Yosuke OMORI

Estimation method of liquefaction strength considering deformation characteristics in gravel-mixed sand

Hirofumi TOYOTA

Liquefaction generally occurs in the loose sandy ground with high groundwater level. However, it has been reported that liquefaction occurred in gravelly ground during the 1995 Kobe Earthquake. The necessity of liquefaction evaluation on gravelly soil has arisen after this earthquake. Although the liquefaction estimation using N-value has been conventionally proposed, it is difficult to apply N-value to the liquefaction estimation of gravelly ground. Therefore, liquefaction estimation technique was renovated for the ground containing wide range of grain-size in this study. Effects of gravel content on liquefaction properties were examined with taking attention to the relevance of wave propagation properties and small deformation characteristics. Moreover, the experiments were conducted using the specimens with relative density of 35%, and the effects of density on liquefaction properties were also discussed in comparison with the results of relative density of 75%, which was conducted last year. The conclusions obtained by the study are indicated below.

(a) Liquefaction characteristic of gravel-mixed sand:
When the gravel content exceeds 20% in mass, the increased tendency of liquefaction strength with gravel content accelerates. The similar tendency was obtained irrespective of relative densities. The liquefaction strength increases with over consolidation ratio, and this tendency is the same irrespective of relative densities.
(b) Small deformation properties of gravel-mixed sand:
The initial shear modulus increases with an increase of gravel content, but hardly increases with over consolidation ratio. The decline of secant shear modulus with shear strain becomes small compared with that of normally consolidated specimen because elastic region expands by the effect of over consolidation.
(c) Initial shear modulus obtained from wave propagation and small deformation tests:
The initial shear modulus obtained from small deformation tests is larger than that from shear wave propagation. The difference of the shear modulus between two methods becomes bigger when gravel content increases.
(d) Wave propagation and small deformation properties related to liquefaction strength:
The liquefaction strength has close relation with a secant shear modulus of about 0.008%-0.015% shear strain. Therefore, there is a possibility that liquefaction strength can be precisely estimated using this kind of secant shear modulus in the gravelly grounds with various densities.
When secant shear moduli are normalized by the initial shear moduli, the difference induced by relative density becomes uncertain. However, the difference induced by over consolidation remains in this normalized relations. Finally, estimation method of liquefaction was proposed in the gravelly grounds with various density using this normalized relation and shear wave velocity obtained from in-situ investigation.

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