Performance of Raft Foundation for a High-Rise Building

Abstract

In Singapore, high-rise buildings are predominantly supported by piles or piled raft foundations. Only very limited high-rise-buildings are supported by un-piled raft foundations. Amongst the scarce amount of un-piled raft foundations, almost all of them have a deep embedment depth with at least several basement levels. This paper aims to present the performance of an un-piled raft foundation with shallow embedment depth for a 30-storey high-rise building. Close monitoring of building settlement has been carried out during and post construction. The recorded maximum settlement of the 30-storey Tower One was on the order of 20 mm. This value was well within the design expectation and found to be very satisfactory. Back analysis has been carried out to determine the elastic modulus using finite element method program, known as Plaxis 3D. The back-analysed elastic modulus of the competent residual soils was found to be on the order of 500 MPa and compared with the elastic modulus derived from extensive in situ field tests (plate load and pressuremeter) and laboratory tests (oedometer and triaxial). The results showed that the second loading cycle of field tests generally give the more reasonable estimate as compared to the laboratory tests. Using limited data, an empirical relationship between elastic modulus and standard penetration testing blow counts has been proposed under small-strain foundation loading. Finally, raft foundation is recommended for future high-rise buildings to be constructed on competent residual soils.