Structural Method Statement (Structural Design Philosophy)

Prepared by:

Graham Spittle &

Ian Neads BSc(Hons) CEng MIStructE MICE

Job number: N350

Revision A - September 2017

Rev A: Excavation sequence sketches added depicting excavation strategy (Refer to Appendix 7.3).

Structural Method Statement (Structural Design Philosophy)

1a Wolsey Road Land to rear of 121 Mildmay Road, London N1 4PT

Contents

1. Introduction and Construction Summary.

2. Appointed Design Team.

3. Site Conditions and Topography.

3.1 General Topographical Description.

3.2 Existing Building.

3.3 Geotechnical Site Conditions.

4. Proposed Development.

4.1 Proposed Development, Basement Design.

4.2 Proposed Development, Superstructure Design.

5. Adjacent Properties.

5.1 Structural Description of Adjacent Buildings and Structures.

5.2 Adjacent Properties with Basements.

5.3 Impact on Adjacent Properties and Groundwater Regime.

5.4 Structural Stability of Adjacent Properties.

5.5 Foundations Near Trees.

5.6 Highways.

6. Design Criteria.

6.1 Design Loads and Structural Load Paths.

6.2 Design Loading for Basement Heave.

6.3 Design Loading for Hydrostatic Pressure.

6.4 Floatation.

6.5 Ground Movement Statement.

6.6 Movement Monitoring.

6.7 Waterproofing Philosophy.

6.8 Opening Up Works and Surveys Prior to the Start of Construction Activities.

6.9 Drainage Statement.

6.10 Party Wall Act.

7. Appendix

7.1 Loading Design Data Sheets.

7.2 Outline Construction Sequence.

7.3 Structural Scheme Drawings Including Excavation Sequence Sketches Depicting

Excavation Strategy.

1. Introduction and Construction Summary

1.1 This report was commissioned by the building’s owner to outline a suitable method to construct a new two storey house with basement (lower ground floor) in the land to the rear of 121 Mildmay Road.

1.2 The method statement included in this report demonstrates how the property can be constructed taking all reasonable steps to minimise the effects of ground movement in order to protect the existing fabric of adjacent properties during excavation and construction of the basement.

1.3 This report includes an assessment considering the effects of a proposed basement construction on ground stability, surface water and groundwater regime at the property.

1.4 Basement construction methods have been selected to minimise movement of the basement perimeter retaining walls. These measures include installation of reinforced concrete underpinned retaining walls and a ground floor transfer slab which will prop the basement walls and ensure perimeter wall movement is minimised.

2. Appointed Design Team

The following organisations and Professionals have been appointed to design the proposed property:

Organisation, Contact details Contact:

Principle designer Architect.

Norton Mayfield Architects Harland Works, Unit 7, 70 John Street, Sheffield. S2 4QU

Christopher Bradley [email protected] 0114 270 0014

Structural Engineer.

Alan Baxter Partnership LLP The Clock Building, Pympes Court, Busbridge Road, Loose, Maidstone, Kent, ME15 0HZ

Graham Spittle [email protected] 01622 744263

mailto:[email protected]

mailto:[email protected]

3. Site Conditions and Topography

3.1 General Topographical Description

The proposed property will occupy a corner site at the junction of Mildmay Road and Wolsey Road.

The topography of the site and surrounding pavements lies at a level of approximately +27m AOD.

3.2 Existing Building.

3.2.1 The site is currently occupied by a garage and single storey outbuildings within the rear garden area to 121 Mildmay Road. The existing building is a lean-to construction sharing a wall with 1 Wolsey Road.

3.2.2 The adjacent houses at 1 Wolsey Road and 121 Mildmay Road are Georgian terraced houses of solid load bearing brick construction.

3.3 Geotechnical Site Conditions

3.3.1 SI Reports

At the time of writing this report the fieldwork associated with a dedicated geotechnical site

investigation is still to be undertaken. Preliminary geotechnical assessment of the site is therefore

made with initial desk study information based on site investigations from other developments in the

vicinity and British Geological survey information.

3.3.2 General Stratification

Borehole logs from a site at the west end of Mildmay Road, held by the British Geological Survey,

have been reviewed for an indication of the ground conditions that might be expected at the site.

Bore hole logs in the general area indicate that the stratification generally comprises made ground of

varying thicknesses (commonly 1 to 2 metres) overlying bands of clay and medium dense sands and

gravels to a depth of approximately 5 to 8 metres below which there is the formation of London Clay.

The ground sequence expected is as follows:

Strata Depth to top of strata (approx)

Description

Made ground. 0m Silty gravelly sand with brick fragments.

Sand and Gravel. 1 to 2m Medium dense becoming dense silty fine to coarse sand and fine to coarse sub angular flint gravel.

Bands of Clay 1 to 2m Bands of stiff Clay within sands and gravel.

London Clay. 5 to 8m Stiff and then very stiff fissured silty clay with occasional partings of silty fine sands and scattered gypsum crystals.

Examining the British Geological Survey geology map, it can be seen that the subject site lies at the

junction of superficial deposits from the “Langley silt member” (Clay and silt) and “Hackney Gravel

Member” (Sand and gravel).

Site specific geotechnical site investigation works will be required to confirm the ground conditions

prior to the commencement of basement excavations and foundation works on site.

Extract from British Geological Survey

geological map.

Showing site to be underlain by London

Clay formation, with superficial deposits of

Clay, Sand and Gravel.

3.3.3 Geotechnical Design Parameters

For the soil profile previously described, British standards and design handbooks suggest the range

of parameters to be adopted for the design to be: an Angle of Internal Friction of 20 to 40 degrees

and allowable net bearing pressures of 75 to 150 kN/m2.

For the preliminary foundation and basement design the following parameters have therefore been

adopted:

Angle of Internal Friction (Φ) = 30o

Allowable bearing pressure, at existing foundation level = 100 kN/m2

Allowable bearing pressure at basement level =150 kN/m2

(Including enhancement for removal of soil overburden.)

The proposed basement will adopt a reinforced concrete underpinned retaining wall, the final design

of which will be undertaken with due regard to the results of the site specific ground investigation

report.

3.3.4 Hydrogeology

The hydrogeology of the site is to be investigated as part of the proposed Site Investigation works.

Site investigations in the general area indicate that water strikes have generally been recorded at the

base of the gravel approximately 5 to 8m below ground. However some shallow perched ground

water is expected within the made ground and terrace gravels.

The information obtained from the Ground Investigation will be used to update the ground models

and to confirm scheme and temporary dewatering works designs.

Examining Environmental Agency maps the subject site can be confirmed to be outside the area of

any Groundwater source protected zones. Environmental Agency records also confirm that the

property is not in an area considered vulnerable to flooding.

Extract from Environmental

Agency map.

Showing site to be outside

Groundwater source protected

zones.

4. Proposed Development

4.1 Proposed Development, Basement Design

4.1.1 The proposals include the construction of a basement below the complete footprint of the new

property. The final basement depth is expected to be approximately 3.0m from ground level to

the underside of the basement slab formation.

4.1.2 Excavation for the proposed basement structure will require retaining wall structures to

maintain stability and to prevent any excessive ground movements. In order to maximise the

usable internal space and to provide the most suitable support to the walls of adjoining

properties, it is proposed to construct the perimeter retaining walls using a series of reinforced

concrete underpins, carried out in a traditional ‘hit and miss’ sequence. This method will assist

in maintaining the structural stability of both neighbouring properties and the external

pavement during construction. Underpinned foundations will be installed below the party wall

with 1 Wolsey Road and front elevation wall bounding Wolsey Road.

4.1.3 For the proposed underpinning to the party wall with 1 Wolsey Road, it is noted that the new

foundations are formed exclusively under the lines of the existing foundations where the soils

will have been consolidated by the existing wall loads. The underpinned foundations have

been designed with sufficient width to achieve bearing pressures that do not exceed that

under the existing foundations. As such, the likelihood of any foundation settlement is

considered to have been minimised. It is further expected that there will be some

improvement of bearing pressure at depth due in part to removal of overburden loads

associated with material excavated. 1 Wolsey Road has an existing basement and as such

the underpinning will only be required to extend the depth of existing basement walls to suit

the depth of the new basement.

4.1.4 At the north end of the site, at the junction with 121 Mildmay Road, underpinning will be

required to the corner of the adjacent property, under the single storey entrance vestibule.

Underpinning should be formed in diminishing depths, stepping up from basement level to

form a transition to existing foundations. Stepped foundation will help prevent hard spots

being created where different parts of the building bear on foundations of inconsistent depth.

4.1.5 Where the proposed basement bounds adjacent garden areas, reinforced concrete cantilever

retaining walls will be constructed formed within battered excavations.

4.1.6 Once underpinned foundations have been installed, the basement will be excavated to

formation level. A reinforced concrete basement slab will be constructed, including slab

thickenings to act as pad foundations under perimeter support lines. The slab thickenings will

be designed based on safe ground bearing pressure and settlement assessments.

4.1.7 The basement slab will comprise a reinforced concrete raft, propping perimeter retaining walls

and tying the basement level together. The basement slab will be designed to accommodate

loads associated with vertical loading from the building and basement use, and in addition

uplift forces associated with hydrostatic pressure and clay heave. Reinforcement connection

with the perimeter retaining wall will be made to resist both vertical loading on the slab and

uplift forces. As the basement area is to have an open garden area to the rear, this will have

the effect of relieving any pressures that might be associated with clay heave and pore water

pressure.

4.1.8 The ground floor slab will offer a rigid support propping the top of the perimeter retaining walls

in the final condition.

4.1.9 The ground movements associated with this method of basement construction are considered

to be relatively small and this will assist in maintaining the structural stability of this and

neighbouring properties during construction. An outline construction sequence for the

basement is included in the appendix of this report.

4.1.10 Uplift hydrostatic forces from water-pressures are in accordance with the Design Loading for

Hydrostatic Pressure noted in 6.3 below. These hydrostatic forces are considered to be

comparable to or in excess of forces likely to be associated with clay heave and therefore

form the governing criteria for the design of the basement slab.

Proposed sections through the underpinned basement retaining walls.

4.2 Proposed Development, Superstructure Design

4.2.1 A reinforced concrete suspended ground floor slab will be constructed. This slab will take

support from basement perimeter walls and internal loadbearing walls. The supporting walls

include the reinforced retaining wall to the front elevation, the rear cantilever retaining wall,

internal walls adjacent to the proposed stair and external walls adjacent to the basement

landscaped garden. Along the line of the party wall with 1 Wolsey Road, the slab will take

support from loadbearing blockwork built from the basement raft slab. This method of support

along this boundary will be used to avoid the need to cut supports into the party wall.

4.2.2 The ground floor slab will be designed to cantilever over part of the rear basement garden,

supporting perimeter walls above ground floor, which in turn carry the roof structure. Along the

front Wolsey Road elevation, the building above ground is displaced from the perimeter line.

These walls will also be carried by the suspended ground floor slab.

4.2.3 The ground floor slab is to be designed as a flat slab. The slab design will include the

provision of openings for glazed floor lights, the stair and a future lift to meet life time homes

standards. Level differences between internal and external slab areas will be need to be

achieved within the depth of floor finishes applied to the slab.

4.2.4 At the front of the property, the existing white painted rendered wall is to be retained, to meet

conservation area requirements. This wall is to be underpinned as part of the basement works

and will require temporary lateral support to safeguard it against toppling over during the

construction works. Temporary support will take the form of steel wailing beams and ideally

cantledge support built out over the pavement.

4.2.5 Above ground floor, loadbearing perimeter walls support the roof structure. The roof is in two

parts, a main mono-pitched roof and a flat green roof area. Both roof areas will be supported

using timber joists spanning between loadbearing walls. At the junction between mono-pitch

and flat roof areas, steel beams will be provided spanning the open plan areas below.

4.2.6 In addition to the loads associated with roof finishes and green roof landscaping, the roof

structure is designed to accommodate maintenance access loads and loads associated with

snow; as defined by the British Standards.

5. Adjacent Properties

5.1 Structural Description of Adjacent Buildings and Structures.

5.1.1 The proposed house will be built directly alongside the neighbouring property at 1 Wolsey

Road. The height of the proposed building has been restricted in order that it does not conflict

with the elevations of the adjacent Georgian properties. To the right hand side of the property,

the walls above ground level are located to provide a separation with the existing house at

121 Mildmay Road. At basement level, the building footprint extends to the boundary with 121

Mildmay Road in order to maximise internal space.

5.1.2 Adjacent properties on Wolsey Road and Mildmay Road are terraced properties dating from

the Georgian period. The properties on Mildmay Road are four storeys in height including an

original basement and elevated ground floor. Properties in Wolsey Road constructed in the

same era have a smaller footprint and are three storeys in height, also including an original

basement and elevated ground floor. The properties are of load bearing masonry construction

with timber floors, and pitched “butterfly” roofs behind a roof parapet.

5.2 Adjacent Properties with Basements.

5.2.1 A search of London Borough of Islington’s planning portal shows that a number of properties

in close proximity to the subject property have had planning permission for basement

construction works in recent years. These basement works generally involve extending the

size of existing basements.

5.2.2 The adjacent properties at 121 Mildmay Road and 1 Wolsey Road both have existing

basements. These basements are approximately 1.8m below ground level, in both cases the

properties having a ground floor elevated above the surrounding street level.

Street view image showing the site and adjacent properties on Wolsey Road.

Street view image showing the site and adjacent properties on Mildmay Road.

5.3 Impact on Adjacent Properties and Groundwater Regime

5.3.1 In accordance with normal planning guidance for subterranean developments, the proposed

construction shall mitigate against the effects of forming the basement including minimising

any impact on neighbouring properties. The development shall also mitigate against changing

the ground water regime.

5.3.2 Existing basements within the adjacent properties are relatively shallow being only about

1.8m below ground level. The proposed basement for the subject property will be limited to a

single below ground storey. It is therefore considered that any damming effect to groundwater

flow of this and original basements to the adjacent houses will be minimal. It is considered

that even if mobile water was forced to find alternative routes as a consequence of the

basement construction, any increase in the level of that water is likely to be significantly less

than the natural variations associated with seasonal changes and rises in levels from extreme

rainfall events.

5.4 Structural Stability of Adjacent Properties

5.4.1 The proposed basement construction methods have been particularly selected to minimise

ground movement. As such the impact on adjacent properties is expected to be minimal. We

do not envisage the formation of the basement having any significantly detrimental effects on

the adjacent properties. As both of the adjacent properties already have basement

constructions of their own, susceptibility to the effects of basement excavation are expected to

be further limited. Refer to Ground movement statement in section 6.5.

5.4.2 At the depths being excavated no significant groundwater is expected, however careful

control of any groundwater inflow during construction will be required to ensure it does not

lead to undermining and settlement of adjacent buildings.

5.5 Foundations Near Trees

5.5.1 There are a number of trees in the adjacent rear gardens. When building near trees account

needs to be taken of their effect on shrinkable soils. Soil moisture contents vary seasonally

and are influenced by a number of factors including the action of tree roots. The resultant

shrinkage or swelling of the soil can cause damage to foundations. For the proposed

development all new foundations are located below the near basement at a depth of

approximately 3.0m and as such are not considered to be at risk of damage from the tree

roots.

5.5.2 Any trees in close proximity to the works will need to be protected from damage by a fence or

barrier throughout the construction works. The root spread of mature trees is quite extensive

and as such parts of the trees root system will need to be cut to form the basement

excavation. In this respect the advice of an arboriculturist is recommended to assess the most

appropriate means to minimise root damage and ensure the stability of the trees is not

impaired.

5.6 Highways

5.6.1 The basement walls adjacent to the public highway are to be designed to support surcharge

loads associated with vehicular use of the road. A retaining wall alongside the highway is to

be installed using a sequenced “hit and miss” underpinning technique to minimise any

movement to the road and pavement. Tight controls on ground movement are to be

implemented to minimise the risk of damage to underground services within the pavement.

5.6.2 The excavations must be carried out in a manner that ensures traffic and construction activity

does not harm pedestrian, cycle, vehicular and road safety. This is to include temporary works

to maintain the stability of the existing front wall to the Wolsey Road elevation during the

works. Ideally the temporary works will incorporate a pavement closure to keep pedestrians

isolated from the works.

6. Design Criteria

6.1 Design Loads and Structural Load Paths

6.1.1 Loads on building structure have been assessed in accordance with BS6399 loadings for

buildings.

6.1.2 External walls loads are to be carried by the basement slab acting as a raft foundation. Loads

from the internal loadbearing walls are similarly supported by the basement slab.

6.2 Design Loading for Basement Heave

6.2.1 The excavation of the new basement to a formation depth of approximately 3.0m below

ground level will entail the excavation of an average of a 2.4m thickness of soil, taking

account of existing variation in ground levels between the front and rear of the plot. This

excavation will result in a net unloading of around 43 kN/m2.

6.2.2 This will result in a measure of short term heave and long term swelling of the underlying

London Clay, which theoretically takes a number of years to complete. These movements will

to a certain extent be mitigated by the continued load applied to the party wall and the existing

basements in the adjacent properties.

6.2.3 The new basement slab is to be designed to withstand the potential heave forces and

movements. About 50% of total movement would normally be expected to occur prior to

construction of the slab (for a normal construction programme). If it is (reasonably) assumed

that the relationship between heave movement and pressure is linear, the maximum heave

pressure for an infinitely stiff slab could therefore be about 21 kN/m2 for the fully constrained

condition. Heave pressure below the new basement slab will however be substantially less

due to the open landscaped area being created at basement level. This provides the

opportunity for unrestrained movement of the soil and therefore alleviates uplift forces on the

basement slab.

6.3 Design Loading for Hydrostatic Pressure

6.3.1 The hydrostatic pressures to be resisted by the basement enclosure walls and basement slab

shall be determined in accordance with:

The guidance and recommendations of BS8102:2009.

Geotechnical/hydrological Site Investigation.

Future predicted hydrological regime during the life of the structure.

Potential future accidental loading (e.g. Burst Water Mains etc.).

6.3.2 Subject to confirmation from a dedicated SI to be undertaken for this site and observations to

be made as part of ongoing groundwater monitoring, it is considered that the following

hydrostatic load conditions are appropriate for the basement slabs and enclosure walls:

A worst case scenario, with water at 1 m below ground level (recommendation of BS 8102)

adopting an ultimate factor of 1.20 on the pressure.

6.3.3 Based on the worst case design loading, the potential hydrostatic ground water pressure on

the basement retaining walls would be 20 kN/m2 [( 3.0m to basement slab formation – 1.0m) x

10 kN/m3]. For the basement slab a reduced hydrostatic ground water pressure is considered

to be appropriate as any pressure will be relieved by the slab opening for the basement

landscaped area.

6.3.4 It is noted that water pressure is not additional to any soil heave pressures but will be the

minimum uplift pressure for design purposes.

6.3.5 At the proposed basement depth it is considered unlikely that, in practice, there will be any

free flowing groundwater, which would result in an influx of water through the landscaped

basement area. This will however need to be subject to confirmation from the dedicated SI to

be undertaken for this site. Should free flowing water be identified the basement slab design

would be revised to include a monolithic slab under the landscaped garden area to exclude

mobile ground water. This scenario would increase the uplift forces the basement slab needs

to be designed for as heave and hydrostatic uplift forces would no longer be relieved by an

opening in the basement slab.

6.4 Floatation

In the final construction condition any buoyancy of the basement box will be overcome by the self

weight of the building.

6.5 Ground Movement Statement

6.5.1 The proposed basement construction methods have been particularly selected to minimise

ground movement. It is anticipated that ground settlement will be very small due the nature of

the proposed construction.

6.5.2 For the purpose of a “Stage1, ground movement assessment”, maximum ground slopes of

1/500 and settlements of less than 10mm are generally considered to have negligible risk of

damage. The design criteria for the basement perimeter retaining walls should be such that

ground movement around the basement is within these limits. The potential for damage to

adjacent buildings should thereby be limited to class 0 & class 1 (Burland et al) negligible,

aesthetic damage.

6.5.3 The excavation depth and modest dimensions of the site are such that heave associated with

unloading of the clay is unlikely to exceed a few millimetres or to have any discernible effect

outside the site boundaries. Any movement that does occur will be further mitigated by the

necessarily slow rate of the excavation and construction.

6.6 Movement Monitoring

6.6.1 With a new ground floor slab acting as a permanent prop across the basement, the potential

for any horizontal movement is extremely limited. There however remains a risk of some slight

vertical ground movement due to deflection of the retaining walls and clay heave effects as

the basement excavation progresses. With this in mind the following monitoring regime is

proposed to safeguard adjacent properties.

6.6.2 Prior to commencement of any underpinned retaining walls or basement excavations, a series

of “targets” or level pins should be installed at ground level at regular intervals around the

building perimeter (Approx. 2m centres). Targets should be installed within the pavement

alongside the site and should be fixed to the face of the party wall with 1 Wolsey Road. These

targets will need to be protected as they will need to remain in place through the full

excavation and basement construction process.

6.6.3 The level, relative to a fixed datum (remote from excavation works on the site) of each “target”

should be accurately measured using a Total Station or accurate optical level, prior to

commencement of basement activities. The level of each “target” should then be measured

and recorded at regular intervals throughout the basement construction process. Initially, it is

suggested that levels should be recorded on a weekly basis; however the interval between

measurements may need to be adjusted depending on the rate of basement excavation and

progress of other construction activities.

6.6.4 Ground level displacements relative to the original level datum should be monitored, with a

“traffic light” action level system put in place to safeguard the construction and prevent

damage to adjacent buildings in the event that greater than expected displacements are

recorded. The suggested action levels are outlined in the table below. These action levels will

need to be reviewed following the detailed design of the basement and adjusted accordingly.

Proposed ground displacement actions

Action

Level.

Recorded

Displacement.

Action to be taken.

Green Less than 5mm.

Works to be progressed as normal.

Amber 5 to 10mm. Increase frequency of monitoring.

Red In excess of 10mm Suspend all excavation works.

Review options for reducing displacement.

Options might include:

Installing additional propping within basement.

Adjusting sequence of excavation, creating

temporary berms etc.

6.7 Waterproofing Philosophy

6.7.1 In response to the Client Brief, the completed basement enclosures shall be designed in

accordance with BS 8102 Table 2 Grade 3 adopting a Type C drained cavity system.

6.7.2 The completed basement retaining walls form a reinforced concrete barrier which forms the

first line of defence against ground water penetration. Within the basement concrete box, a

drained cavity system will then be installed, this comprising an “egg crate” MS20 type

drainage membrane under screed to the floor and blockwork liner walls around the perimeter.

6.7.3 The structural enclosure itself will be designed to achieve a basic level of performance,

equivalent to BS8102 grade 1, prior to the installation of a drained cavity system to enhance

the completed basement environment to a BS8102 grade 3. Achieving a basic level of

performance within the structure is considered critical to the overall performance of the

waterproofing system, in that by defining structural performance the reliance on drainage

systems to maintain the basement environment is reduced. In this regard, it is considered that

the specification and quality of construction of the basement slab and construction joints are

of critical importance in achieving a base level of water resistance.

6.8 Opening Up Works and Surveys Prior to the Start of Construction Activities

6.8.1 Prior to the start of construction activities on site, it is proposed that a number of perimeter

trial pits are excavated to confirm the form and depth of existing foundations. This trial pit

information will be used to inform the requirements and specification of underpinning and any

temporary shoring requirements for excavations.

6.8.2 An inspection is to be made to confirm the condition of the existing party wall and retained

front wall. An assessment is to be made in respect of any temporary support requirements

needed prior to commencement of foundation works.

6.8.3 The appointed contractor should be a ground-works specialist familiar and experienced in

underpinning techniques. Their works should be overseen by a suitably qualified person at

key stages including:

Installation of temporary works

Commencement of underpinning works.

Basement excavation.

Inspection of formation prior to placing basement raft slab.

Inspection of slab reinforcement prior to placing concrete.

6.9 Drainage Statement

6.9.1 Prior to commencement of the works a full CCTV survey of existing drainage from the house

to the main sewer is to be undertaken to check and record condition. A similar survey is to be

undertaken upon completion of the works to ensure no inadvertent damage has occurred.

6.9.2 All above ground drainage will continue to outfall to the main sewer under gravity. It is

anticipated that drainage from the new basement will also be by gravity to the main sewer.

Should the sewer depths prove to be too high for an adequate gravity outfall from the

basement, a localised pumped drainage system will be installed to lift foul water to the ground

floor drainage system.

6.9.3 No drains, other than those to the existing house are understood to pass through the site and,

therefore, the drainage and sewerage for the nearby houses will be unaffected by the

proposed scheme.

6.9.4 Surface water drainage from the basement landscaped area will be collected in gulleys prior

to discharge by gravity to the main sewer. Should the sewer depths prove to be too high for

an adequate gravity outfall, a pumped drainage system separate to the foul drainage will be

installed to lift surface water to the main sewer. No ground water is to be discharged to the

main sewer.

6.10 Party Wall Act

6.10.1 These works fall within the scope of the Party Wall Act 1996 and a Party Wall Surveyor will

need to be appointed by the building’s owner to prepare and serve the necessary notice. It is

recommended that Party Wall Awards be agreed, signed, published and the 14 day notice

period has been given prior to commencement of the works.

6.10.2 The Party Wall Awards should include visual condition surveys of the adjacent buildings,

agreed by the Party Wall Surveyors. This is required in order that any damage identified after

the commencement of works on site cannot unfairly be attributed to the construction activities.

7. Appendix

7.1 Loading Design Data Sheets

ALAN BAXTER PARTNERSHIP LLP

CONSULTING STRUCTURAL ENGINEERS

Sheet No: Data 1 Project No: N350

By IN Date

Checked: GS Date

LOCATION

STRUCTURAL DESIGN DATA SHEET

Relevant Design Standards and guidance notes

BS 8103-1: 1995 Structural Design of Low Rise Buildings

BS 648: 1964 Schedule of Weights of Building Materials

BS 6399: Part 1: 1996 Loading for Buildings: Code of Practice for Dead and Imposed Loads

BS 6399: Part 3: 1988 Loading for Buildings: Code of Practice for Imposed Roof Loads

BS8102:2009 Protection of below ground structures

BS 8110;Part 1: 1997 Structural use of Concrete

BS 5628-1: 2005 Structural use of unreinforced masonry

BS 5950: Part 1: 2000 Structural use of Steelwork in Building

BS 5268-2: 2002 Structural use of timber

Tata steel section Interactive 'Blue book'

CALCULATIONS OPTIONS

Dec-15

Land to rear of 121 Mildmay Road, London N1

New house

DESIGN DATA SHEETS & LOADING DATA. for

New House

Land to rear of 121 Mildmay Road London N1 4PT




By IN Date

Checked: GS Date

LOCATION

Loading Data Basement and Floor Slabs

Basement slab

Dead mm thk. Slab

screed and cavity drainage

Floor coverings tiled

insulation

Total kN/m2

Live kN/m2

Ground Floor suspended slab

Dead mm thk. Slab

screed

Floor coverings tiled

12.5mm plasterbaord + skim on battens

partition allowance

Total kN/m2

Live kN/m2

Ground Floor suspended slab, external front courtyard

Dead mm thk. Slab

screed

Paving slabs


insulation

Waterproofing

Total kN/m2

Live kN/m2

Flat roof terrace, at ground floor, rear,

Dead mm thk. Slab

screed

Paving slabs


Waterproofing

insulation

Total kN/m2

Live (allowing full balcony access) kN/m23.00

250

250

250

0.02

250 6.00

1.10

1.20

0.10

8.60

0.18

0.02

Dec-15


New house


1.00

1.50

6.00

1.10

0.30

0.18

8.58

1.70

0.02

8.02

1.50

0.30

6.00

8.60

3.00

6.00

1.10

1.20

0.18

0.10




By IN Date

Checked: GS Date

LOCATION

Loading Data Roofs

Mono-Pitched roof

Dead Standing seam metal profile roof

18mm plywood

Timbers joists

Battens

12.5mm plasterboard + skim

Insulation & Roofing membrane

Total kN/m2

Pitch= o Dead load on plan = /cos = kN/m2

BS6399-3

cl 4.3.2 Live at o pitch = 0.75 x (60- )/30 = kN/m

2

Flat roof, green roof

Dead Timbers joists

12.5mm plasterbaord + skim

sarking board

Green roof growing medium #

Roof finish including waterproofing

insulation

Total kN/m2

# allows for Biodiverse 'Extensive' green roof with saturated green roof growing

medium weight of 100Kg/m2

Live (allowing maintenance access) kN/m2

Glazed Roof lights

Dead 2 x 6mm glass

frame allowance, including waterproofing

Total kN/m2

BS6399-3

cl 4.3.2 Live Flat roof kN/m20.75

0.02

1.63

1.50

0.15

0.36

0.51

30 0.7530

0.16

0.17

0.15

1.00

0.13

Dec-15


New house


0.58

30

0.12

0.08

0.03

0.18

0.05

0.58 30 0.67

0.12

Typical detail of standing seam metal roof buildup. For Loading purposes only. Refer to Architects details and specification for final construction requirements.




By IN Date

Checked: GS Date

LOCATION

Loading Data Walls

External solid walls (Rendered)

Dead Medium density concrete blocks( mm thk) #

Insulated plasterbaord + skim

External render

Total kN/m2

External solid walls (Metal clad)

Dead Medium density concrete blocks( mm thk) #

Insulated plasterbaord + skim

18mm plywood

Standing seam metal profile

Total kN/m2

Existing Party Wall (solid brick)

Dead Brickwork( mm thk)

Internal plaster finish

Total kN/m2

Basement loadbearing wall (alongside Party Wall)

Dead Medium density concrete blocks( mm thk)

Plasterboard on dot and dabs

Cavity drainage membrane & insulation

Total kN/m2

New Internal wall (Blockwork)

Dead Medium density concrete blocks( mm thk)

12.5mm plasterbaord + skim (both sides)

Total kN/m2

Partition wall (studwork)

Dead Timbers studs

12.5mm plasterbaord + skim (both sides)

insulation

Total kN/m2

Perimeter basement walls (Lining walls to underpinning)

Dead Concrete wall ( mm ave.)

Cavity drainage membrane & insulation

Blockwork liner wall ( 7.3 N/mm2 aerated)

Plasterboard on dot and dabs

Total kN/m2

# note: medium density concrete blocks assumed for loading purposes only.

Final block selection to meet thermal performance, to Architects specification.

100

190 3.42

0.17

0.05

3.64

Dec-15

0.19

3.87

7.20


New house


215

215

0.19

0.12

4.30

0.12

300

330 5.94

0.17

6.11

0.17

8.17

0.02

0.45

2.14

1.80

3.87

0.34

0.30

4.36

0.09

0.34

0.05

0.75

7.2 Outline Construction Sequence.

1. Site establishment. Including erecting hoarding to the front elevation, initiating a pavement

closure and establishing site welfare facilities.

2. Demolition of existing lean-to outbuildings and site clearance. The existing lean-to buildings

and ground floor slabs are to be removed to provide access for the foundation construction

and basement excavation. The front rendered wall is to be retained. The main access to the

site will be through the existing garage door opening on Wolsey Road.

3. Survey. Upon completion of demolition activities, an inspection is to be made to confirm the

condition of the existing party wall and retained front wall. An assessment is to be made in

respect of any temporary support requirements needed prior to commencement of foundation

works.

4. Temporary support. Provide temporary lateral support to the front rendered wall to safeguard

it against toppling over during the construction works. Temporary support will take the form of

steel wailing beams and ideally kentledge support built out over the pavement.

5. Retaining Walls along the Wolsey Road elevation. A basement enclosure wall is to be

constructed below the retained perimeter wall. This will take the form of reinforced concrete

retaining walls constructed as a series of reinforced concrete underpins, carried out in a

traditional ‘hit and miss’ sequence.

6. Ground movement monitoring. Level targets around the building perimeter are to be regularly

monitored as the basement excavation is progressed, to ensure there is no adverse

movement. In the event that ground movement or wall deflections exceed predefined limits,

the excavation will be stopped until additional propping and strutting systems are put in place.

7. Commence basement excavation. Upon completion of the underpinned retaining wall to the

front elevation, the site is to be excavated to a level coincident with that of the adjacent

basement to 1 Wolsey Road.

8. Underpinning to Party Wall. A basement enclosure wall is to be constructed below the party

wall. This will take the form of reinforced concrete retaining walls constructed as a series of

reinforced concrete underpins, carried out in a traditional ‘hit and miss’ sequence.

9. Cantilever retaining walls to areas adjacent to rear garden. Form battered excavation and

construct reinforced concrete retaining walls to the rear basement elevations.

10. Complete basement excavation. With the perimeter retaining walls in place, continue

excavation of the basement.

11. Basement slab. Once excavated to basement formation level, a reinforced concrete

basement slab will be constructed including slab thickenings to act as pad foundations under

internal support lines.

12. Basement walls. Construct loadbearing blockwork walls at basement level.

13. Ground floor slab. The reinforced concrete ground floor slab is then constructed taking

support on the perimeter retaining walls and loadbearing block walls. The ground floor slab is

to be designed to support the load bearing wall arrangement above ground level, including

cantilevers over the basement garden and the offset location of walls along the front

elevation.

14. Commence superstructure works. Once the ground floor slab has been installed, this will act

as a working platform allowing commencement of superstructure works concurrent with works

within the basement space.

15. Walls above ground. Construct perimeter walls above ground floor. Install steel beams to

carry the roof over open plan areas.

16. Roof structure. The mono-pitch and flat roof areas will be constructed with timber joists.

17. Basement waterproofing. A drained cavity system will be installed, this comprising an “egg

crate” MS20 type drainage membrane under screed to the floor and blockwork liner walls

around the perimeter.

18. New Internal walls. Internal non load bearing blockwork and studwork partition walls are

constructed to the basement areas.

19. Internal fit out activities. Upon completion of the structural frame, internal fit out operations will

commence, including installation of the staircase, plumbing, electrics and all internal finishes.

20. External landscaping.

7.3 Structural Scheme Drawings.

Including Excavation Sequence Sketches Depicting Excavation Strategy.

Sketch 01 - Basement structural proposals.

Sketch 02 - Ground floor structural proposals.

Sketch 03 - Roof structural proposals.

Sketch 04 - Basement sections (through underpinned retaining walls).

Sketch 05 - Excavation Sequence Pavement Boundary.

Sketch 06 - Excavation Sequence to Party Wall.

Land to rear of 121 Mildmay Road. N1 4PT

Structural scheme.

Basement

N350– SK1 Rev A 9/12/15

Underpinning to adjacent property. Underpinning formed in diminishing depths, stepping up from basement level form transition to existing foundations.

Denotes Reinforced underpin forming retaining wall.

Denotes Reinforced Cantilever retaining wall. In battered excavation.

250mm thk. slab

Thickening provided at edge of basement slab to form foundation to wall.

Toe of free standing retaining wall (below level of landscaping).

Underpinning to party wall. Underpinning required to extend the depth of existing basement walls to suit the depth of the new basement.

Underpinning to freestanding wall on front elevation. Temporary restraints to be put in place to maintain the stability of the wall throughout the works.


Structural scheme.

Ground floor

N350– SK2 7/12/15

Ground floor slab supports walls over.

Slab cantilevers to support perimeter walls

250mm thk. Slab.

Openings for roof lights formed in ground floor slab.

Opening for roof light formed in ground floor slab.

Opening for future lift provision formed in ground floor slab. Opening in-filled with timber floor joists.


Structural scheme.

Roof

N350– SK3 7/12/15

Roof structure comprising 225 x 47 (C16) timber joists at 400mm centres. With 18mm Plywood / OSB forming sheathing to roof below insulation and roof finishes.

Green roof structure comprising 225 x 47 (C16) timber joists at 400mm centres. With 18mm Plywood / OSB forming sheathing to roof below insulation and roof finishes.

Steel beams support roof over open spaces below.


Structural scheme.

Basement Sections

N350– SK4 9/12/15

Section Through Underpinning to

Party wall.

Reinforced Underpinning forming retaining wall under freestanding wall on front elevation.

Existing rendered freestanding wall on front elevation.

Blockwork liner wall.

Load bearing Blockwork wall supporting ground floor slab and structure over.

Reinforced Underpinning forming retaining wall under party wall.

Heal on underpin to match width of existing spread foundation in order to mirror existing bearing conditions..

Existing basement to 1 Wolsey Road

Existing elevated ground floor to 1 Wolsey Road

Existing Party Wall shared with 1 Wolsey Road

Suspended ground floor slab.

Basement slab with finishes including drained cavity system.


Basement slab with finishes including drained cavity system.

Section Basement Retaining Wall

Under Front Wall.

1a Wolsey Road.

(Land to rear of 121 Mildmay Road. N1 4PT)

Structural scheme. - Sections

Excavation Sequence Pavement Boundary.

N350– SK5 10/09/17

1. Retaining Walls along the Wolsey Road elevation.

A basement enclosure wall is to be constructed below the retained

perimeter wall. This will take the form of reinforced concrete

retaining walls constructed as a series of reinforced concrete

underpins, carried out in a traditional ‘hit and miss’ sequence.

2. Ground movement monitoring.

Level targets around the building perimeter are to be regularly

monitored as the basement excavation is progressed, to ensure

there is no adverse movement. In the event that ground movement

or wall deflections exceed predefined limits, the excavation will be

stopped until additional propping and strutting systems are put in

place.

Reinforced Underpinning forming retaining wall under freestanding wall on front elevation.

Existing rendered freestanding wall on front elevation.

Ground floor slab installed, forming permanent prop to retaining wall

Step 1 – Existing. Step 2 – Excavate in Underpin Sequence.

Step 3 – Form Retaining wall Segments.

Step 4 – Thrust Blocks and Propping.

Step 5 – Basement Slab. Step 6 – Ground floor Slab.

Props with kentledge to provide restraint to freestanding wall.

Excavate 1m wide pits in underpin sequence. Trench props installed prior

to excavating to form next section of retaining wall.

Local excavations for concrete thrust blocks, below proposed basement slab.

Steel wailing beam and raking props to restrain new retaining wall.

Propping to be maintained and monitored throughout construction works.

Excavate to basement slab formation level. Form reinforced concrete basement slab.

1a Wolsey Road.

(Land to rear of 121 Mildmay Road. N1 4PT)

Structural scheme. - Sections

Excavation Sequence to Party Wall.

N350– SK6 10/09/17

1. Underpinning to Party Wall.

A basement enclosure wall is to be constructed below the party wall.

This will take the form of reinforced concrete retaining walls

constructed as a series of reinforced concrete underpins, carried out

in a traditional ‘hit and miss’ sequence.

2. Ground movement monitoring.

Level targets around the building perimeter are to be regularly

monitored as the basement excavation is progressed, to ensure there

is no adverse movement. In the event that ground movement or wall

deflections exceed predefined limits, the excavation will be stopped

until additional propping and strutting systems are put in place.

Reinforced Underpinning forming retaining wall under party wall.

Step 1 – Existing. Step 2 – Excavate.

Step 4 – Form Retaining wall Segments.

Step 5 – Basement Slab.

Excavate 1m wide pits in underpin sequence.

Excavate to adjacent basement level.

Excavate to basement slab formation level. Form reinforced concrete basement slab.

Heal on underpin to match width of existing spread foundation in order to mirror existing bearing conditions..

Existing basement to 1 Wolsey Road

Existing elevated ground floor to 1 Wolsey Road

Existing Party Wall shared with 1 Wolsey Road


Step 3 – Excavate in Underpin Sequence.

Documents

Structural Method Statement (Structural Design Philosophy)