Laboratory Studies of Two Common Saprolitic Soils in Hong Kong

Two commonly encountered saprolitic soils in Hong Kong, weathered volcanic tuff (WT) and weathered granite (WG), were studied using high-quality intact samples. The intact samples exhibited quasi-preconsolidation pressure or yield stress under isotropic compression due to their bonded structures, but the yield was progressive and not abrupt. As the stress increased, significant volumetric changes were measured. These changes resembled clay-type behavior. The soils also exhibited anisotropic deformation under isotropic loading and unloading, which was associated with the features of their parent rocks. During the drained tests, shearing at the in situ stress-state produced peak strength and volumetric dilation. Undrained shearing showed complicated stress paths and dilatancy behavior in these soils. Phase transformation states and dilative shear failure were readily seen, which resembles typical sand-type behavior. Distinct shear band(s) appeared in the WT specimens during shearing, whereas a bulging type of failure appeared in the WG specimens. The soils ultimately approached the corresponding state guided by a unique critical state line, regardless of their complex initial states in relation to the bonded structure and drainage conditions.