Old Marylebone Road, London

Mark Creighton

Mark Creighton

Director

Tel: 020 3096 7567
Email

Client

Gilbert Ash/Simpson Associates
on behalf of Whitbread plc

Services

  • Restricted Access Ground Investigation
  • Pile Condition Report
  • Geoenvironmental Assessment
  • Geotechnical and Foundation Reuse Risk Review
  • Piled Raft Settlement Analysis

Enabling reuse of piled foundations

CGL was engaged to provide a site investigation at 191 Old Marylebone Road to assess the viability of reusing the existing piled foundations and incorporating them into a proposed new 14 storey concrete frame building with much higher foundation loads than the original structure.

The proposed building will be used for a new ‘hub by Premier Inn’ hotel. CGL’s investigation was carried out in coordination with the demolition contractor on site to allow the existing piles to be tested and to gather geotechnical and geoenvironmental samples. The works were undertaken within the basement of the existing nine storey office building using cutdown cable percussive boreholes and hand excavated trial pits in order to install specialist testing equipment and provide access.

Before commencing work onsite, copies of the original piling design were acquired from the structural engineer of the existing building and used to identify the likely position and dimensions of existing piles.

Using intrusive investigation and parallel seismic testing, CGL established the strength, durability and length of a sample of the existing piles, which were found to be shorter than indicated on the original scheme drawings. The information acquired was used with PLAXIS  3D software to develop a finite element model of the existing and proposed structure. This allowed the complex soil structure interaction of the piles and a proposed raft foundation to be assessed. The analysis showed that total settlements of the proposed structure would be acceptable using the bearing capacity on the underside of the raft slab and the existing piles in conjunction with  strategically placed supplementary piles.

Given the limited information and uncertainty surrounding the existing piles a sensitivity check was undertaken to mitigate the risks and assess the potential impacts should any of the existing piles prove to be defective.

CGL used the initial site investigation to recommend an appropriate testing regime once the existing structure is demolished to verify the position of piles and the condition of their reinforcement. These investigations will mitigate the risk of incorporating the existing piles into the proposed substructure.

Reuse of existing piles following the principles of the BRE RuFUS Framework (reuse of foundations for urban sites) can represent  significant cost savings for developers, providing a suitable analysis is undertaken to mitigate risks and provide a robust solution. As urban development becomes more congested, reuse of foundations supports sustainability and future development potential as well as
minimising usage of new materials.