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Technical paper: Bitumen slip coated trial piles and prototype underreamed trial pile at Angel Square, Islington

by L J Whitworth, Soletanche-Fondaco and A J Turner and R G Lee, Ove Arup & Partners

This paper was first published in GE’s January/February 1993 issue.

Introduction

This paper describes the construction, testing and analysis of load tests carried out on one slip coated trial pile and one non slip coated trial pile at the Angel Square site, Islington. In addition, a prototype underream pile was constructed to determine if such piles could safely be constructed at the junction of the London Clay and Woolwich & Reading Clay. Using these results, it was possible to determine the effectiveness of a bitumen coated slip liner and confirm that underreamed piles were a suitable foundation option for the structure.

The Angel Square project for Balfour Beatty Developments (BICC Developments) and Charlecote Estates consisted of a seven storey office and retail development with a two storey basement. The foundation scheme was originally conceived as a conventional straight shafted pile solution. However, during the project design stage, the developers were approached by London Underground Limited which was planning to upgrade its Angel underground station. Part of this upgrading involved constructing a ticket hall within the ground floor of Angel Square development with an escalator shaft from the hall down to station level, see Figures 1 and 2.

This raised the prospect of the tunnelling, which was to commence about one year after the Angel Square piling contract works were completed, severely affecting the performance of the piled foundations. The structural loads from the building ranged from 4600kN to 8250kN per pile. To these loads would now be added the effects of the construction of a 4m diameter pilot tunnel and a 7.85 to 8.6m diameter tunnel between rows of bearing piles.

With the likelihood of the London Clay acting in negative skin friction against the pile shaft as the clay mass moved relative to the piles, the decision was taken to slip coat the piles through the London Clay and support the loads on the base of underreamed piles at the top of the Woolwich & Reading Clay. This was a similar approach as the one used at Cannon treet, when it was thought construction of the proposed Fleet Line could affect previously constructed bearing piles (Ground Engineering, November 1974).

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