South Thames College

Pile Type: CFA Secant Wall Piles

Number of Piles: 679no.         

Diameter: 750mm - 900mm

Depth: 28.5m

South Thames College is a new residential and commercial development consisting of four blocks. Three of these blocks share a single-storey basement. The site is located within the one-way system of Wandsworth. Whilst access and working area was relatively unrestricted, the presence of a Thames Water Sewer at a shallow depth running through the centre of the site meant additional consideration in pile construction was required. 

In addition, the secant piled wall which follows the line of the sewer on one elevation was located in close proximity to the 'pile exclusion zone' set by the service provider.

Relic piles and old concrete foundation were left in the ground where it was not viable to remove. This meant the secant wall, which, by its nature must for an interlock, would need to be installed taking in to account positional restrictions of sewer and requirements to not deviate the piled wall in to the basement footprint if obstructions were encountered. 

Our in-house design team designed a 750mm diameter secant bored piled wall, installed using the CFA technique with our Soilmec SF-65. To deal with the relic piles and obstructions in the line of the wall we mobilised our Bauer BG-20 bored piling rig to site to core out the obstructed positions utilising rotary coring methods. 

A return visit in July allowed all remaining 750mm diameter bearing piles could be installed from a reduced level within the basement footprint straight in to the London Clay. In turn this meant rotary bored piles could be utilised to achieve the high working loads in on 750mm diameter piles installed to a depth 28.5m.

Savings were made as a result of:

• Decreasing pile diameters from 900mm, as originally detailed, to 750mm as proposed by Franki design team.
• Piling at a reduced level where London Clay was present meant reduction in concrete quantity and reinforcement as piles would have otherwise been constructed from ground level. The higher platform level would habe meant CFA piles or temporary casings to depth would be needed as a consequence (due to the presence of granular drift deposits).
• Even through significant additional relic piles and obstructions were encountered Franki still delivered the scheme within budget and to the original proposed programme.