Settlement of building foundations based on field pile load tests

Serviceability limit requirements are an important concern in pile foundation design. In places such as Hong Kong, where depths of competent founding soils/bedrock vary greatly, the pile lengths can vary from a few meters to over 100 m. Following the current practice of pile design and acceptance, the settlement of pile foundations can be considerably different from one site to another. The objective of this paper is to evaluate ranges of the settlement of buildings supported by driven pile foundations. Settlement measurements from full-scale static load tests on a large number of piles are reported. From the load-displacement curves of these load tests, the distributions of the settlement of single piles under the intended design load, twice the design load, and the failure load are determined. The settlements of single piles at the intended design load vary from less than 10 mm for relatively short piles to about 30 mm for long piles and the settlements at twice the design load vary from 18 mm to 85 mm. The single pile settlement follows approximately a lognormal distribution. The settlement of a pile group is then obtained by multiplying the single pile settlement by a group settlement ratio that is defined as the ratio of the pile group settlement to the single pile settlement under the same load per pile. The distribution of the settlement ratio for freestanding pile groups in sand is established based on data from some field and model pile group tests. The results show that considerable pile group settlement can occur even if all individual piles satisfy specified acceptance criteria. Settlement problems may be particularly serious for buildings supported by very long piles or piles founded on undulating bedrocks.