Gmax of Fine-Grained Soils at Wide Void Ratio Range, Focusing on Time-Dependent Behavior

Effects of creep as well as initial water content at deposition on the pseudo-elastic shear modulus, Gmax, of fine-grained soil are studied in a consolidometer equipped with bender elements. The state boundary (SB) concept is discussed in void ratio (e)-Gmax-Effective Vertical Stress (σ′v) space and the condition of metastability associated with short-term creep and long-term natural creep are exemplified from laboratory creep tests and in-situ soil test results, respectively. In addition to the conventional laboratory reconstitution method adopted for almost all clay samples that use an initial water content of twice the liquid limit, the soil samples from Minato-Mirai (MM) site were also prepared by directly consolidating the slurry with a water content ranging from 1.5 to 5.0 times the liquid limit. The larger primary metastable region or higher degree of on-depositional structuration associated with the samples at higher initial void ratio is confirmed. However, no appreciable difference was found in the increase in Gmax with time, represented by Ng, between the samples prepared by the two different methods. In addition to the highly plastic MM clay, the study of Ng also covered other clayey soils of different origin and plasticity index range from 29 to 78. Metastability index, MI(Gmax)e that manifested the aspect of structuration and déstructuration reaches a maximum value at the end of sustained loading and vanishes slowly with any increase in stress level. By using liquidity index instead of void ratio in the e-log Gmax plot, the metastability index, MI(Gmax)LI, is found to represent a wider variety of soils with minimum scatter. A slight stress level dependency of metastability increase was observed yielding smaller values at higher stresses. For the present test conditions and duration, subsequent stressing of 1.5-2.0 times the creep stress brought about the complete déstructuration of the creep-added soil-structure formed in the previous creep step.