ESTIMATE OF ELASTIC SHEAR MODULUS IN HOLOCENE SOIL DEPOSITS

抄録

An empirical equation in use for estimating the pseudo-elastic shear modulus, G_f, of subsoil, associated with shear strains less than 0.001% is proposed in this paper. In a series of in-situ seismic cone tests performed nationwide, the profiles of both G_f and the in-situ void ratio, e₀, with depth were successfully characterised at five sites, each comprising a soft clay layer deposited in the Holocene era. The database which comprised the original data from the field and laboratory tests, coupled with similar information on well-documented Holocene clay deposits in Europe, was statistically analyzed in attempts to determine a generalised relationship with which G_f of soft clay may be reasonably estimated only from routinely available borehole data; that is e₀ and the current geostatic effective overburden pressure, σ'_v. An empirical relationship, G_f=5, 000 e₀^-1.5√(σ'_v) (kPa), was derived from the statistical analysis applied to data from seven different clays worldwide, for which e₀ extended over a range between 1 and 5, and the overconsolidation ratio ranged roughly between 1 and 2. The applicability of the proposed relationship was evaluated for two case records, each in which the clay exhibited unusual behavior; i.e., the undrained shear strength remained more or less constant with depth due to the existence of artesian pressure at one site, and, at the other, G_f decreased, whereas e₀ increased, with depth. It was demonstrated that even in these clay deposits exhibiting exceptional profiles, the proposed relationship was capable of predicting G_f with a reasonable accuracy by determining the profiles of e₀ and σ'_v with depth. In addition, the prediction when compared to G_max from carefully performed laboratory cyclic tests, yielded a better estimate of G_f from the in-situ seismic survey. Despite the fact that the empirical relationship was initially designated to estimate G_f of soft clays, it may be equally applicable to sandy deposits. This was verified by comparing it to similar, and well-established, relationships developed for sands. A case record as such is also described for a loose sand deposit at Higashi-Ohgishima in Tokyo Bay which was placed in 1960's by land reclamation.