Performance of raft foundations for high-rise buildings on the Bouldery Clay in Singapore

The Bouldery Clay in Singapore consists of sandstone boulders that are embedded in a matrix of very stiff to hard silty and sandy clay. The strength and deformation characteristics of the soil matrix are close to those of a weak rock or a heavily overconsolidated clay. The average value of the second cycle moduli from the pressuremeter tests and the plate load tests is 480 MPa, and the average value of the undrained shear strength from in situ tests is 1000 kPa. The performance of raft foundations for six high-rise buildings founded on the Bouldery Clay has been very satisfactory, and the measured maximum and differential settlements are within allowable limits. The average equivalent undrained modulus of elasticity back-calculated from settlement observations is 540 MPa, compared with the average modulus of 480 MPa from in situ tests after unloading and reloading. It is therefore recommended that unloading and reloading of at least two cycles should be carried out for in situ tests so that a more representative modulus of elasticity for the Bouldery Clay and other very stiff or hard soils can be obtained.