Lecture 7 The Fundamentals of Foundation Design

or

Which Foundations and Why

Prepared by

Scott Stewart Atkins Middle East PhD BE

Introduction

Tuesday 27th March Part 1 Basic Principles and Design Relationships Part 2 Design Examples

Tuesday 3rd April Part 3 Fundamentals of Foundations (Which Foundations & Why) Part 4 IStructE Exam questions

Part 3 Fundamentals of Foundations (Which Foundations & Why) 1. Key Issues for Consideration 2. Foundation Types 3. Excavations 4. Retaining Walls

Key Issues for Consideration Uncertainties Significant and varying degrees of uncertainty are inherent in the geotechnical design process and allowances must be made for these uncertainties. Examples include :

estimating loads variability of the groundwater and ground conditions evaluation of geotechnical properties and behaviour of the founding strata compared to the analytical model chosen

Engineering judgement and experience are an essential part of geotechnical engineering and vital for controlling the safety of geotechnical structures. Get your geotechnical engineer involved!

Key Issues for Consideration

Loading Movement / Serviceability Stratigraphy Groundwater Previous use of the site Topography and Geomorphology Site Access Environmental Considerations Economy / Reliability / Durability Construction Design Mgmt (CDM) / Health and Safety

Think about

Vertical Compression Tension

Horizontal

Seismic / Dynamic (V, H)

Combinations of above

V

H

M

Key Issues for Consideration Loading

Total settlement

Differential movement

Rotation and angular distortion

Horizontal Strain

Key Issues for Consideration Movements

Excavations Movements Caused by;

Relaxation of soil towards excavated material Vertical and horizontal

movements

Groundwater drawdown

Heave and settlement

Water drawdown = settlement

Excavation Flexible vs Stiff walls Propping vs Excavation access

Dewatering

Ground water table drawdown Erosion during dewatering (piping)

Horizontal Strain

Key Issues for Consideration Movements

Key Issues for Consideration Movements

Key Issues for Consideration Movements

Limiting values

Values of relative rotation (angular distortion)

Type of structure Type of damage

Skempton and MacDonald

Meyerhof

Framed buildings and reinforced load bearing walls

Structural damage

Cracking in walls and partitions

1/150

1/300 (but 1/500 recommended)

1/250

1/500

Values of deflexion ratio D / L

Burland and Wroth

Cracking by sagging At L/H = 1 D / L = 0.4 X 10 -3 At L/H = 5 D / L = 0.8 X 10 -3

Unreinforced load- bearing walls

Cracking by hogging At L/H = 1 D / L = 0.2 X 10 -3 At L/H = 5 D / L = 0.4 X 10 -3

Key Issues for Consideration Stratigraphy How much information is available ? Soil or rock ? Variable or uniform ? Depth to founding strata Sources of information geological maps, borehole records etc.

Key Issues for Consideration Groundwater

Have a sound understanding of the hydrogeological conditions

Consider seasonal and long term water pressure changes

Check on the location and size of any man-made water sources

Consider the potential rise in water level caused by global damming effect of the structure

Is groundwater flow rate static?

Key Issues for Consideration Previous Site Use

Sources historical maps, archive records etc.

Often sites will have had a long and varied past.

- previous residential use - industrial / old infrastructure - historic mining Implications - obstructions - aggressive ground /contamination

Present Day

1900s

Key Issues for Consideration Topography & Geomorphology

Is the site level or sloping ?

What are the ground surface conditions? (boggy, hardstanding etc. )

Key Issues for Consideration Site Access and Constraints Is the site green field or urban

Head room restrictions e.g.

overhead power lines

Key Issues for Consideration Environmental Considerations

Pollution

BS5228 Part 4 1992, Code of practice for noise and vibration control applicable to piling operations

CIRIA TN142, 1992, Ground-borne vibrations arising from piling

Sensitivity of people ~ 0.15 mm / sec

Sensitivity of equipment ~ 5mm / sec for telephone exchange

Sensitivity of buildings > 10 mm / sec

Dust / Contamination

Key Issues for Consideration Economy / Reliability / Durability

Economy

selection of the most appropriate foundation solution

Reliability / Durability

linked to construction methods / workmanship

aggressive ground and groundwater conditions

Key Issues for Consideration Construction Design Management (CDM) Regulations (Health and Safety)

CIRIA Report 166, Feb 1997, CDM Regulations work sector guidance for designers

Plant Lifting/pitching piling equipment Open bores

Health and Safety issues associated with ground contamination

Foundation

Shallow Foundation

Deep Foundation

Pad/Strip Footings

Balanced Foundations

Raft Common Types

Uncommon Types

Piles, Anchors, etc

Foundation Types

Foundation Types Shallow Foundations

Typically 1m - 3m deep and generally more economical if competent strata are located near the ground surface. Require excavations so generally used when the water table is at depth. Types

Mass Footings Pads Strips Rafts

Foundation Types Shallow Foundations Things to Note

Frost susceptibility - expansion of soils when frozen due to formation of ice.

Keep foundation depth at least 450mm below ground level (BS8004 : 1996)

Change in ground moisture content leading to shrinking and

swelling seasonal wetting and drying (min depth of foundation 900mm

(BS8004 : 1996) Trees (removal will change conditions, e,g, heave, softening)

Inadequate bearing capacity

Excessive settlements

Swelling or shrinking clays

Founded on non engineered fill

High ground water

Foundation Types Deep Foundations

Large diameter bored pile

Barrette / diaphragm wall

Pre-bored H-pile Driven Pile Mini-pile

Foundation Types - Deep Foundations

Small diameter bored piles < 0.3m dia

Small rigs < 2m high

Capacities up to 1MN

Confined spaces / poor access

Drilling water / settling ponds required

Note: Durability

Foundation Types Deep Foundations Mini piles

Precast concrete piles (typically 0.3m square) - capacities up to 3MN

Steel piles bearing piles (H) or tubular piles

- capacities up to 8MN

Cheap and quick to install Need to consider installation forces (pile

head damage, bending in shaft) Noise and vibration issues Unsuitable for soils with boulders Fixed lengths Progressively harder to drive groups

Foundation Types Deep Foundations Driven Piles

Capacities up to 25MN straight shafted 0.45m - 2.5m

diameter under-reamed (up to 3 x shaft

diameter) base or shaft grouted (for extra

capacity)

Need support fluid where drilling below the water table

Casings (temporary or permanent) Can reinforce full depth

Foundation Types Deep Foundations Bored Piles

Capacities up to 7MN Sizes up to 0.9m diameter Relatively fast and cheap Reinforcement needs to be plunged

typically 12m max. No need for casings or support fluid Requires experienced drilling crew Not able to check base cleanness

Foundation Types Deep Foundations Continuous Flight Auger (CFA) Piles

Foundation Design Primary objectives of engineering design are :

Safety Serviceability Economy

Safety and serviceability can be improved by increasing the design margins or factors of safety (FoS) with the aim of reducing the probability of failure. To assist the engineer and to ensure compliance with a minimum specified level of technical quality we have:

Codes of practice such as BS:8004 and Eurocode 7 (EC7) Formulation of the geological and geotechnical models Evaluation of geotechnical design parameters and Choice of appropriate design methods.

London District Surveyors Association guidance note for piles in London Clay, 2000.

* Assumes: mean cu line based on metal

U100 samples) piles concreted within 12 hours independent supervision not for bentonite or CFA piles

Foundation Design Pile Foundations Factors of Safety (FoS)

Occurs when adjacent ground settles by more than pile Relative movement between fill and pile shaft Relative movement between underlying compressible stratum Consolidation of compressible layers Dewatering

All soil layers above settling layer impose negative skin friction. Measures to counteract

Tolerate additional pile settlement Sleeve piles Accelerate process so as to be complete by time pile is installed

E.g. Pre-loading, ground improvement Install longer piles to resist

Foundation Design Pile Foundations Negative Skin Friction

ICE 1996 Specification for piling and embedded retaining walls

Particular specifications only to be

filled out

Need to agree concrete and reinforcement requirements

Foundation Design Pile Foundations Specification

75mm in plan normal Out-of-position piles have moment induced in

head M = V d therefore more reinforcement required

1 in 75 verticality normal

Out-of-vertical piles have lateral force on head H = V tan

Effects reduce with depth (lateral pile analysis)

d

V

H

Foundation Design Pile Foundations Tolerances

ICE 1999 Specification for piling and embedded retaining walls, section 10.0 Static Load Testing of Piles

Provides requirements for:

Testing / monitoring equipment

and procedures Data to be reported by

contractor

Foundation Design Pile Foundations Testing

Key terms:

SWL - the specified working load of the pile (for design conditions)

DVL - design verification load of the pile is a substitute SWL for the conditions at the time of the pile test e.g additional pile length, variable groundwater conditions

MLT - maintained load test, with hold points at each load increment

CRP - constant rate of penetration test, pile loaded continuously at a prescribed rate to failure

Foundation Design Pile Foundations Load Testing

Proof load test (or contract pile test) - on working piles to prove

settlement for design method chosen

Proof Load = DVL + 50% SWL

Extended proof test (preliminary pile test) - to failure in order to establish pile design parameters

Proof load + increments of 25% SWL to failure

Foundation Design Pile Foundations Load Testing

Impulse testing Does not require any cast in

materials Detects cracks Good for near surface defects

only (has limited depth range)

Foundation Design Pile Foundations Integrity Testing

Sonic echo testing (Sonic logging) can pick up voids, reduced concrete strength

requires tubes cast into pile (not possible for

CFA piles)

increase tubes gives better coverage

does not give information on material in cover zone (i.e. outside of tubes)

Foundation Design Pile Foundations Integrity Testing

Consultant or contractor ?

Design responsibility must be clearly allocated

For contractor designed piles, there will be a performance requirement e.g. settlement at working load, therefore pile testing should strongly be considered

ICE 1996 Specification for piling and embedded retaining walls, Clause 1.2(j)

Foundation Design Pile Foundations Design Responsibility

First published by the BSi in 1995 as DD ENV 1997-1 : 1995

Superseded BS 5930, 6031,8002, 8004, 8006, 8081

BSi is not updating or maintaining superseded standards

BS8004 is referred to and in use in UAE

Foundation Design EC7

Adoption of limit-state design principals

Compatibility with EN1990 (Eurocode - Basis for Structural Design)

Partial factors applied to material strength, actions and resistances

National Application Standards for the individual member states to include local practices

Foundation Design EC7 Philosophy

Foundation Design - Summary

Excavations Open Cut Slope stability in fine grained soils

BUT pore water changes cause softening in clay => cu reduces and allowable height reduces.

Benching to avoid slope for material to fall into excavation

Material properties are very important in slopes and can have significant effects on stability esp. with pore water present.

u

ccH 4=Critical height,

Excavations Open Cut Slope stability in coarse grained soils

If soil is dry, soil can be angled at

natural angle of repose

Pore water will have major impact on slope stability

If shallow, water bearing soils, sheet piling may be used Nb flexible wall requires many

anchors at increasing depths to minimise ground movements

Retaining walls Cantilever (Pile) wall

Reducing pile spacing

Reducing wall flexibility (=> reduced ground movements)

King post Basic, cheap Timber/concrete lagging Watch passive resistance

Contiguous pile

Tangent pile Shotcrete finishing

Secant pile

Ground water cutoff Hard-soft (soft is unreinforced Hard-hard (all reinforced)

I beam or rectangular rebar cage in primary

Retaining walls Diaphragm Wall

Ground water cutoff

Can use as permanent wall Grab or Hydrofraise/Mill

Better verticality with hydrofraise

Panels installed in threes

Primary, secondary and closing

Hydrofraise/Mill Grab

Retaining walls Propping Raked struts

Horizontal Struts

Effect on construction access

Top Down Construction Use floor slab as strut to control

ground movements Detail temporary openings in slabs E.g. urban environments (metros,

towers, etc)

Retaining walls Anchoring

Ease of construction

Need to consider obstructions behind wall (e.g. utilities, basements, other retaining walls)

Prestressed anchors can act to minimise ground movements Nb effect of reverse curvature

on structure

Part 4 Workshop Using Previous Exam questions

Quickly review each question. Identify main geotechnical elements. Interpret geotechnical data. Assess structural implications due to the geotechnical setting. Undertake some simple geotechnical assessments.

Q1. Office Building Next to Existing Stone Tower Clients requirements 1. An existing stone tower is to be used in a new office development; see Figure Q1. 2. The tower is made from stone set in mortar and cannot be used to support the new office structure in any form. The new building is to be set partially into the interior of the tower as shown on Figure Q1. 3. The Architect wishes to retain the smallest floor depth possible and to have the building clad entirely in glass. The Architect has also stipulated that there is to be no visible structure around the glazed perimeter other than columns and the floor plate. Columns are to be spaced at least 8.0m apart. 4. The building is to have a 3.1m clear height between each floor and ceiling and is to be 4 storeys high. The height of the tower is 16.5m. The Architect has requested that the maximum level of the roof line of the building matches the height of the tower. 5. The existing stone tower is founded at a constant depth of 1.0m below ground level. The foundation of the tower does not extend beyond its plan area. Imposed loading 6. Roof 2.5kN/m2, Floor loading 6.0kN/m2, Loadings include an allowance for partitions, finishes, services and ceilings. Site conditions 7. The site is level and located in a park in the centre of a town. Basic wind speed is 40m/s based on a 3 second gust; the equivalent mean hourly wind speed is 20m/s.

Q1. Office Building Next to Existing Stone Tower Borehole 1 0 1.0m: Made ground 1.0m to depth: Rock. Allowable bearing pressure = 1000kPa Borehole 2 0 5.0m Made ground 5.0m - 8.0m Stiff clay

C = 80kPa Below 8.0m Rock.

Allowable bearing pressure = 1000kPa

Q1. Office Building Next to Existing Stone Tower

Stone tower is settlement sensitive. Glass cladding is movement sensitive faade. New foundation could cause existing stone tower to settle. Variable ground conditions! Rock is at different depth between boreholes.

Additional SI required to understand variability (i.e. what about third dimension?) . Make clear assumption but state further SI required.

Plan for new foundations to be below level of existing. Raft would require too much excavation. Placing foundation on clay gives SABP = 2*cu = 160kPa. E = 150*cu = 12MPa.

Nb can use benefit of surcharge to improve bearing capacity assessment. Loads are roughly 50kPa and assuming 8m c/c spacing gives 3200kN column load

i.e. 4.5m square pad. Settlement of a 4.5m pad = qB/E = 50kPa*4.5m/12MPa = 19mm. If bearing capacity not enough or settlement too large use piled foundations.

Q2. Hazardous Liquid Storage Building Clients requirements 1. A waterproof building is required to store two tanks containing hazardous liquids. 2. The tanks are 2 metres in diameter, 5 metres long and each weigh 400kN. 3. The tanks must be stored so that the underside of each tank is at an elevation of 5.0m above

ground level. No internal columns are permitted either within the building or beneath the tanks. A 6.0 m clear space is required around each tank for inspection and the tanks must have at least 4.0m clear space between them.

4. The building is to be situated at the centre of an island approximately 100.0m square protected by sheet piling.

5. The tanks will be delivered by barge at a minimum distance of 5.0m from the edge of the island and then stored in the building for approximately one month. During this period the access doors of the building must be kept shut.

Imposed loading 6. Roof loading 1.5kN/m2 (including imposed and services)

Q2. Hazardous Liquid Storage Building Site conditions The site is located in a river estuary. Basic wind speed is 46m/s based on a 3 second gust; the equivalent mean hourly wind speed is 23m/s. Mobile crane capacity on the island is limited to 20 tonnes due to the poor ground conditions. No suitable barge mounted cranes are available in this location. Borehole 1 at island edge Ground level 1.5 m River silt 1.5 m to 6.0 m Soft clay C = 25 kN/m2

6.0 m - depth Rock allowable safe bearing pressure 1000 kN/m2

Water was found at 4.0m depth Borehole 2 at island centre Ground level 0.5 m Topsoil 0.5 m 3.0 m Soft clay C = 25 kN/m2

3.0 m - depth Rock allowable safe bearing pressure 1000 kN/m2

Water was found at 2.0m depth

Q2. Hazardous Liquid Storage Building

Varying elevations of ground conditions. River Silt and Topsoil not suitable as founding stratum. Is Soft clay suitable as founding stratum? Placing foundation on clay gives S.A.B.P. = 2*cu = 50kPa. E = 150*cu =

3.75MPa. Loads are roughly 400kN and assuming 50kPa bearing capacity gives 2.8m

square pad. Settlement of a 2.8m pad = qB/E = 50kPa*2.8m/3.75MPa = 37mm. What about a raft? Settlement of a raft 17m wide loaded to 30kPa = qB/E =

30kPa*17m/3.75MPa = 136mm! If bearing capacity not enough or settlement too large use piled

foundations.

Q4. Commercial Building Clients requirements 1. A seven-storey commercial building on a square site 45.0m x 45.0m: see Fig. Q4. 2. The facade at the south-east corner is to be inclined between level 2 and the roof. All other facades are to be vertical. All facades are required to be fully-glazed between level 2 and the roof. 3. To provide flexibility for building entry points, the clear distance between external columns on level 1 must be a minimum of 8.0m. External columns on level 2 and above, if required, must be evenly-spaced. No column is permitted on any level at the north-west corner of the building. 4. Neither external nor internal structural walls are permitted. A clear distance of at least 7.0m is required between an internal column and any other column or external enclosure. The service cores are to be structurally independent of the main building. 5. No foundations may extend beyond the site boundary. 6. Allowable structural floor zones are: Level 2: 1.7m Other levels and roof: 1.2m 7. A minimum fire resistance of 2 hours is required for all structural elements. Imposed loading 8. Roof 2.5kN/m2 All floors 5.0 kN/m2

Q4. Commercial Building

Site conditions 9. The site is level and is located in the suburban area of a town 200km from the sea. Basic wind speed is 40m/s based on a 3 second gust; the equivalent mean hourly wind speed is 20m/s. 10. Ground Conditions 0.0m 2.0m Loose fill 2.0m 5.0m Sandy gravel. N varies from 10 to 20 5.0m 8.0m Weathered rock. Allowable bearing pressure 500kN/m2

Below 8.0m Rock. Allowable bearing pressure 1500kN/m2

Ground water was encountered at 2.5m below ground level.

Q4. Commercial Building Draw cross section. Loose Fill not suitable as founding stratum.

Is Sandy Gravel suitable as founding stratum?

Gravel N = 10 to 20 so = 30 33 degrees

Using Charts - S.A.B.P. = 200 to 250kPa. E = 1 or 2N = 10 40MPa. (say 25MPa)

Loads assume roughly 40kPa and assuming 8m column spacing gives 2560kN, thus

pad size = 2560kN/250kPa = 3.2m square pad.

Settlement qB/E = 250kPa*3.2m/25MPa = 32mm

Q5. Art Gallery Clients requirements 1. A two-storey art gallery is to be constructed on a sloping city-centre site containing a buried culvert: see Fig. Q5. 2. Level 1 is to have plan dimensions of 56.0m x 35.0m with columns at a minimum centre-to-centre spacing of 7.0m in each direction. Level 2 is to have plan dimensions of 50.0m x 12.0m with no internal columns. An allowance for lift and stair cores is included within these plan dimensions. 3. The floor-to-floor height from levels 1 to 2, and the floor-to-eaves height from level 2 to the roof is to be 4.5m. A maximum structural zone of 0.75m is permitted. 4. A flat, level access route of minimum width 3m is to be provided around the perimeter of the building at level 1. 5. A single car park with plan dimensions of 20.0m x 50.0m is required. 6. Access to the site is to be provided at the two locations shown on Fig. Q5, one for vehicles and one for pedestrians. 7. The culvert may be built over but may not be diverted and no additional loads may be applied to it either vertically or laterally. No construction may approach horizontally closer than 4.0m to the centreline of the culvert. Imposed loading 8. Gallery floors, levels 1 and 2 5.0kN/m2

Roof 1.5kN/m2

Car park 2.5kN/m2

Q5. Art Gallery

Site conditions 9. The site is located in a city 100km from the sea. Basic wind speed is 46m/s based on a 3 second gust; the equivalent mean hourly wind speed is 23m/s. 10. Ground conditions: Datum level - 12.0m sandy clay, C =100kN/m2,

=15 degrees Below -12.0m rock, allowable bearing capacity

= 2000kN/m2

Groundwater was found at 3.0m below ground level. The soil strata and ground water level may be assumed to follow the slope of the ground.

Q5. Art Gallery How to deal with the culvert? Placing a load bearing structure above it will induce a stress on the culvert which is not

allowed. Placing foundations to the side of the culvert will also impart a horizontal stress if it is too

close. No construction can be placed within 4m of the culvert. Options are: 1. Excavate for shallow foundations and place them below the level of the culvert. Will have

to deal with groundwater (dewatering), and possibly temporary shoring to the culvert using sheet piles.

2. Place shallow foundations above water table and a calculated distance from the edge of the culvert to ensure that no horizontal stress increase on culvert, i.e. use a conservative 45 degree distribution. Structure will have to cantilever or bridge over.

3. Use two rows of sleeved piles either side of the culvert 4m away and bridge over the culvert. The upper portion of the pile above culvert level has a frictionless sleeve to make sure no stresses imparted onto the culvert. Frictionless sleeve is either bitumen coating or an outer casing with a gap between the actual pile.

List of references BS5228 Part 4 (1992). Code of practice for noise and vibration control applicable to

piling operations BS 5930 (1999). Code of Practice for Site Investigations BS 6031 (2009). Code of Practice for Earthworks BS 8002 (1994). Code of Practice for Earth Retaining Structures BS 8004 (1986). Code of Practice for Foundations BS 8006 (1995). Code of Practice for Strengthened Reinforced Soils and Other Fills BS 8081 (1989). Code of Practice for Ground Anchorages BS EN 1997-1 (2004). Eurocode (EC)7: Geotechnical Design CIRIA R166 (1997) CDM Regulations work sector guidance for designers CIRIA TN142 (1992) Ground-borne vibrations arising from piling ICE (1996) Specification for piling and embedded retaining walls

Special thanks to Esad Porovic (Arup) and Imraan Motara (WSP).

Any Questions?

What is the active earth pressure for undrained conditions? Refer to BS 8004 For active conditions;

Total active thrust normal to the wall = Pan zcKzP uacan =

2.21

Take cw/cu = 0.75 => Kac = 2.6

=> Assuming v = .z

GOOD LUCK!

THE END

Lecture 7The Fundamentals of Foundation DesignorWhich Foundations and WhyIntroductionPart 3 Fundamentals of Foundations (Which Foundations & Why)Slide Number 4Slide Number 5Slide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Key Issues for Consideration Slide Number 17Slide Number 18Slide Number 19Foundation Types Slide Number 21Slide Number 22Slide Number 23Slide Number 24Slide Number 25Slide Number 26Slide Number 27Slide Number 28Slide Number 29Slide Number 30Slide Number 31Slide Number 32Slide Number 33Slide Number 34Slide Number 35Slide Number 36Slide Number 37Slide Number 38Slide Number 39Slide Number 40Slide Number 41Slide Number 42Slide Number 43Slide Number 44Slide Number 45Slide Number 46Slide Number 47Slide Number 48Part 4 Workshop Using Previous Exam questionsQ1. Office Building Next to Existing Stone TowerQ1. Office Building Next to Existing Stone TowerQ1. Office Building Next to Existing Stone TowerQ2. Hazardous Liquid Storage BuildingQ2. Hazardous Liquid Storage BuildingQ2. Hazardous Liquid Storage BuildingQ4. Commercial BuildingQ4. Commercial BuildingQ4. Commercial BuildingQ5. Art GalleryQ5. Art GalleryQ5. Art GalleryList of referencesAny Questions?What is the active earth pressure for undrained conditions?GOOD LUCK!