Leading the re-use of foundations

5th February 2025

Geotechnical • Specialist Services

Piling is generally the most expensive type of building foundation. They are often used if you have poor soil conditions and if a subsidence risk exists. A client designing a three storey extension in Southeast London had found old piling from a previous building under its site. Ground & Water was commissioned to find out if they could be re-used.

Our Client

Our client was a firm of architects.

Preliminary-Assessment

A preliminary desk study was conducted. The BGS Geological Map for the West Dulwich area (South London Map, Map number 270) revealed that the site was located on bedrock deposits of the London Clay Formation. Superficial Head Deposits were noted ~150m north-west of the site. An area of Made Ground was noted ~100m north of the site. Given London Clay is highly susceptable to shrinkage as it dries out it was thought piles were used as foundations for the former building due to the close proximity of mature trees. Given the cost of removing the old piles, and replacing with another foundation solution and the fact they were directly under the site of the proposed extension, it made financial and practical sense to try to re-use them.

 

Our Challenge

Due to the history of the site, previously installed piles were present in the location of the proposed end terrace extension. Given their age there were no as built drawings available. Ground & Water’s challenge was to find out their, condition, depth and whether they were suitable to re-use as part of the foundations for the new build.

The Ground & Water Approach

A thorough site investigation was required, which included uncovering the pile caps, low strain Sonic Integrity Testing (SIT), drilling a Cable Percussion Borehole with UT100s and Standard Penetration Testing to determine the condition and bearing capacity of the installed piles.

The low strain SIT was undertaken on the four existing 350mm nominal diameter concrete piles. Continuous signals were recorded to depths between 8.50m and 9.50m across the piles (see table below).

The next step was to understand the ground conditions.

 

Site works comprised the drilling of one Cable Percussion Borehole to a depth of 20.45m bgl. Alternating UT100s and Standard Penetration Testing (SPT) were undertaken at 1.00m intervals for the full depth of the borehole.

This revealed a thin capping of Made Ground to be underlain by the medium to very high undrained shear strength London Clay.

 

The Outcome

Based on the ground model produced Ground & Water was able to determine the load bearing capacity of the installed piles. The client and its structural engineer were ultimately able to save time and money by re-using some of the piles onsite.

This was thanks to our thorough Site Investigation and extensive knowledge of ground engineering.

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