValidSC8.xls. Comments These results are considered to validate ReSlope for use in calculating the stability factor for reinforced slopes with user defined failure surfaces.


8.12 Issue 1

8.12. Validation exercise TB1

Computer files ReSlope file reference - ValidTB1.rsp Excel file reference - ValidTB1.xls Object To demonstrate that ReSlope calculated stability factors can be verified using spreadsheet calculations Description The data describes a vertical wall of reinforced soil 5m high with four layers of reinforcement. Results The stability factor of 1.07 for the critical wedge calculated by ReSlope is also calculated within the spreadsheet. Comments This demonstrates that ReSlope is correctly analysing reinforced slopes using partial factors and tie back wedge analysis.


Issue 1 8.13

8.13. Validation exercise TB2

Computer files ReSlope file reference – ValidTB2.rsp Object To verify that ReSlope calculations for tie back wedges give the same results as a published example Description The data describes an example titled “5.1.4 Design example using the tie back wedge method” on page 22 of the document “The design of Reinforced Soil Structures using Tensar Geogrids” from Netlon Ltd in the Spring 97 revision of the document. Results The results for the wedge with a toe at the base of the wall (wedge angle 68 degrees) in ReSlope gives a reinforcement restoring force of 202kN/m and a disturbing force of 93kN/m. The published results are restoring 204kN/m and disturbing 92.5kN/m. Comments These results are considered to validate the use of the tie back wedge method in ReSlope.


8.14 Issue 1

8.14. Validation exercise TP1

Computer files ReSlope file reference - ValidTP1.rsp Excel file reference - ValidTP1.xls Object To demonstrate that ReSlope can accurately analyse two part wedges and compare the results with a published example. Description The data describes an example titled “6.1.1 Design example using the HA68/94 method” on page 35 of the document “The design of Reinforced Soil Structures using Tensar Geogrids” from Netlon Ltd in the Spring 97 revision of the document. Results The results for the wedge with a toe at the base of the wall in ReSlope gives a reinforcement restoring force of 538kN/m and a disturbing force of 287kN/m. The published results are restoring 538kN/m and disturbing 288kN/m. A spreadsheet calculation has also been undertaken giving identical results to the ReSlope calculation. Comments The spreadsheet calculation uses the formula for reinforcement tension given in HA68/94. ReSlope internally calculates forces from basic wedge geometry. The results are the same in either case. These results are considered to validate the use of ReSlope to calculate the stability factor of slopes using the Two Part Wedge method.

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Authorisation Code

Note:

Please remember to include your fax number in order that codes are returned ASAP.

Timed Demo Version Authorisation Code Request Fax Sheet



Program Name

Code Entry No.

Comp. ID No. Days

Authorisation Code

Demo Code

Module Name

Module Authorisation Code


Limited Run Demo Version

Authorisation Code Request Fax Sheet



Program Name

Code Entry No.

Comp. ID No. Runs

Authorisation Code

Demo Code

Module Name Module Authorisation Code


Customer Enquiry Fax Sheet

To: CADS Ltd. Fax: +44(0) 1202 690284 Number of sheets …………. (inclusive)

From: Name: Company: Phone: Fax:

CADS Application: Version:

Processor type and speed: Memory size: Hard disk size: Operating system:

Other applications running:

Details of the enquiry:

Note: Please remember to enter your fax number so that we may reply as soon as possible.

CADS Address Details

If you would like to contact CADS, please do so on the following:

Computer and Design Services LtdArrowsmith Court

BroadstoneDorset

UKBH18 8AX

Tel. (Sales): +44 (0) 1202 603031Tel. (Support): i+44 (0) 1202 603733Fax (Support): i+44 (0) 1202 690284Email (Sales): I [email protected] (Support): [email protected]: www.cads.co.uk

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