Field performance of long bored piles within piled rafts

In this study, well-instrumented field tests on bored piles were conducted at two construction sites in Zhejiang, China. At the first site, three bored piles in clay deposits overlying rock were load-tested to investigate the load transfer and deformation behaviour of a single pile. The lengths of the 1·1 m diameter test piles ranged from 88 m to 120 m, allowing particular insight into the in situ performance of long piles. In addition, the load–settlement behaviour of these piles within a piled raft group was observed during and after construction of the building they supported. At the second site, four bored piles 32 m long and 0·6 m in diameter were observed during single-pile static load tests, and during and after construction of the overlying building. The pile-head settlement was observed to be due mainly to the compression of the pile shaft for the long pile at the first site. The threshold of slip displacement for fully mobilising the shaft resistance was found to be 15–20 mm and 10–15 mm in clayey and silty deposits respectively. The measured load–deformation behaviour at the pile base demonstrates that the base resistance of single piles in a piled raft system is greater than that in the single-pile condition, and the pile-group settlement ratio is related to the applied pile-head load. The bearing pressure under the perimeter of the raft was measured and found to be greater than the bearing pressure beneath the central part of the raft, although the ratio between these values was found to vary with the applied structural load. In the cases studied, approximately 80% of the applied structural load was carried by the piles; the remainder was carried by the raft in bearing against the soil.