Modeling Small-Strain Behavior of Hong Kong CDG and Its Application to Finite-Element Study of Basement-Raft Footing

Stress-strain responses, including small-strain behavior, of Hong Kong completely decomposed granite (CDG) were investigated using triaxial specimens prepared from Mazier samples in a manner in which structure in the soil was essentially preserved. The observed behavior was different from what has been reported by previous researchers, which was attributed to the preservation of structure in the specimens. In particular, obvious shearing dilatancy (as measured by d epsilon vol/d epsilon 1) was observed for the tests conducted with confining stress larger than the in situ stress. It was found that the hardening soil small-strain (HSS) model was able to capture most of the observed prefailure behavior, and most of the intrinsic assumptions of the HSS model were satisfied. All input parameters in the HSS model could be determined objectively from the triaxial tests, and the maximum shear modulus at the very-small-strain region could be obtained from the precise small-strain measurements without the need of bender-element testing. The HSS model was then incorporated into a finite-element (FE) study of a basement-raft footing on Hong Kong CDG. This study shows that (1) neglecting the small-strain component in the model could lead to an overestimation of settlement by 55%, and (2) inclusion of cap yield is significant. The FE analysis results clearly demonstrate basement-raft footings can support middle to high-rise buildings in Hong Kong without excessive settlement.