Enabling the 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 the 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 on site, 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 current piles prove 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 theexisting piles into the proposed substructure.

Following planning consent, Simpson, on behalf of the contractor Gilbert Ash, again approached CGL for Ground Movement Reports for party wall approval and for further advice on an appropriate post-demolition pile testing strategy to verify the position of piles, concrete strength, condition and identifying presence of reinforcement required. These investigations mitigated the risk of incorporating the existing piles into the proposed substructure. CGL further collaborated with the design team in providing the detailed construction design of supplemental settlement control piles and the conglomerate of existing and proposed piles under the onsite tower crane. A novel solution was also promoted for the proposed drainage attenuation within a contiguous piled ring.

Following existing pile cap removal and piling works, static load tests were undertaken on new and existing piles. CGL reviewed the pile test data and other site records supporting our robust qualitative pile testing regime. There was a satisfactory agreement between the as-built pile location and condition, needing no significant remedial interventions/remedies on site. There was also good alignment of static working pile load tests with CGL’s design predictions.

This reuse of existing piles, following the principles of the BRE RuFUS Framework (reuse of foundations for urban sites), represented significant cost savings, all facilitated by CGL’s advanced FE analysis undertaken to mitigate risks and provide a robust design solution. As urban development becomes more congested, the reuse of foundations supports sustainability and future development potential as well as significant carbon savings through lower concrete and transportation requirements compared to a replacement foundation scheme.