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821/01/Rev1 5th October 2021 No.33 Lanark Road, W9 – Pavement Vaults Construction Method Statement 1. Introduction 1.1) The owners of the site at No. 33 Lanark Road, in the City of Westminster, are

proposing to lower the floor level within the existing pavement vaults at the front of the property.

1.2) We have been appointed by the owners to provide structural engineering input in

relation to the proposed works and to prepare this Construction Method Statement. The statement has been prepared in accordance with the City of Westminster’s planning policy and sets out the existing arrangement and condition of the site, and goes on to explain the proposed structural works and how the proposed construction methodology has been developed to limit the impact on the existing site conditions.

1.3) This note forms part of the planning application submitted by Ian Wylie Architects

to the City of Westminster. It should be read in conjunction with all other relevant drawings and reports submitted as part of the planning application.

1.4) Our understanding of the building and site has been informed by our internal

desktop study, the architect’s existing structure drawings, and our visit to site on the 15th September 2021 to look at a trial pit in one of the vaults.

2. Existing Building

2.1) No.33 Lanark Road is a mid-terrace house comprising two storeys above ground with a single storey lower ground floor level and habitable loft space.

2.2) The building is not listed but is within the London Borough of Westminster’s Maida Vale Conservation area. From the appearance of the elevation the building is part of a terrace that dates from the mid-1800s.

2.3) Based on our experience of projects with buildings of a similar age we expect the basic structure to be load bearing brickwork masonry walls with timber partitions and floors from ground floor level and above. The roof structure is also likely to be timber.

2.4) The available British Geological Survey maps show that the ground conditions on the site consist of a thin layer of made ground over London Clay which extends to depth. Logs from nearby boreholes on Lanark Road corroborate this information.

2.5) The existing house is likely to have shallow brick corbel footings that bear onto the top of the London Clay.

2.6) At the front of the property there are two brick barrel pavement vaults. The floor within the vaults comprise non-original ground bearing concrete slabs. The slabs appear to be unreinforced.

Stand Consulting Engineers

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2.7) Trial pit investigations carried out in September and recorded during our visit show that the brickwork walls of the vaults do not have meaningful footings, with the walls founded just below the slab level and built off reworked London Clay containing shell, brick and tile fragments.

2.8) The building and pavement vaults are both in a reasonable condition for their age and type of construction.

2.9) A summary of the existing structure of the pavement vaults is shown on SK01 and the results of the trail pit investigations are shown on SK02. Photo of the pit are included in Appendix B.

3. Proposals

3.1) The proposed works within the pavement vaults comprise the lowering of the existing floor level by 600mm to increase the ceiling height, thus allowing the vaults to be used as a habitable space.

3.2) The door opening to one vault is to be increased in width. 3.3) The existing opening between the vaults is to be increased in with to allow for the

integration of utilities. The proposed opening is below the springing of the vaults.

3.4) The structural engineering works required to implement the proposals comprise the underpinning of the existing pavement vault walls (1m deep), the formation of a new reinforced concrete floor slab, and the installation of new lintels to form the new opening.

3.5) The proposed scope of the structural works is shown on drawings SK03.

4. Structural Engineering Comments on Construction and Methodology

4.1) Underpinning is a well-established practice for lowering the foundation level of existing masonry walls.

4.2) It is carried out in a ‘hit-and-miss’ sequence, in lengths of approximately 0.9m – 1m, with no adjacent pins excavated and cast in at the same time.

4.3) The width of the underpins are to be at least the width of the existing wall above to mitigate the risk of increasing the bearing pressure at the base of the wall, and therefore the risk of movement.

4.4) The quality of underpinning is linked with the performance of the contractor; as a result, it is important that such works be undertaken by a contractor with a proven record of projects dealing with underpinning and basement constructions near existing buildings.

4.5) It is impossible to entirely eliminate the risk of ground movements during such works. However, given the existing vaults and adjacent building are in a reasonable condition for their age and type of construction, we do not anticipate the underpinning of the vault to have any significant structural impact on the vaults or the adjacent buildings.

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4.6) Movements as a result of the installation of the underpins are not expected to exceed 1 to 2 mm. This level of movement would correspond to Burland damage category of 0 or 1.

4.7) The structural engineering specification for the underpinning is included within Appendix A.

4.8) The sequence of construction assumed in the preparation of this note are as follows:

I) Carry out underpinning in a hit and miss sequence in accordance with the

underpinning specification. Note, the section of wall where the opening is to be formed is not to be underpinned.

II) Carefully install new precast concrete lintels in sequence into chases formed

in the existing brickwork above the proposed widened openings.

III) Carefully form opening below new lintels and make good the brickwork.

IV) Excavate the vaults to the formation level of the new slab by hand. Remove spoil from site as required.

V) Locally excavate and cast new strip footing below newly formed openings.

VI) Cast new reinforced concrete ground bearing slab.

VII) Complete architectural fit out, including waterproofing etc.

5. Conclusion 5.1) A structural engineering construction method statement has been undertaken by

Stand Consulting Engineers for the proposed excavation of the existing pavement vaults at No.33 Lanark Road.

5.2) The engineering rationale and construction issues associated with the proposed

works have been explored and summarised in this report. A structural scheme design has been prepared, which aims, as far as practicable, to maintain the status quo for the existing site. The buildability of the proposed scheme has also been explored and the principles for the sequence of construction defined.

5.3) In summary, we do not envisage any significant engineering issues from the

proposed development.

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Appendix A Structural Engineering Drawings and Underpinning

Specification

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Appendix B Record Photos of the trail pit Investigations

No.33 Lanark Road

Underpinning Specification

Stand Consulting Engineers 52 Foundling Court The Brunswick Centre Marchmont Street London WC1N 1AN T: 020 7278 6136 W: www.standengineers.co.uk

Prepared by Stuart Tappin

Issued by Robert Walton

Project No 821

Rev P1

Date Sept 2021

Consulting

Engineers

This specification is to be read in conjunction with the Architect’s and Engineer’s drawings

and the contract documents.

1.0 GENERAL

1.1 The contractor is responsible for verifying all site and setting out dimensions and levels,

including ‘as built’ portions of retained structure and temporary works, before

commencing the works.

1.2 The Contractor shall ensure that the stability and structural integrity of the existing

building and all adjacent structures is maintained at all stages of the works. They shall

design, install and maintain all necessary temporary works, and programme the works

accordingly.

1.3 The Contractor shall prepare his own proposals for the sequence of construction for

which they shall remain entirely responsible.

1.4 The Contractor shall take all necessary site measurements to prepare fabrication

drawings and other elements of the new structure.

1.5 The Contractor shall ensure that sufficient tolerances are provided and integrated

throughout all elements of the works.

1.6 Any scaffolding erected shall comply with BS EN 12811 and BS 5975 2008. Any metal props

and struts required shall comply with BS 4074 2000. Any temporary shoring, propping or

strutting shall be provided with an adequate foundation.

2.0 EXCAVATING AND FILLING

2.1 WATER: Keep excavations free from water until foundations and below ground constructions are completed.

2.2 BACKFILLING TO FOUNDATIONS:

- Under oversite concrete and pavings: Hardcore as clause 65.

- Under grassed or landscaped areas: Material excavated from the trench, laid and compacted in 300 mm layers.

2.3 HARDCORE:

- Granular material, free from harmful matter and excessive dust or clay, well graded, passing a 75 mm BS sieve and one of the following:

- Crushed hard rock or quarry waste (other than chalk).

- Crushed concrete, brick or tile, free from plaster.

- Gravel or hoggin.

- Spread and level both backfilling and general filling in layers not exceeding 150 mm. Thoroughly compact each layer with a vibratory roller, vibrating plate compactor, vibro- tamper, power rammer or other suitable means.

270 FOUNDATIONS GENERALLY:

Obtain instructions if a natural bearing formation of undisturbed subsoil is not obtained at the depth shown on the drawings or the formation contains soft or hard spots or highly variable material.

DESIGNATED MIX FOR BLINDING AND TRENCH FILL (NON AGGRESSIVE SOIL)

- Mix GEN 2 to BS 8500-1:2006

- The concrete will be unreinforced.

- Nominal maximum size of aggregate: 20 mm.

- Admixtures: Do not use

310 UNSTABLE GROUND: Inform CA without delay if any newly excavated face will not remain unsupported sufficiently long to allow the necessary earthwork support to be inserted. If the instability is likely to affect adjacent structures or roadways, take appropriate emergency action until instructions are obtained.

3.0 UNDERPINNING SPECIFICATION

3.1 The contractor shall be responsible for ensuring that his operations do not in any way impair the safety or condition of the existing structure or the adjacent properties. They shall provide any temporary supports required for this purpose and shall carefully inspect the condition of the structure both before and during the execution of the work and immediately inform the Engineer if they consider that any more stringent procedure than that specified is necessary.

3.2 Underpinning is to be carried out in short sections not exceeding 1000mm in length.

3.3 Should any movements of the existing structure or adjacent properties take place, work should be stopped immediately and the CA be notified for inspection prior to commencement of the works.

3.4 The underside of the footings are to be cleaned and hacked free of any dirt, soil or loose materials before underpinning.

3.5 The body of the underpinning is to be constructed in grade C35 (35N/mm2) sulphate resisting concrete is to be cast to the widths and depths shown on the drawings. As far as practicable the excavation and concreting of any section of underpinning shall be carried out on the same day.

3.6 Un-concreted sections shall be kept covered to prevent the ingress of water.

3.7 The mass concrete is to be stopped off approximately 75mm below the underside of the existing footing and the final pinning up over the whole extent of the latter is to be carried out with a dry pack comprising a 1:3 cement:sand with sufficient water to

achieve a set. The dry pack is to be well rammed in as soon as possible after the foundation has set hard so that the gap between the top and bottom surfaces is completely filled.

3.8 Excavation of any sections of underpinning shall not be commenced until at least 48 hours after completion of any adjacent section of work.

3.9 Form a rough surface between adjacent sections of underpinning.

3.10 Unless otherwise noted mild steel rods 16mm in diameter and 600mm long shall be cast equally into each section to cross the joint, at the rate of 2 No. for every 600mm depth.

3.11 The contractor shall provide for the maintenance of drainage services during the underpinning operation and for the reinstatement of any services interrupted or disturbed by the excavations.

3.12 The contractor shall prepare a method statement and submit it to the C.A for comment at least 10 working days prior to the commencement of the work. This is to include:

(i) The underpinning sequence

(ii) Method of excavation in the gravels and proposed shoring

(iii) How dry pack will be placed

3.13 The contractor shall remove all waste material on completion of the works.