All done in time for tea and marmalade sandwiches

by | Feb 14th 2024

All done in time for tea and marmalade sandwiches

14th February 2024

Geoenvironmental • Geotechnical • Specialist Services

Ground & Water was commissioned to undertake a full suite of reporting at a proposed basement development in Paddington including a Phase 1 Desk Study, Phase 2 Site Investigation Report, a Basement Impact Assessment and Ground Movement Assessment for the site. Great client/supplier teamwork resulted in an excellent outcome.

Our Client

Our client was a firm of structural engineers.

Preliminary-Assessment

The initial Phase 1 site investigation identified limited redevelopment during the site’s history as well as the surrounding site environs. Bomb damage was flagged as a potential off-site source of contamination given there were 12 bomb impact records within a 250m radius of the site. The closest of which was 18m to the south-east. The desk study also identified the existing building, garden walls and gateways to be Grade II listed. A review of the TFL Asset Register also identified the Bakerloo Line 30m south of the site running beneath the road in front of the property at a depth of 19.00m bgl. The corresponding London Underground Limited Zone of Influence ran parallel to the line and encroached from the front boundary of the property extending back covering the full footprint of the building. The desk study provided key considerations as to where we would target our Phase 2 Site Investigation in order to satisfy our contamination and geotechnical risk assessments.

 

Our Challenge

The existing site was occupied by a four-storey semi-detached residential property. The site was noted to be split across three levels with differing front garden level, lower ground floor level and rear garden level making access tricky but not impossible!

The proposed development comprised the construction of a full footprint basement, extending to both the front and rear of the property. The dimensions of the basement are approximately 9.20m wide in the centre of the basement by 23.00m long (including wall thicknesses). The basement would be founded at a depth of ~4.69m below existing lower ground floor level (blgf). The proposal from the outset was to have a combined foundation approach including underpinning existing wall sections as well as a small section of piled retaining walls where the basement extended out beyond the existing footprint of the building.

The Ground & Water Approach

The Phase 2 site investigation comprised several trial pits across the site, including foundation exposures to understand the composition of the existing foundations that would require underpinning. A borehole, using a modular windowless sampler, was undertaken to a depth of 12.45m bgl with SPTs at 1.00m intervals, as well as super heavy dynamic probing through the base of the borehole to a depth of 15.00m bgl in order to provide geotechnical parameters to 5.00m below the depth of the proposed piles. As part of the Phase 2 site works a detailed UXO risk assessment was undertaken to bottom out the risk of UXO being encountered on-site, particularly given the proposed piling works.

A suite of soil testing, ground gas monitoring and vapour monitoring was undertaken on-site and determined that no remediation requirements were required for the proposed development.

Basement Impact and Ground Movement Assessments

 Upon submission of the findings of the Phase 2 Site Investigation our client was then able to progress with the initial geotechnical design elements of the basement and issue Ground & Water with finalised basement plans including detailed drawings showing the dimensions and loadings of retaining walls and internal slab loaded points. From this Ground & Water was able to produce an exact layout of the proposed basement including the piled wall section as shown in the figure below. The output of the settlement and heave analysis allowed Ground & Water to tailor the requirement for heave protection measures to defined areas.

 

The Ground Movement Assessment was also tailored to take into consideration the detailed foundation scheme. Using typical analysis incorporating both the horizontal and vertical CIRIA curves for installation within earlier models identified Category 2 damage, in accordance with the Burland Damage Categories, to the front and rear neighbouring property walls. This was considered to be overly conservative given the underpinning method for the vast majority of the basement construction. Therefore, the utilisation of an excavation only curve for the installation reduced the damage to Category 1 (Very Slight). This was thought to be a more realistic analysis, in line with the Construction Method Statement.

 

The Outcome

The project enabled Ground & Water to undertake a comprehensive assessment of the proposed basement from both a contamination and geotechnical perspective. With proactive communication and working closely with our client we were able to influence the design of the basement from an early stage whilst also producing refined accurate modelling of the final scheme.

Find out how we conduct BIAs and GMAs HERE Back to case studies

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