Croft Structural Engineers Clock Shop Mews Structural

Job Number: 160601

Revision Date Comment

- June 2016 First Issue

Croft Structural Engineers

Clock Shop Mews

Rear of 60 Saxon Road

London SE25 5EH

Structural Method Statement

Site Details

37 Calabria Road

London

N5 1HZ

Client’s Details

Mr Kevin O’Sullivan

Structural Design Reviewed by

Chris Tomlin MEng CEng MIStructE

Above Ground Drainage Reviewed by

Phil Henry BEng MEng MICE

Report compiled by

Noma Manzini MTech. CEng

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W:\Project File\Project Storage\2016\160601-37 Calabria Road\2.0.Calcs\BIA\160610-CMS.docx

Contents Non-technical Summary .................................................................................................................................... 4

1. Introduction .................................................................................................................................................. 5

2. Desk Study ..................................................................................................................................................... 6

2.1. Proposed Works ................................................................................................................................... 6

2.2. Age of Property & Site History ........................................................................................................... 7

Age of Property ............................................................................................................................................ 7

Site History ..................................................................................................................................................... 7

2.3. Listed Buildings ..................................................................................................................................... 7

2.4. Adjacent Properties ............................................................................................................................ 8

2.5. Topography ....................................................................................................................................... 10

2.6. Highways, Rail and London Underground .................................................................................... 12

2.7. Trees .................................................................................................................................................... 12

3. Site Investigation, Existing foundations and Ground Water ................................................................ 14

3.1. Desk Study Geology ......................................................................................................................... 14

Local Borehole information ...................................................................................................................... 16

3.2. Summary of Site investigation ......................................................................................................... 16

Existing Foundations/ Trial Pit Results ....................................................................................................... 17

Conclusion .................................................................................................................................................. 17

Slope Stability .............................................................................................................................................. 17

3.3. Ground Water Desk Study ............................................................................................................... 17

River & Water Course ................................................................................................................................ 19

Historic Water Course ................................................................................................................................ 19

Flood Risk ..................................................................................................................................................... 19

Fluvial Flood Risk ......................................................................................................................................... 19

Surface Water ............................................................................................................................................. 20

Ground water ............................................................................................................................................. 20

3.4. Ground Water Assessment .............................................................................................................. 21

Ground Water and Cumulative Effects ................................................................................................. 21

Surface Water & SuDS Desk Study ........................................................................................................... 22

Surface Water Proposals & SuDS Proposals ........................................................................................... 23

4. Ground Movement Assessment & Predicted Damage Category .................................................... 24

Burland Scale .............................................................................................................................................. 29

5. Engineer Works ........................................................................................................................................... 30

Loading (BS 6399-1) ................................................................................................................................... 32

Progressive Collapse ..................................................................................................................................... 32

Lateral Stability ............................................................................................................................................... 33

Stability Design ........................................................................................................................................... 33

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Lateral Actions ............................................................................................................................................ 33

Roads and Adjacent Loads ..................................................................................................................... 34

Basement Design ........................................................................................................................................... 35

Temporary Works ........................................................................................................................................... 37

Appendix A..................................................................................................................................................... 39

Structural Design Calculations ..................................................................................................................... 39

wall 1 and wall 2 (temporary case) ............................................................................................................... 41

Retaining wall analysis & design (BS8002) ..................................................................................................... 42

RETAINING WALL ANALYSIS (BS 8002:1994) ................................................................................................ 42

RETAINING WALL DESIGN (BS 8002:1994) ................................................................................................... 45

Design of reinforced concrete retaining wall toe (BS 8002:1994) ...................................................... 45

Design of reinforced concrete retaining wall heel (BS 8002:1994) .................................................... 45

Design of reinforced concrete retaining wall stem (BS 8002:1994) ................................................... 46

Indicative retaining wall reinforcement diagram................................................................................. 48

wall 1 and wall 2 (permanent case) .............................................................................................................. 49








wall 3 wall 4 (temporary case) ........................................................................................................................ 55








wall 3 and wall 4 (permanent case) .............................................................................................................. 62








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6. Noise Vibration and Dust .......................................................................................................................... 68

1. Preparation of site to fully contain the area ..................................................................................... 68

2. Management and hours of working................................................................................................... 70

3. Excavation of basement ...................................................................................................................... 70

Appendix B ......................................................................................................................................................... 72

Basement Method Statement ..................................................................................................................... 72

37 Calaria Road ................................................................................................................................................ 73

1. Basement Formation Suggested Method Statement ..................................................................... 73

2. Enabling Works ...................................................................................................................................... 74

3. Basement Sequencing ......................................................................................................................... 74

4. Underpinning and Cantilevered Walls .............................................................................................. 76

5. Floor Support .......................................................................................................................................... 84

Timber Floor ................................................................................................................................................. 84

6. Concrete Ground bearing slabs .................................................................................................... 84

7. Supporting existing walls above basement excavation ................................................................ 84

8. Approval ................................................................................................................................................. 85

9. Basement Temporary Works Design Lateral Propping .................................................................... 86

Trench Sheet Design ..................................................................................................................................... 87

Cross Props ..................................................................................................................................................... 91

Appendix C – Structural Plans & Details ........................................................................................................ 94

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Non-technical Summary Croft Structural Engineers has produced a Structural Method Statement to

support the planning application for a new basement excavation at 37 Calabria Road.

Croft Structural Engineers carried out a desk study of the property and the surrounding area. The following points were noted:

No listed buildings were identified immediately adjacent to the site. The site was not used for industrial purposes in the past No basements were identified in the neighbouring properties. No significant electrical power assets close by No larger water features (ex: ponds) in the locality. Examination of maps from the Environment Agency show that the

site is not in risk of flooding due to rivers & seas, ground water and surface water

The underlying soil to be London Clay formation The property is not in the conservation area.

Site investigations consisted of walk over surveys. The walk over survey visually confirmed the following:

The findings of the desk study relating to the above ground features were correct

No trees are present in the property and neighbouring properties which get affected by basement construction.

The structure has no cracks on the external wall The neighbouring structures are in good condition and no cracks

were identified on the external wall.

Croft Structural Engineers produced a scheme design for the new basement structure. The following features were incorporated into the design:

Lateral loads from soil, water and surcharge.

Vertical loads from existing building

Temporary works and construction method statement

Movement to the structure and to the neighbouring properties due to the construction of new basement

Given the above, the basement construction has no significant impacts on the neighbouring properties and the local area.

After the planning, the project should be adequately monitored for the duration of the construction. To help achieve this, a suitably qualified professional with relevant experience should be appointed by the client to ensure that the planning recommendations of this Structural Method

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Statement are followed.

1. Introduction

The Structural Method Statement SMS has been produced following the Supplementary Planning Document -January 2016 (SPD) for Basement Development in Islington Borough.

The Key Elements of the report are;

Desk Study

Site investigation t

Engineering Design Work Completed by a Structural Engineer

Temporary works and Construction Sequence

After the planning, the project should be adequately monitored for the duration of the construction. To help achieve this, a suitably qualified professional with relevant experience should be appointed by the client to ensure that the planning recommendations of this Structural Method Statement are followed.

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2. Desk Study

2.1. Proposed Works

The proposed work constitutes excavation of a new basement under the footprint of the property. This will be constructed in reinforced concrete retaining walls underpinning the existing external walls. Light wells will be created at the front and side.

Croft Structural Engineers Ltd has extensive knowledge of the design and construction of new basements. Over the last 4 years we have completed over 400 basements in and around the area. The method developed is:

1. Excavate front to allow for conveyor to be inserted.

2. Form ‘front of basement’ with cantilevered retaining walls

3. Slowly work from the front to the rear inserting 1000 long cantilevered retaining walls sequentially.

4. Cast ground slab

5. Waterproof internal space with a drained cavity system.

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2.2. Age of Property & Site History

Age of Property The Property is Victorian era property.

Site History What was the previous usage of the site?

Historical maps of the area show the area used for residential purposes at least from past100 years. It is unlikely therefore that the land under this site had industrial uses at any time in its history.

Fig. 1: Map of London & Suburbs 1868 map

Local Bombing An extract from world war II bomb census (below) shows that bomb is located near the site. Hence care should be taken during excavation.

Fig 2: Extract from Bomb sight map

2.3. Listed Buildings

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Is the building listed

No. 37 Calabria Road

Figure 3: Extract from English Heritage maps

Are the adjacent buildings listed?

No. Adjacent buildings are not listed.

2.4. Adjacent Properties

The condition of the adjacent buildings has been inspected to consider whether the basement will significantly affect their structure.

Visual inspections of the internal facades have been undertaken of the properties.

Nos 35–Calabria Road –Property to the left

Property Age: Victorian era property

Property use: Residential

Number of storeys: Three

Is a basement present? No.

Structural Defects Noted

There are no defects noted from outside

Structural Assessment of ongoing movement:

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There are no signs of movement

Figure 4: 35 Calaria Road

Property to Right There is road to the right side of the property

Nos 39 Calabria Road – Property to Rear

Property Age Victorian Era:

Property use: Residential

Number of storeys: Three

Is a basement present? No.

Structural Defects Noted: No defects noted from outside

Structural Assessment of ongoing movement: No signs of ongoing movement in the property.

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Figure 5: 39 Calabria Road

2.5. Topography

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Figure 6: Extract from Lidar Topography map

From the topography map it is seen that the area is on same level ground.

Slope Stability Does the existing site include slopes, natural or manmade greater than 7o (approximately 1 in 8)?

No. Difference in height between the rear garden and front is less than 1 in 8 slopes (approx. flat). There are no major falls within 20m which will increase the risk of land slip.

Will the proposed re profiling of landscaping at site change slopes at the property boundary to more than 7o (approximately 1in 8)?

No. The proposed landscaping does not affect the slope.

Does the development neighbour land including railway cuttings and the like with a slope greater than 7o (approximately 1 in 8)?

No. There are no railway cuttings adjacent to the property.

Is the site within a wider hillside setting in which the general slope is greater than 7o (approximately 1 in 8)?

No. The slope of the wider hillside setting is as per the property, approximately flat.

Is the London Clay the shallowest strata on site?

Yes. London Clay Formation is the shallowest strata.

Will any tree/s be felled as part of the proposed development

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and/or are any of the works proposed within any tree protection zones where trees are to be retained.

No. No local trees are to be felled.

Is there a history of seasonal shrink-swell subsidence in the local area, and/ or evidence of such effects at the site?

No. Subsidence not considered as an issue on this site.

Is the site within an area of previously worked ground?

No. From the historical maps, the site has been residential for at least 100 years

2.6. Highways, Rail and London Underground

Highways Is the site within 5m of a highway or pedestrian footway?

Yes. Site is within 5m of the footpath/alleyway and the road surface is within 5m from the front light well.

Highways loading allow:

10kN/m2 if within 45° of road

100kN point loads if under road or with in 1.5m

5kN/m2 if within 45° of Pavement

Garden Surcharge 2.5kN/m2

Surcharge for adjacent property 1.5kN/m2 + 4kN/m2 for concrete ground bearing slab

London Underground and Network Rail

Is the site over (or within the exclusion zone) of any tunnels, e.g. railway lines?

No. Nearest is the Overground Rail, +/- 640m from site.

UK Power Networks Will the basement works affect any UK Power Network Assets?

(Substations etc)

No. There are no Power Network Assets (sub stations) near the property.

2.7. Trees

Vicinity of Trees Some shrubbery and general vegetation in the neighbouring garden..

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Are any trees to be removed due to the basement?

No trees to be removed due to the basement development

Conservation Area Is the site in Conservation Area?

No. The site is not in Conservation Area

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3. Site Investigation, Existing foundations and

Ground Water

3.1. Desk Study Geology The British Geological Survey (BGS) maps shows that the underlying strata is

London Clay Formation: Clay, Silt and Sand

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Figure 7: Extract from BGS maps

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Local Borehole information

Local Bore Hole Information for site nearby from BGS indicates that the soil is Clay and there is no water found in the borehole.

TQ38NW371

Figure 8: Extract from BGS Borehole scans

3.2. Summary of Site investigation

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BGS maps and the borehole scans of the surrounding areas (from BGS)

indicate that the underlying soil to be London Clay formation.

Existing Foundations/ Trial Pit Results

As the property is Victorian era building the foundations are with brick corbels at shallow depths. However, this can be confirmed with trial pits during detailed design stage.

Conclusion The clay substrate is good to excavate through and create basements in. The inherent nature of the clay to form a clean face results in more stable excavations in the short term. The Trenches must be fully propped during the works.

With Clays water will cause volumetric change to the ground allowing it to swell and shrink. As such the area has an elevated risk of subsidence. The items that increase the risk of subsidence are drainages and Trees. Trees affect the Clays by removing water from the soil in the summer causing it to shrink. The affect can be noticed up to 30m from some trees. Placing a basement under the building will reduce the effects of tree on the property and decrease the risk of subsidence. Adding a new re-inforce concrete structure below the existing property will therefore enhance the structural integrity of the existing building.

As party wall is to be underpinned and will leave the party wall with a deeper footing than the neighbours other walls, the design should look at the available bearing capacity. As part of the Party Wall agreement a pre-condition survey will be carried out. The design will consider the impact of the deeper footings.

Slope Stability Design overall stability to Ka & Kp values. Lateral movement necessary to achieve Ka mobilisation is height/500 (from Tomlinson). This is tighter than the deflection limits of the concrete wall.

For the retaining wall and angle of friction of Ø= 30º is used.

3.3. Ground Water Desk Study

Groundwater flow

Is the site located directly above an aquifer?

No. The Environment Agency maps show the site is not above an aquifer. The site is located above the London Aquifer which is to be found at depth below the London clay at around 100m.

The site is not near boundary of soil interfaces. It is not considered that the

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new basement will cause new springs to appear.

Is the site within 100m of a watercourse, well used/disused or potential spring line?

No. OS maps and local walkover survey show no wells, watercourses or potential spring lines within 100m of the site.

Is the site within an aquifer? If so will the proposed basement extend beneath the water table such that dewatering may be required during construction?

No. The Environment Agency maps show the site is not above an aquifer. The site is located above the London Aquifer which is to be found at depth below the London clay at around 100m.

Site Water Table Unknown – Knowledge of groundwater table required. Trial pit will be completed prior to undertaking the work. The design of the foundations will be to the new EuroCodes which requires the water table to be considered to full height this allows for local flooding/burst water mains etc.

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River & Water Course

Figure 9: Water courses in Islington

Historic Water Course

Are there any Historic(Lost ) water courses in the vicinity?

No. There are no Historic water courses near site.

Flood Risk

Fluvial Flood Risk Is the Area in a Flood Risk Zone?

No. The area is not in floor risk zone.

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Figure 10: Extract from Environmental agency map

Surface Water Is the Site having risk of flooding due to surface water?

No. Environmental Agency maps shows that the property has no risk due to surface water flooding.

Figure 11: Extract from Environmental Agency maps

Ground water Is the site in an area known to be at risk from ground water flooding?

No. The site is not in risk of flooding due to ground water

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Figure 12: Extract from Appendix E of SPD 2016

3.4. Ground Water Assessment

Ground Water and Cumulative Effects

As clays are encountered at depth then a 150mm layer of compacted type I should be provided to prevent damming.

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Figure 13 Extract from Arup report on ground water flow

The reinforced retaining walls have been designed to withstand ground water flooding.

Surface Water & SuDS Desk Study

As part of the proposed site drainage, will surface water flows (e.g. volume of rainfall and peak run-off) be materially changed from the existing route?

No. The rainwater run-off will still percolate into the ground.

Will the proposed basement development result in a change to the hard surfaced /paved external areas?

No. The amount of hard standing is very little.

Will the proposed basement result in changes to the inflows (instantaneous and long term of surface water being received by adjacent properties or downstream watercourses?

No. The proposed development will enter the current drainage system.

Will the proposed basement result in changes to the quality of surface water being received by adjacent properties or downstream watercourses?

No. The quality of water is unlikely to be altered; the route it uses to reach the adjacent land will be altered. Proposed development will enter the current drainage system.

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As part of the site drainage will more surface water (e.g. rainfall and run-off) than at present be discharged to the ground?

No. Existing roof drainage will run into the existing drainage system. Surface water will still discharge to ground.

Surface Water Proposals & SuDS Proposals

The basement is under the existing footprint of the building and there is no increase in the run off from the property.

The groundwater has been assumed to be at full height of the retaining walls which have been designed for localized failure (Burst Mains etc)

Place a 150mm deep drainage layer across the site to prevent damming. Provide a 150mm deep by 1000mm wide Type (i) wrapped in terram below one line of pins across the width of the property.

This proposal is not considered to be in an area at risk of flooding.

The flow of surface water (above the basement) will need to be considered. A 150mm high protrusion of the wall from the light well will minimise the risk of localized flooding though the light well.

Control of water in basement

It is not intended for water to enter the basement. However, should water enter it will need to be removed. To accommodate this into the scheme the following will be incorporated;

Sump pump required with positive pressure pumping. Dual pumps with automatic float values Battery backup for 24 hours to allow for continuous working if power

supply fails.

If external drainage is low and gravity drainage is possible then any drainage must incorporate one-way flow values.

Drainage and Damp proofing

Assumed that drainage and damp proofing is by others: Details are not provided within our brief.

Our recommendation is that drained cavity systems are used to habitable basements with pumped sumps. This is a specialist contractor design item.

Concrete is not designed BS 8007. But where possible BS 8007 detailing is observed to help limit crack widths of concrete.

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Is a Flood Risk Assessment needed? No

Is a SuDS Design needed? No

4. Ground Movement Assessment & Predicted

Damage Category This assessment covers both short term and long term movements relating

to the construction and the performance of the permanent works. The design and construction methodology aims to limit damage to the existing building on the site and to all adjoining buildings to Category 2(max.) as set out in Table 2.5 of CIRIA report C 580 .

This assessment has used empirical means as set out in CIRIA2 C 580 Embedded Retaining Walls: Guidance for Economic Design.

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Ref

Movement

Width, L= 4800 mm

of neighbouring building

Existing building

Height H= 8000 mm

L/H = 0.60

New Basement Basement Hb= 2.5 m

Potential movement due to installation of wall using parameters from Table 2.2 of CIRIA C580

Horizontal Surface Movement / Wall Depth = -0.05%

max h = -0.05% x 2.5 = -1.25 mm

Distance behind wall wall to neglibible movement (mulitple of wall depth) = 1.5

L = 2.5 x 1.5 = 3.75 m

Horizontal Movement gradient due to installation = -0.3 mm/m

x = 0 x = 3.75 m (distances are measured from underpinned wall)

(deflection graphs are indicative and not to scale)

h = -1.3 mm h = 0.0 mm h -0.0260%at x = 0 at x = L

Vertical Surface Movement / Wall Depth = -0.05%max v = -0.05% x 2.5 = -1.25 mm


L = 2.5 x 1.5 = 3.75 m

Movement Assessment CIRIA C580: Embedded retaining walls - guidance for ecomonic design

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Vertical Movement gradient due to installation = -0.3 mm/m

x = 0 x = 3.75 m (distances are measured from underpinned wall)

= -1.3 mm = 0.0 mm /L -0.0260%at x = 0 at x = L

Potential movement due to excavation of wall using parameters from Table 2.4 of CIRIA C580

(excavation will be propped during construction)

Horizontal Surface Movement / Wall Depth = -0.15%

max h = -0.15% x 2.5 = -3.75 mm

Distance behind wall wall to neglibible movement (mulitple of wall depth) = 4

L = 2.5 x 4 = 10 m

Horizontal Movement gradient due to excavation = -0.4 mm/m

x = 0 x = 10000 mm (distances are measured from underpinned wall)

(deflection graphs are indicative and not to scale)

h = -3.8 mm h = -2.0 mm h -0.04%at x = 0 at x = L

Vertical Surface Movement / Wall Depth = -0.10%max v = -0.10% x 2.5 = -2.5 mm


L = 2.5 x 3.5 = 8.75 m

Vertical Movement gradient due to excavation = -0.3 mm/m

x = 0 x = 8750 mm (distances are measured from underpinned wall)

= -2.5 mm = -1.1 mm /L -0.03%at x = 0 at x = L

Total movement at wall location (excavation and installation)Total Horizontal Movement (excavation and installation h = -5.0 mmTotal Vertical Movement (excavation and installation) = -3.8 mm

TOTAL STRAIN (EXCAVATION AND INSTALLATION)

Table 2.5 CIRIA C580Category of Damage Normal Degree Limiting Tensile Strain %

0 Negligible 0.00% - 0.05%1 Very slight 0.05% - 0.075%2 Slight 0.075% - 0.15%3 Moderate 0.15% - 0.30%

4 to 5 Severe to Very Servere > 0.30%

Max. Anticipated Damage may be categorised as 'Slight' ; Category 2lim = 0.120%

h = -0.064% h/lim = 0.53/L = -0.055% /L/lim = 0.46

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0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2

(‐) D/L/e

lim

eh/elim

Fig 2.18b from CIRIA C580

L/H = 0.5

L/H = 1

L/H = 1.5

L/H = 2

L/H = 4

building

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Any ground works pose an elevated risk to adjacent properties. The proposed works undermines the adjacent property along the party wall line:

The party wall is to be underpinned. Underpinning the party wall will remove the risk of the movement to the adjacent property.

The works must be carried out in accordance with the party wall act and condition surveys will be necessary at the beginning and end of the works.

The method statement provided at the end of this report has been formulated with our experience of over 400 basements completed without error.

The design of the retaining walls is completed to KO lateral design stress values. This increase the design stresses on the concrete retaining walls and limits the overall deflection of the retaining wall.

It is not expected that any cracking will be occurring during the works. However, our experience informs us that there is a risk of movement to the neighbours.

To reduce the risk, the development:

Employ a reputable firm for extensive knowledge of basement works.

Employ suitably qualified consultants. Croft Structural engineer has completed over 400 basements in the last 4 years.

Design the underpins to the stable without the need for elaborate temporary propping or needing the floor slab to be present.

Provide method statements for the contractors to follow

Investigate the ground, now completed.

Record and monitor the external properties. This is completed by a

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condition survey on under the Party Wall Act before and after the works are completed. See end of method statement.

Allow for unforeseen ground conditions: Loose ground is always a concern. The method statement and drawings show the use of precast lintels to areas of soft ground; this follows the guidance by the underpinning association.

With the above the maximum level of cracking anticipated is Hairline cracking which can be repaired with decorative cracking and can be repaired with decorative repairs. Under the party wall Act damage is allowed (although unwanted) to occur to a neighbouring property as long as repairs are suitability undertaken to rectify this. To mitigate this risk The Party Wall Act is to be followed and a Party Wall Surveyor will be appointed.

Burland Scale Extract from The Institution of Structural Engineers “Subsidence of Low-Rise Buildings”

Table 6.2 Classification of visible damage to walls with particular reference to type of repair, and rectification consideration

Category of Damage

Approximate crack width

Limiting Tensile strain

Definitions of cracks and repair types/considerations

0 Up to 0.1 0.0-0.05

HAIRLINE – Internally cracks can be filled or covered by wall covering, and redecorated. Externally, cracks rarely visible and remedial works rarely justified.

1 0.2 to 2 0.05-0.075

FINE – Internally cracks can be filled or covered by wall covering, and redecorated. Externally, cracks may be visible, sometimes repairs required for weather tightness or aesthetics.

NOTE: Plaster cracks may, in time, become visible again if not covered by a wall covering.

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5. Engineer Works

Structural Summary

This report is for planning purposes only and is not for construction: The information, drawings, calculations, method statement and other information in this report are for planning purposes. Croft provide no design warranty or insurances for the final design. Further information and design considerations must be undertaken before building regulations submission. The information provided in this document is not for construction.

37 Calabria Road is a single-occupancy three storey Victorian era terraced building. It has masonry external load bearing walls. The floor is made of timber. The internal walls are load bearing timber /masonry walls..

Figure 14: 37 Calaria Road Property

Structural Defects Noted There are damp signs on the party wall in the kitchen. There are also damp signs in the study room rear wall.

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Figure 15: Damp on party wall in kitchen

Figure 16: Damp in study room

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Intended use of structure and user requirements

Family/domestic use

Loading (BS 6399-1) UDL

kN/m2

Concentrated Loads kN

Domestic Single Dwellings 1.5 1.4

Is Live Load Reduction included in design No

Soil above garden structures

Islington require 1000mm depth in gardens.

Allow for 18kN/m3 for Dead loads from soils.

The current basement proposal does not have any garden basement.

Progressive Collapse

Number of Storeys 3+New Basement

Is the Building Multi Occupancy? No

Part A3 Progressive collapse

EN 1991-1-7:1996 Table A1

Class 1

Single occupancy houses not exceeding 4 storeys

Agricultural buildings

Progressive collapse Change of use

To NHBC guidance compliance is only required to other floors if a material change of use occurs to the property.

Initial Building Class 1

Proposed Building Class 1

If class has changed material change has occurred

No

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Additional Design Requirements to Comply with Progressive Collapse

Class1 – Design to satisfy EN 1990 to EN 1999 stability requirements

Lateral Stability

Exposure and wind loading conditions

0.6 kN/m2

Stability Design

The existing masonry walls which carry the stability of the house are not being altered. The reinforced concrete retaining walls are designed to carry the lateral loading applied from above.

The lateral earth pressure exerts a horizontal force on the retaining walls. They will be checked for resistance to overturning this produces.

Lateral Actions

Lateral Forces applied from;

Soil loads

Hydrostatic pressure

Surcharge loading.

These produce retaining wall thrust; this is restrained by the opposing retaining wall.

Retained soil Design overall stability to Ka & Kp values. Lateral movement necessary to achieve Ka mobilisation is height/500 (from Tomlinson). This is tighter than

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Parameters the deflection limits of the concrete wall.

Roads and Adjacent Loads

Check for

Highways loading allow:

10kN/m2 if within 45° of road

100kN point loads if under road or with in 1.5m

5kN/m2 if within 45° of Pavement

Garden Surcharge 2.5kN/m2

Surcharge for adjacent property 1.5kN/m2 + 4kN/m2 for concrete ground bearing slab

CE Marking of construction products

All products used in construction must have CE marking to demonstrate compliance where either a harmonized European standard or European Technical Assessment (ETA) is in force.

1. Consequence Class Table B.1 – Definition of Consequence Classes

Consequence Class

Description Examples

CC2 Medium consequence for loss of human life; economic, social or environmental consequences considerable

Residential and office buildings, public buildings where consequences of failure are medium

2. Service Category Table B.1 – Suggested Criteria for Service Categories

Categories Criteria

SC1 Buildings and components designed for quasi static actions only

Structures and components with their connections designed for seismic actions in regions with low seismic activity and in DCL

Structures and components designed for fatigue actions from cranes

(class S0).

3. Production Category Table B.2 – Suggested Criteria for Service Categories

Categories Criteria

PC1 Non welded components manufactured from any steel grade products

Welded components manufactured from steel grade products below

S355

PC2 Welded components manufactured from steel grade products from S355 and above

Components essential for structural integrity that are assembled by

welding on construction site

Components with hot forming manufacturing or receiving thermic

treatment during manufacturing

Components of CHS lattice girders requiring end profile cuts

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4. Production Category

Table B.3 – Recommended Matrix for Determination of Execution Classes

Consequence classes

CC1 CC2 CC3

Service categories SC1 SC2 SC1 SC2 SC1 SC2

Production PC1 EXC1 EXC2 EXC2 EXC3 EXC3/EXC4 EXC3/EXC4

Categories PC2 EXC2 EXC2 EXC2 EXC3 EXC3/EXC4 EXC4

Execution Class 2 (EXC2) will be appropriate for majority of building constructed in the UK. If execution class is not specified on a project EXC2 applies.

Basement Design

Our design considers a risk managed approach. With over 400 basements completed we have moved away from solely relying on soil testing of 2-3 discrete points as providing a reasonable description of the ground. Our design now considers what we assume to be the worst case ground that the building will encounter. Our design now has a risk managed approach considering the worst cases that may be found.

This report is for planning purposes only and is not for construction: The information, drawings, calculations, method statement and other information in this report are for planning purposes. Croft provide no design warranty or insurances for the final design. Further information and design considerations must be undertaken before building regulations submission. The information provided in this document is not for construction.

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Loads on Retaining wall

Water Table Has a soil investigation been carried out No

Design Permanent condition for water table level:

If deeper than existing, design reinforcement for water table at full basement depth to allow for local failure of water mains, drainage and storm water.

Global uplift forces can be ignored when water table lower than basement. BS8102 only indicates guidance.

Drainage and Damp proofing

Assumed that drainage and damp proofing is by others: Details are not provided within our brief.

It is recommended that a water proofing specialist is employed to ensure all the water proofing requirements are met. Croft structural engineers are not the waterproofing designer nor act as the structural waterproof designer.

Croft are not the structural waterproofer. The waterproofing specialist must name who is their structural waterproofer. The Structural waterproofer must inspect the structural details and confirm that are happy with the

Prop

52.5 kN/m2 52.5 kN/m2

5 kN/m25 kN/m2

55.9 kN/m2

1850

1675

1500 350

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robustness.

Due to the construction nature of the segmental basement it is not possible to water proof the joints. All water proofing must be made by the waterproofing specialist. They should make review of our details and recommend to us if water bars and stops are necessary.

The waterproof design must not assume that the structure is watertight. To help reduce water floor through joints in the segmental pins all faces should be;

Cleaned of all debris and detritus Faces between pins should be needle hammered to improve key All pipe work and other penetrations should have puddle flanges

or hydrophilic strips

Localised Dewatering

Localised dewater to pins may be necessary.

Some engineers may raise the theoretical questions about pumping of water causing localised settlement. We believe that this argument is a red herring when applied to single storey basements and our reason for stating this is:

The water table in the area is variable, The water level naturally rises and falls over time and does not lead to

subsidence The water table has naturally been rising and falling for over the last

20,000 years, any fines that will have been removed from the soil would have done so already.

If the water table rises and falls naturally why does this not cause subsidence due to fine removals every year? It does not because the soil has been naturally consolidated by the rise and fall of the water table in the area.

The effect of local pumping for small excavations will not affect the local area.

There is only a risk of subsidence from large scale pumping of soil which lowers the water table below its natural lowest level.

Temporary Works Walls are designed to be temporarily stable. Temporary propping details will be required for the ground and soil and this must be provided by the contractor. Their details should be forwarded to Croft Structural Engineers.

Particular attention should be paid to the point loads from above.

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Critical areas where point loads are present from above

Cross wall

Chimney Stack

Door openings

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Appendix A Structural Design Calculations

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WALL 1 AND WALL 2 (TEMPORARY CASE)

Location Area Type L Load Load kN

L W m2 kN/m2 Dead % Live Total

wall 1

roof DL 4.8 1.0 4.8 gk 1.20 5.8

roof LL qk 0.75 3.6

loft fl DL 4.8 1.0 4.8 gk 0.63 3.0

loft fl LL qk 1.50 7.2

2nd fl partitions 3.0 1.0 3.0 gk 0.55 1.7

1st fl DL 4.8 1.0 4.8 gk 0.63 3.0

1st fl LL qk 1.50 7.2

1st fl partitions 3.0 1.0 3.0 gk 1.50 4.5

ground fl DL 4.8 1.0 4.8 gk 0.63 3.0

ground fl LL qk 1.50 7.2

grd partitions DL 3.0 1.0 3.0 gk 1.50 4.5

grd to 1st wall DL 3.0 1.0 3.0 gk 7.60 22.8

1st to roof wall 6.0 1.0 6.0 gk 5.00 30.0

78.3 kN/m 25.2 kN/m

wall 2

roof DL 4.8 1.0 4.8 gk 1.20 5.8

roof LL qk 0.75 3.6

loft fl DL 4.8 1.0 4.8 gk 0.63 3.0

loft fl LL qk 1.50 7.2

2nd fl partitions 3.0 1.0 3.0 gk 0.55 1.7

1st fl DL 4.8 1.0 4.8 gk 0.63 3.0

1st fl LL qk 1.50 7.2

1st fl partitions 3.0 1.0 3.0 gk 1.50 4.5

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ground fl DL 4.8 1.0 4.8 gk 0.63 3.0


grd partitions DL 3.0 1.0 3.0 gk 1.50 4.5

grd to 1st wall DL 3.0 1.0 3.0 gk 7.60 22.8

1st to roof wall 6.0 1.0 6.0 gk 5.00 30.0

78.3 kN/m 25.2 kN/m

RETAINING WALL ANALYSIS & DESIGN (BS8002)

Loadings

Dead loadDL=79kN/m

Live loadLL=26kN/m

RETAINING WALL ANALYSIS (BS 8002:1994) TEDDS calculation version 1.2.01.06

Wall details

Retaining wall type Cantilever

Height of wall stem hstem = 3000 mm Wall stem thickness twall = 350 mm

Length of toe ltoe = 1500 mm Length of heel lheel = 300 mm

10 kN/m2105 kN/m

1600

Prop

2150

1500 350 300

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Overall length of base lbase = 2150 mm Base thickness tbase = 400 mm

Height of retaining wall hwall = 3400 mm

Depth of downstand dds = 0 mm Thickness of downstand tds = 400 mm

Position of downstand lds = 1250 mm

Depth of cover in front of wall dcover = 0 mm Unplanned excavation depth dexc = 0 mm

Height of ground water hwater = 0 mm Density of water water = 9.81 kN/m3

Density of wall construction wall = 23.6 kN/m3 Density of base construction base = 23.6 kN/m3

Angle of soil surface = 0.0 deg Effective height at back of wall heff = 3400 mm

Mobilisation factor M = 1.5

Moist density m = 18.0 kN/m3 Saturated density s = 21.0 kN/m3

Design shear strength ' = 24.2 deg Angle of wall friction = 0.0 deg

Design shear strength 'b = 24.2 deg Design base friction b = 18.6 deg

Moist density mb = 18.0 kN/m3 Allowable bearing Pbearing = 100 kN/m2

Using Coulomb theory

Active pressure Ka =0.419 Passive pressure Kp = 4.187

At-rest pressure K0 = 0.590

Loading details

Surcharge load Surcharge = 10.0 kN/m2

Vertical dead load Wdead = 79.0 kN/m Vertical live load Wlive = 26.0 kN/m

Horizontal dead load Fdead = 0.0 kN/m Horizontal live load Flive = 0.0 kN/m

Position of vertical load lload = 1600 mm Height of horizontal load hload = 0 mm

Loads shown in kN/m, pressures shown in kN/m2

Calculate propping force

Propping force Fprop = 4.9 kN/m

Check bearing pressure

Total vertical reaction R = 169.3 kN/m Distance to reaction xbar = 1159 mm

Eccentricity of reaction e = 84 mm

Reaction acts within middle third of base

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Bearing pressure at toe ptoe = 60.3 kN/m2 Bearing pressure at heel pheel = 97.2 kN/m2

PASS - Maximum bearing pressure is less than allowable bearing pressure

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RETAINING WALL DESIGN (BS 8002:1994)

TEDDS calculation version 1.2.01.06

Ultimate limit state load factors

Dead load factor f_d = 1.4 Live load factor f_l = 1.6

Earth pressure factor f_e = 1.4



Design of reinforced concrete retaining wall toe (BS 8002:1994)

Material properties

Strength of concrete fcu = 35 N/mm2 Strength of reinforcement fy = 500 N/mm2

Base details

Minimum reinforcement k = 0.13 % Cover in toe ctoe = 30 mm

Design of retaining wall toe

Shear at heel Vtoe = 164.8 kN/m Moment at heel Mtoe = 162.5 kNm/m

Compression reinforcement is not required

Check toe in bending

Reinforcement provided 16 mm dia.bars @ 150 mm centres

Area required As_toe_req = 1086.2 mm2/m Area provided As_toe_prov = 1340

mm2/m

PASS - Reinforcement provided at the retaining wall toe is adequate

Check shear resistance at toe

Design shear stress vtoe = 0.455 N/mm2 Allowable shear stress vadm = 4.733 N/mm2

PASS - Design shear stress is less than maximum shear stress

Concrete shear stress vc_toe = 0.521 N/mm2

vtoe < vc_toe - No shear reinforcement required

Design of reinforced concrete retaining wall heel (BS 8002:1994)

Material properties


Base details

Minimum reinforcement k = 0.13 % Cover in heel cheel = 30 mm

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Design of retaining wall heel

Shear at heel Vheel = 6.2 kN/m Moment at heel Mheel = 0.9 kNm/m


Check heel in bending


Area required As_heel_req = 520.0 mm2/m Area provided As_heel_prov = 754

mm2/m

PASS - Reinforcement provided at the retaining wall heel is adequate

Check shear resistance at heel

Design shear stress vheel = 0.017 N/mm2 Allowable shear stress vadm = 4.733 N/mm2


Concrete shear stress vc_heel = 0.428 N/mm2

vheel < vc_heel - No shear reinforcement required

Design of reinforced concrete retaining wall stem (BS 8002:1994)

Material properties


Wall details

Minimum reinforcement k = 0.13 %

Cover in stem cstem = 30 mm Cover in wall cwall = 30 mm

Design of retaining wall stem

Shear at base of stem Vstem = 51.3 kN/m Moment at base of stem Mstem = 128.4

kNm/m


350 31

2

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Check wall stem in bending


Area required As_stem_req = 996.2 mm2/m Area provided As_stem_prov = 1340

mm2/m

PASS - Reinforcement provided at the retaining wall stem is adequate

Check shear resistance at wall stem

Design shear stress vstem = 0.164 N/mm2 Allowable shear stress vadm = 4.733 N/mm2


Concrete shear stress vc_stem = 0.568 N/mm2

vstem < vc_stem - No shear reinforcement required

Check retaining wall deflection

Max span/depth ratio ratiomax = 9.88 Actual span/depth ratio ratioact = 9.62

PASS - Span to depth ratio is acceptable

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Indicative retaining wall reinforcement diagram

Toe bars - 16 mm dia.@ 150 mm centres - (1340 mm2/m)

Heel bars - 12 mm dia.@ 150 mm centres - (754 mm2/m)

Stem bars - 16 mm dia.@ 150 mm centres - (1340 mm2/m)

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WALL 1 AND WALL 2 (PERMANENT CASE)



Wall details



















10 kN/m2105 kN/m

1600

Prop

2150

1500 350 300

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Loading details














10105

Prop

99.2 60.44.2 3.0 14.0 29.428.6

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Material properties


Base details








mm2/m








Material properties


Base details


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mm2/m








Material properties


Wall details





kNm/m


350 31

2

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mm2/m










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WALL 3 WALL 4 (TEMPORARY CASE)

Location Area Type L Load Load kN

L W m2 kN/m2 Dead % Live Total

wall 3

ground fl DL 1.0 1.0 1.0 gk 1.00 1.0


1.0 kN/m 2.0 kN/m

wall 4

ground fl DL 1.0 1.0 1.0 gk 1.00 1.0


1.0 kN/m 2.0 kN/m


Loadings

Dead loadDL=1kN/m

Live loadLL=2kN/m


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Wall details




















Loading details





400

3000

3400

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103

Prop

50.5 3.94.2 25.628.6

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Material properties


Base details








mm2/m








Material properties


Base details


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mm2/m








Material properties


Wall details





kNm/m


350 31

2

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mm2/m










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WALL 3 AND WALL 4 (PERMANENT CASE)



Wall details



















400

3000

3000

3400

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Loading details











Reaction acts outside middle third of base



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Material properties


Base details








mm2/m








Material properties


Base details


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mm2/m








Material properties


Wall details





kNm/m


350 31

2

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mm2/m










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6. Noise Vibration and Dust Best construction method should be chosen to reduce the unnecessary

Noise, Vibration and dust. The following table is a guidance to minimise the effect of the same.

Borehole test, soil investigation, Construction Traffic Management Plan (CTMP) and Construction Method Statement (CMS). Full investigations and reports have to be carried out ahead of building works to formalize the best practical means to be used

CONSTRUCTION

METHOD

Measure,

NOISE DUST VIBRATION

In accordance with the best practical means, to be used

To minimize, noise, vibration and dust during the construction of the basement, including the excavation, that is likely to affect adjacent residential premises and school (if any)

1. Preparation of site to fully contain the area

Boarding to front of house enclosing entrance, and windows kept in place for complete duration of construction

Boarding keeps noise inside the house and keeps house more rigid stopping attenuation, absorbs sound and

Stops airborne sound escaping

Dust from debris stored internally is contained within boarded up house preventing it from escaping to neighbours before collection.

Any internal vibration is further reduced by additional boarding to absorb before emitting to neighbour: as timber absorbs vibration better than metal or glass. The house is also more rigid, stopping vibration

Windows retained and sealed shut during construction, including front door and terrace doors kept closed

Airborne noise is contained within development

Airborne dust is contained within the development

Windows being sealed shut (taped) stops any rattling of windows or accentuation of any vibrations on site

Hording and sheeting to cover roof terrace.

Covering with hording and sheeting restricts airborne noise from escaping as best can be.

Sheeting to roof terrace stops window blowing up dust from excavation and any dust generated from

Hording and sheeting stops vibration as best is practicable.

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CONSTRUCTION

METHOD

Measure,


works escaping to vicinity.

Retention of internal floors and structure during excavation works

Keeping the internal floors in situ during works allows the house to work as a buffer to contain noise and reduces the site area to the smallest volume reducing the effect noise can have.

Dust is contained to a smaller area and has several filters (ie floors and walls) to pass through and thus get stopped before it can affect neighbours, thus reduced.

Retaining the existing structure reduces vibration by keeping the house rigid and secondly by having a mix of materials all with different attenuation frequencies; vibration is absorbed and not accentuated, lastly floors and walls act as a break in otherwise continuous structure which acts as a buffer to stop vibration continuing out to neighbours.

Temporary works and structure

Temporary works allow the house to be kept rigid and allow for small scale, less noise emitting methods of construction to be used.

Temporary works keep the house rigid and safe so stop other areas of the house degenerating through works and thus dust being created.

Temporary works keep the house rigid which stops vibrations.

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CONSTRUCTION

METHOD

Measure,


2. Management and hours of working

Project manager to manage all works on site, member of Considerate Contractors Scheme

Hours of working are restricted and staff supervised to use tools appropriately. No radio on site.

Small team working reducing noise. Coordination between workers ensured.

Hours of working are restricted and staff supervised to use tools appropriately with appropriate guarding to prevent dust migration.

Hours of working are restricted and staff supervised to use tools appropriately and reduced use of power tools to minimize vibration.

3. Excavation of basement

Non-percussive tools used for excavation (ie hand dug)

Hand tools are quieter. Method chosen reduces need for any heavy noisy machinery

Less dust generated by hand tools than fast repetitive motor driven tools.

Vibration is minimized by not using percussive tools

Excavation limited to 1m runs and shuttered for reinforced concrete foundations.

Each underpin is restricted to 1m lengths containing noise and amount of work that can be done at once to small area thus reducing overall hubbub. Method is quieter than piling or machine methods.

Dust is contained within shuttering, area is dampened with water to allow digging and eliminate dust.

Shuttering contains any subsequent vibration from excavation and keeping surrounding area soil intact.

Removal of spoil

All spoil is hand bagged and stored internally by hand so no noise from skip or large refuse area, removed as per CTMP by small van and hand loaded

Spoil hand bagged, not using electric conveyor belt, and reducing emission of dust.

Spoil bagged by hand (ie shovel) so no machinery to transmit vibration

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CONSTRUCTION

METHOD

Measure,


Removal of debris

Bagged debris is stored internally in a covered area and removed by waiting small van as per CTMP timed to cause least disruption

Debris removed by hand; dust contained within refuse sack, sealed shut.

Debris removed by hand, vibration minimized, in bags.

Mixing and pouring of concrete for underpins

Concrete is mixed on site for small quantities for underpin, contained within the site for noise and for short period of time once underpin and shuttering formed (ie

Separate activity)

Area set aside and shuttered off for mixing concrete to contain dust. Only small quantities mixed at time. Only small amounts of dry concrete

Stored on site in internal area to avoid unnecessary dust.

Concrete mixer put on level base in clear working area to avoid vibration.

Delivery of concrete for floor reinforced floor slabs

Large quantities are not mixed on site but delivered and pumped by specialist lorry to site in speedy low noise method from front of house through hording

No dust emitted from delivery of liquid concrete, area of road washed down before and after delivery. Area cordoned off as per CTMP (approx. ½ hour).

Large quantities of concrete mixed off site to reduce continuous vibration and delivered to site.

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Appendix B

Basement Method Statement

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37 Calaria Road

1. Basement Formation Suggested Method Statement

1.1. This method statement provides an approach that will allow the basement design to be correctly considered during construction. The statement also contains proposals for the temporary support to be provided during the works. The Contractor is responsible for the works on site and the final temporary works methodology and design on this site and any adjacent sites

1.2. This method statement has been written by a Chartered Engineer. The sequencing has been developed using guidance from ASUC (Association of Specialist Underpinning Contractors).

1.3. This method has been produced to allow for improved costings and for inclusion in the Party Wall Award. Final site conditions need there to be flexibility in the method statement: Should the site staff require alterations to the Method statement this is allowed once an alternative methodology, of the changes is provided, and an Addendum to the Party Wall Award will be required.

1.4. Contact Party Wall Surveyors to inform them of any changes to this method statement.

1.5. On this development, the approach is: construct the underpin segments that will support the permanent steel work insert the new steelwork remove load from above and place it onto new supporting steelwork cast the remainder of the retaining walls that will form the perimeter of the basement.

1.6. On this project, the cantilever pins are designed to be inherently stable without lateral support to the top of the wall. However, temporary props will be provided near the head and will provide support until the concrete has gained sufficient strength. The base benefits from propping. This is provided in the final condition by the ground slab. In the temporary condition, the edge of the slab is buttressed against the soil in the middle of the property. Also the skin friction between the concrete base and the soil provides further resistance. The central soil mass is to be removed in 1/3 portions and cross propping subsequently added as the central soil ass is removed

1.7. BGS maps shows that the underlying soil as London Clay Formation.

1.8. The bearing pressures have been limited to 100kN/m2. This is standard loading for the local ground conditions and acceptable to Building Control and their approvals

1.9. The water table level at site is unknown now.

1.10. The structural waterproofer (not Croft) must comment on the proposed design and ensure that he is satisfied that the proposals will provide adequate waterproofing.

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1.1. Provide engineers with concrete mix, supplier, delivery and placement methods two weeks

prior to the first pour. Site mixing of concrete should not be employed apart from in small sections (less than 1m3). The contractor must provide a method on how to achieve site mixing to the correct specification. The contractor must undertake toolbox talks with staff to ensure site quality is maintained.

2. Enabling Works

2.1. The site is to be hoarded with ply board sheets, at least 2.2m high, to prevent unauthorised public access.

2.2. Licences for skips and conveyors should be posted on the hoarding.

2.3. Provide protection to public where conveyor extends over footpath. Depending on the requirements of the local authority, construct a plywood bulkhead over the pavement. Hoarding to have a plywood roof covering over the footpath, night-lights and safety notices.

2.4. Dewater: If water is present,

2.4.1. Place a bore hole to the front of the property down to a depth of 6m 2.4.2. Pump water away from site.

If water table is not present,

2.4.3. No significant dewatering is expected. Localised removal of water may be required to deal with rain from perched water or localised water. This is to be dealt with by localised pumping. Typically achieved by a small sump pump in a bucket.

2.5. On commencement of construction, the contractor will determine the foundation type, width and depth. Any discrepancies will be reported to the structural engineer in order that the detailed design may be modified as necessary.

3. Basement Sequencing

3.1. Begin by placing cantilevered walls as noted on plans. (Cantilevered walls to be placed in accordance with Section 4.)

3.2. Needle and prop the floor/ walls over.

3.3. Insert steel over and sit on cantilevered walls.

3.3.1. Beams over 6m to be jacked on site to reduce deflections of floors.

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3.3.2. Dry pack to steelwork. Ensure a minimum of 24 hours from casting cantilevered walls to dry-packing. Grout column bases

3.4. Excavate lightwell to front of property down to 600mm below external ground level.

3.5. Excavate first front corner of lightwell. (Follow methodology in Section 4)

3.6. Excavate second front corner of lightwell. (Follow methodology in Section 4)

3.7. Continue excavating section pins to form front lightwell. (Follow methodology in Section 4)

3.8. Place cantilevered retaining wall to the left side of front opening. After 48 hours place cantilevered retaining wall to the right side of front opening.

3.9. Needle and prop bay. Insert support

Figure 2 Example of needling to existing wall

3.10. Excavate out first 1.2m around front opening, prop floor and erect conveyor.

3.11. Continue cantilevered wall formation around perimeter of basement following the numbering sequence on the drawings.

3.11.1. Excavation for the next numbered sequential sections of underpinning shall not commence until at least 48 hours after drypacking of previous works. Excavation of adjacent pin to not commence until 48 hours after drypacking. (24hours possible due to inclusion of Conbextra 100 cement accelerator to dry pack mix). No more than

3.11.2. Floor over to be propped as excavation progresses. Steelwork to support floor to be inserted as works progress.

3.12. Excavate and cast floor slab

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3.12.1. Excavate 1/3 of the middle section of basement floor. As excavation

proceeds, place Slim Shore props at a maximum of 2.5m c/c across the basement. Locate props at a third of the height of the wall.

3.12.2. Continue excavating the next 1/3 and prop then repeat for the final 1/3.

3.12.3. Place below-slab drainage. Croft recommends that all drainage is encased in concrete below the slab and cast monolithically with the slab. Placing drainage on pea shingle below the slab allows greater penetration for water ingress.

3.12.4. Place reinforcement for basement slab.

3.12.5. Building Control Officer and Engineer are to be informed five working days before reinforcement is ready and invited for inspection.

3.12.6. Once inspected, pour concrete.

3.13. Provide structure to ground floor and water proofing to retaining walls as required. It is recommended to leave 3-4 weeks between completion of the basement and installing drained cavity. This period should be used to locate and fill any localised leakage of the basement

4. Underpinning and Cantilevered Walls 4.1. Prior to installation of new structural beams in the superstructure, the contractor may

undertake the local exploration of specific areas in the superstructure. This will confirm the exact form and location of the temporary works that are required. The permanent structural work can then be undertaken whilst ensuring that the full integrity of the structure above is maintained.

4.2. Provide propping to floor where necessary.

4.3. Excavate first section of retaining wall (no more than 1000mm wide). Where excavation is greater than 1.2m deep, provide temporary propping to sides of excavation to prevent earth

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collapse (Health and Safety). A 1000mm width wall has a lower risk of collapse to the heel face.

4.4. Excavation of pins involves working in confined spaces and the following measures should be applied:

o Operatives must wear a harness and there must be a winch above the excavation.

o An attendant must be present at all times, at ground level, while excavation is occupied.

o A rescue plan must be produced prior to the works as well as a task-specific risk and method statement.

o Working in the confined space should require a permit to work.

Figure 3 – Schematic Plan view of soil propping

Figure 4 Propping examples

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Figure 5 Examples of excavations of pins

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Figure 6 Examples of completed walls and back propping to central soil mass

4.5. Backpropping of rear face: Rear face to be propped in the temporary conditions with a minimum of 2 trench sheets. Trench sheets are to extend over entire height of excavation. Trench sheets can be placed in short sections as the excavation progresses.

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Figure 7 Example of trench sheet back propping

4.5.1. If the ground is stable, trench sheets can be removed as the wall reinforcement is placed and the shuttering is constructed.

4.5.2. Where trench sheets are left in a slight over spill may occur past the neighbours boundary wall line. Where this slight over spill is not allowed by the Party Wall Surveyors then cement particle board should be used as noted below.

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4.5.3. Where soft spots are encountered, leave in trench sheets or alternatively back prop with precast lintels or sacrificial boards. If the soil support to the ends of the lintels is insufficient, then brace the ends of the PC lintels with 150x150 C24 timbers and prop with Acrows diagonally back to the ground.

4.5.4. Where voids are present behind the lintels or trench sheeting, grout voids behind sacrificial propping. Grout to be 3:1 sand/cement packed into voids.

4.5.5. Prior to casting, place layer of DPM between trench sheeting (or PC lintels) and new concrete. The lintels are to be cut into the soil by 150mm either side of the pin. A site stock of a minimum of 10 lintels should be present to prevent delays due to ordering.

4.6. If cut face is not straight, or sacrificial boards noted previously have been used, place a 15mm cement particle board between sacrificial sheets or against the soil prior to casting. Cement particle board is to line up with the adjacent owner’s face of wall. The method adopted, to prevent localised collapse of the soil, is to install these progressively, one at a time. Cement particle board must be used in any condition where overspill onto the adjacent owner’s land is possible.

4.7. Underpins can be completed in segmental lifts (e.g. top section of wall followed by bottom section of wall).

Croft’s recommendation is that walls with high vertical loads or which are susceptible to settlement, and all party walls, should be completed as first pin top first pin bottom, next pin top next pin bottom. We do not recommend for such conditions that all the top sections for every pin completed, followed by all the lower pins: such a sequencing can result in the existing wall being left on a narrower section than the original footing for too long, resulting in settlement.

4.7.1. Place reinforcement for retaining wall segmental lift

4.7.1.1. At lift sections, reinforcement needs to be driven in. This is to be completed by pre-drilling holes and inserting the reinforcement into the predrilled hole.

4.7.1.2. Underside of the wall to be cast with chamfer to allow concrete for lower lift to be cast and no packing to be required.

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Figure 8 Segmental lift construction

4.8. Excavate base. If soil over is unstable, prop top with PC lintel and sacrificial prop.

4.9. Visually inspect the footings and provide propping to local brickwork. If necessary install sacrificial Acrow, or pit props, and cast into the retaining wall.

4.10. Clear underside of existing footing.

4.11. Local Authority inspection to be carried out for approval of excavation base.

4.12. Place blinding.

4.13. Place reinforcement for retaining wall base and stem. Drive H16 Bars U-bars into soil along centre line of stem to act as shear ties to adjacent wall underpin.

4.14. Site supervisor to inspect and sign off works before proceeding to next stage.

4.14.1. For pins 1, 3 and 5, inform the engineer five days before the reinforcement is ready, to allow for inspection of the reinforcement prior to casting.

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4.15. Cast base. On short stems it is possible to cast base and wall at the same time. It is essential

that pokers/vibrators are used to compact concrete.

4.16. Concrete Testing:

4.16.1. For first 3 pins take 4 cubes and test at 7 days then at 14 days and inform engineer of results. Test last cube at 28 days. If cube test results are low then action into concrete specification and placement method must be considered.

4.16.2. If results are good from first three pins, then from the 4th pin onwards take 2 cubes of concrete from every third pin and store for testing. Test one at 28 days. If result is low, test second cube. Provide results to client and design team on request or if values are below those required.

4.16.3. A record of dates for the concrete pouring of each pin must be kept on site.

4.16.4. The location of where cubes were taken and their reference number must be recorded.

4.17. Horizontal temporary prop to base of wall to be inserted. Alternatively cast base against soil.

4.18. Place shuttering and pour concrete for retaining wall. Stop a minimum of 75mm from the underside of existing footing. It is essential that pokers/vibrators are used, hitting shutters is not considered adequate.

4.19. 24 hours after pouring the concrete pin, the gap shall be filled using a dry-pack mortar. Ram in dry-pack between the top of the retaining wall and existing masonry.

4.19.1. If gap is greater than 120mm, place a line of engineering bricks to the top of the wall. Dry pack from the engineering bricks to existing masonry.

4.20. After 24 hours, the temporary wall shutters can be removed.

4.21. Trim back existing masonry corbel and concrete on internal face.

4.22. Site supervisor to inspect and sign off for proceeding to the next stage. A record will be kept of the sequence of construction, which will be in strict accordance with recognised industry procedures.

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5. Floor Support Timber Floor 5.1. The timber floor will remain in situ and be supported by a series of steel beams, to provide open

areas in the basement.

5.2. Position 100 x 100mm temporary timber beams, lightly packed, to underside of joists either side of existing sleeper wall and support with vertical Acrow props @ 750 centres. Remove sleeper walls and insert steel beams as a replacement. Steel beams to bear onto concrete padstones built into the masonry walls (refer to Structural Engineer’s details for padstone and beam sizes)

5.3. Dismantle props and remove timber plates on completion of installation of permanent steel beams.

6. Concrete Ground bearing slabs 6.1. The support of the existing concrete floor will be undertaken in conjunction with the

underpinning process. Two opposite pins are constructed and allowed to cure as described elsewhere.

6.2. Locally prop concrete floors with Acrows at 2m centres with timbers between. If the underside is found be in poor condition, then temporary boarding and props are to be introduced.

6.3. Insert steelwork and dry pack to underside of floor

6.4. Between steelwork, place 215wide x 65dp PC lintels at a maximum spacing of 600mm

6.5. If necessary, brick up to 50mm below the underside of the floor

6.6. Dry pack between lintel/brickwork to underside of slab.

6.7. Remove props

This process is to continue one pin width at a time

7. Supporting existing walls above basement excavation 7.1. Where steel beams need to be installed directly under load-bearing walls, temporary works will

be required to enable this installation. Support comprises the temporary installation of steel needle beams at high level, supported on vertical props. This will enable safe removal of brickwork below and installation of the new beams and columns.

7.1.1. The condition of the brickwork must be inspected by the foreman to determine its condition and to assess the centres of needles. The foreman must inspect upstairs to consider where loads are greatest. Point loads between windows should be given greater consideration.

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7.1.2. Needles are to be spaced to prevent the brickwork above ‘saw toothing’. Where brickwork is good, needles must be placed at a maximum of 1100mmcenters. Lighter needles or Strongboys should be placed at tighter centres under door thresholds

7.2. Props are to be placed on sleepers on firm ground or, if necessary, temporary footings will be cast.

7.3. Once the props are fully tightened, the brickwork will be broken out carefully by hand. All necessary platforms and crash decks will be provided during this operation.

7.4. Decking and support platforms to enable handling of steel beams and columns will be provided as required.

7.5. Once full structural bearing is provided via beams and columns down to the new basement floor level, the temporary works will be redundant and can be safely removed.

7.6. Any voids between the top of the permanent steel beams and the underside of the existing walls will be packed out as necessary. Voids will be drypacked with a 1:3 (cement: sharp sand) drypack layer, between the top of the steel and underside of brickwork above.

7.7. Any voids in the brickwork left after removal of needle beams can at this point be repaired by bricking up and/or drypacking, to ensure continuity of the structural fabric.

8. Approval

8.1. Building Control Officer/Approved Inspector to inspect pin bases and reinforcement prior to casting concrete.

8.2. Contractor to keep list of dates of pins inspected and cast.

8.3. One month after the work is completed, the contractor is to contact Adjoining Party Wall Surveyor to attend site and complete final condition survey and to sign off works.

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9. Basement Temporary Works Design Lateral Propping

This calculation has been provided for the trench sheet and prop design of standard underpins in the temporary condition. There are gaps left between the sheeting and as such no water pressure will occur. Any water present will flow through the gaps between the sheeting and will be required to be pumped out.

Trench sheets should be placed at regular centres to deal with the ground. It is expected that the soil between the trench sheeting will arch. Looser soil will require tighter centres. It is typical for underpins to be placed at 1200c/c in this condition the highest load on a trench sheet is when 2 No.s trench sheets are used. It is for this design that these calculations have been provided.

Soil and ground conditions are variable. Typically, one finds that, in the temporary condition, clays are more stable and the Cu (cohesive) values in clay reduce the risk of collapse. It is this cohesive nature that allows clays to be cut into a vertical slope. For these calculations, weak sand and gravels have been assumed. The soil properties are:

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Trench Sheet Design

Soil Depth Dsoil = 3000mm

Surcharge sur = 10kN/m2

Soil Density = 20kN/m3

Angle of Friction = 25

ka = (1 - sin()) / (1 + sin()) = 0.406

kp = 1 / ka = 2.464

Soil pressure bottom soil = ka * *Dsoil = 21.916kN/m2

Surcharge pressure surcharge = sur * ka = 4.059 kN/m2

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Sxx = 15.9 cm3

py = 275N/mm2

Ixx = 26.9cm4

A = (1m * 32.9kg/m2 ) / (7750kg/m3 ) = 4245.161mm2

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CONTINUOUS BEAM ANALYSIS - INPUT

BEAM DETAILS

Number of spans = 3

Material Properties:

Modulus of elasticity = 205 kN/mm2 Material density = 7860 kg/m3

Support Conditions:

Support A Vertically "Restrained" Rotationally "Free"

Support B Vertically "Restrained" Rotationally "Free"

Support C Vertically "Restrained" Rotationally "Free"

Support D Vertically "Free" Rotationally "Free"

Span Definitions:

Span 1 Length = 1000 mm Cross-sectional area = 4245 mm2 Moment of inertia = 269.103 mm4



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LOADING DETAILS

Beam Loads:

Load 1 UDL Dead load 4.1 kN/m

Load 2 VDL Dead load 21.9 kN/m to 0.0 kN/m

LOAD COMBINATIONS

Load combination 1

Span 1 1.4Dead

Span 2 1.4Dead

Span 3 1.4Dead CONTINUOUS BEAM ANALYSIS - RESULTS

Support Reactions - Combination Summary

Support A Max react = -12.3 kN Min react = -12.3 kN Max mom = 0.0 kNm Min mom = 0.0 kNm

Support B Max react = -38.5 kN Min react = -38.5 kN Max mom = 0.0 kNm Min mom = 0.0 kNm

Support C Max react = -24.8 kN Min react = -24.8 kN Max mom = 0.0 kNm Min mom = 0.0 kNm

Support D Max react = 0.0 kN Min react = 0.0 kN Max mom = 0.0 kNm Min mom = 0.0 kNm

Beam Max/Min results - Combination Summary Maximum shear = 18.8 kN Minimum shearFmin = -19.8 kN

Maximum moment = 2.4 kNm Minimum moment = -4.4 kNm

Maximum deflection = 17.1 mm Minimum deflection = -0.1 mm

Number of sheets Nos = 3

Moment M_allowable = Sxx * py * Nos = 13.118kNm

Deflection D = / Nos = 5.699mm

Acro Load Acro = Rmax_B / 2 = -19.272kN

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Acrow Props A or B are acceptable placed 0.5m from top, middle and 1m from bottom

Cross Props

Props should be placed a third up the wall measured from the bottom slab.

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Surcharge sur = 10kN/m2

Soil Density = 20kN/m3

Angle of Friction = 25

Soil Depth Dsoil = 3000mm

ka = (1 - sin()) / (1 + sin()) = 0.406

kp = 1 / ka = 2.464

1 - sin() = 0.577

Soil force bottomsoilforce = ka * * Dsoil * Dsoil / 2 = 36.527kN/m

Surcharge Force Surchargeforce = ka * sur * Dsoil = 12.176kN/m

Place Props every other pin spacing = 2m

Propforce Propforce = spacing * (soilforce + Surchargeforce) = 97.406kN

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Figure 9 Mabey Mass 50 Load Chart

Provide Mabey Mass 50 at 2m Centres at 1/3 the height of the wall.

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Appendix C – Structural Plans & Details

Structural Drawings Plans 1:100

Structural Sections 1:50

Documents

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