Engineering Structural Considerations

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46 Cheyne Walk - November 2012 Full Planning Pack Page No. 292

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Albyns Limited, 41 Paradise Walk, London SW3 4JL, t: 020 7349 6400, f: 020 7349 6410 www.albyns.com Registered in England & Wales. Reg. No. 06793582, VAT No. 946900894.

8.2 Engineering Peer review

Why?We have commissioned a third party engineer to review our engineers full engineering design, calculations and proposed method statement. In addition they also reviewed the Damage risk assessment and the Geotechnical assessment.

This is to provide information that the design is correct and is not just one engineers vision of how the works should be constructed.

Who?The engineers who have reviewed our proposals are The Morton Partnership Ltd. They are a nationally renowned company of consulting engineers who are recognised for their innovative low intervention solutions to structures used in construction. Specialising in historic buildings their work has included some of the most prestigious buildings in the country such as Canterbury Cathedral, St Georges Hall, Liverpool and the mansion House, London.

Peter Corcoran


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Registered in England No. 2727193 THE MORTON PARTNERSHIP LTD.

CONSULTING CIVIL & STRUCTURAL ENGINEERS, HISTORIC BUILDING SPECIALISTS Old Timber Yard House, 55 The Timber Yard Drysdale Street, London N1 6ND

Tel: 020 7324 7270 Fax: 020 7729 1196 email: [email protected]

www.themortonpartnership.co.uk

Registered Office: Leonardo House, 11 Market Place, Halesworth, Suffolk IP19 8BA Tel: (01986) 875651 Fax: (01986) 875085 London Office: Old Timber Yard House, 55 The Timber Yard, Drysdale Street, London N1 6ND Tel: 020 7324 7270 Fax: 020 7729 1196

Essex Office: 8 Church Street, Coggeshall, Essex. CO6 1TU Tel: 01376 563883 Fax: 01376 563894

Our ref: PC\14103~02

30 May 2012

Matthew Parratt Albyns41 Paradise Walk London SW3 4JL by email

Dear Matthew

RE; 46 CHEYNE WALK, LONDON

Thank you for your acceptance of our fee proposal, dated 22 May 2012 for carrying out a desk based review of the engineering proposal for forming a double basement under the above property. Below we outline the findings of our review.

Introduction and Clients Brief

The owner of the property has submitted a planning application to construct a new deep basement extension to the rear of the house under the garden. The basement is over 11m deep.

The client had had architectural drawings prepared for the planning application and an engineering proposal has also been prepared by Hurst, Pierce and Malcolm, Consulting Civil and Structural Engineers to supplement the application.

We have been appointed by the client via Albyns, the Project Managers to carry out an independent desk based assessment of the structural proposals and review the proposed structural works.

We have been issued with the following information:-

Proposed layouts for planning application Proposed structural engineering drawings by Hurst Pierce & Malcolm Structural Planning Report by Hurst Pierce & Malcolm Damage Risk Assessment by Geotechnical Consulting Group Geotechnical Assessment by Listers Geotechnical Consultants

Summary of Structural Proposals

We do not intend to repeat the full technical proposals in this report but below is a summary of the key structural works.

Two of the four walls of the basement are to be formed by installing a secant piled wall. The other two walls are formed by a 4 stage underpinning system under the party wall with No 47. The ground water level was found at 5.4m below ground floor level and the engineers are recommending that a permeation grouting system is installed to the areas where traditional underpinning techniques are to be used.

The upper basement level slab is to be cast before the excavations are carried out below this level to prop the retaining walls. The works below this level are to be carried out in a ‘top down’ construction where a hole is left in this slab and the excavated material is removed through this hole.


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The stage of underpinning directly under the lower basement slab will need to be carefully carried out as the first excavations must be limited to the length of the first underpin. The underpinning and excavations below the slab are to be sequenced carefully to ensure that the props are adequately installed until it is acceptable for them to be removed.

The walls in the long term are to be propped via concrete floors at basement level, lower basement level and underside of pool level. The overall structural form is inherently strong and will provide adequate resistance to the forces applied to it.

The uplift of the concrete subterranean structure is prevented by the secant piled wall and also the tension piles connected to the underside of the raft.

Review of Structural Proposals

Generally it is our opinion that the proposals and report are very thorough and detailed. The investigations, proposed construction method statement, damage risk assessment and sequence drawings are very comprehensive. The information identifies what risks are associated with carrying out subterranean developments of this kind and the proposals attempt to minimise the risk of any movement causing damage to the historic fabric.

Below we have carried out an assessment of what we would expect the risks to be to the Tudor wall adjacent to the site and also the existing adjacent listed buildings and provide a summary of how HPM have attempted to minimise this risk.

Tudor Garden Wall Risk Action to Minimise Risk Accidental damage to wall caused by impact damage

HPM have proposed to fully protect and soft prop the wall prior to works commencing and therefore this risk is minimal. The damage risk assessment highlights that the installation of the pad foundations of the propping system could affect the listed wall however we feel this risk to be minimal. The pads should be kept to small sizes and not deep enough to undermine the wall.

Movement to wall caused by undermining.

The HPM proposals involve only excavating for the pad foundations for the temporary restraint and protection works to the wall but these are not continuous. The guide walls to the piled wall are in close proximity to the wall but are shown as being the same depth of the wall and therefore undermining should not occur. It will be necessary to carefully remove the guide wall so as not to cause damage to the wall. The installation of the secant piled wall avoids having to excavate under the formation of this wall.

Movement caused by installing secant piled wall in close proximity to the wall

The guide wall for the secant piled wall is to be formed close to the wall as mentioned above. The piled wall is to be approximately 20m below ground level and the anticipated settlement at the top of the wall according to GCG is 6mm which could potentially cause movement in the historic wall. The wall is to be monitored during the works and therefore it will be possible to rectify any minor problems that may arise.

Excavations to basement allowing bearing strata to settle causing movement in the wall

The secant piled wall will hold back the bearing strata supporting the wall and is considered one of the most effective methods of forming deep excavations next to sensitive structures with least risk. The contractor is to ensure that as the excavations progress, the struts are to be maintained tight, preventing the secant piled wall from deflecting or settling horizontally.


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Tudor Garden Wall - Coninued Risk Action to Minimise Risk Long term movement effects causing movement in the wall

The design of the concrete box is very robust and it is not anticipated that long term creep deflection will allow the bearing strata under the listed wall to displace, causing movement in the wall.

In conclusion, there is a low risk that the integrity to this wall will be compromised both during the works and for the long term following completion of the works. HPM have included in their CMS that the wall is to be protected, propped and monitored. Every effort has been made at design stage to minimise any movement or damage occurring to this wall however the contractor must ensure that the method statement is closely adhered to.

Existing Listed Buildings Risk Action to Minimise Risk Movement to wall caused by undermining.

The existing party walls will only be undermined locally for a very short period of time during the underpinning process which is normal for traditional underpinning. The pins are to be limited in length to approximately 1m and a sequence has been provided by HPM to ensure that no excessive length of wall is undermined at any one time. Over excavation is to be controlled in the first level dig under the upper basement level slab to form a working space for the first pin at this level.

Movement caused by installing deep underpinning to the existing walls

The geotechnical consultant concludes that the construction of the basement is likely to cause settlement of the adjacent walls of 10mm which is likely to cause minor damage to the adjoining buildings. The only method of potentially reducing this settlement fractionally is to install a secant pile wall within the existing walls. This option would however reduce the footprint of the basement considerably making the development impractical. The level of movement will depend on the workmanship of the contractor and with this in mind we would recommend that a suitably experienced contractor should be employed to carry out the works.

Excavations to basement allowing RC walls to deflect

The reinforced concrete underpinning wall will hold back the retained earth and as the excavation gets progressively deeper this retained load will increase. The contractor is to ensure that the as excavations progress, the struts are to be maintained tight, preventing the wall from deflecting.

Long term movement effects causing movement in the wall

The design of the concrete box is very robust and it is not anticipated that long term creep deflection will allow the structure to move. Uplift has been calculated and this has been designed out by the tension piles attached to the raft.GCG confirm that because of the inclusion of the tension piles movement associated with uplift will be negligible.

In conclusion, there is a risk of minor movements occurring to the party wall with No. 47 as stated in the geotechnical report however this is not entirely avoidable when undertaking a subterranean development of this magnitude. We have assessed the method of forming the basement and are of the opinion that to utilise the maximum available space, the method proposed by HPM is the most suitable. As mentioned above, a suitable contractor should be sourced with relevant experience of similar projects so that workmanship is carried out to the highest standard. A high standard of workmanship will reduce the risk of unnecessary movement occurring.


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We trust the above meets with your satisfaction but if you have any comments or queries then please do not hesitate to contact the undersigned.

Yours sincerely FOR THE MORTON PARTNERSHIP

PETER CORCORAN


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Engineering Structural Considerations

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