Katsunobu TOYOSHIMA

Effects of gravel shape on shear modulus and liquefaction strength in gravel-mixed sand

Hirofumi TOYOTA

Liquefaction generally occurs in the sandy ground, which has loose sedimentation and high groundwater level. However, it has been reported that liquefaction occurred in gravelly ground during the 1995 Kobe Earthquake. Nevertheless, few studies have been conducted on liquefaction of gravelly soil. Although a pre-loading method has been also reported to be effective as countermeasures against liquefaction, the mechanism to prevent the liquefaction is unclear because the method was developed for soft clayey ground against ground subsidence. Therefore, it is important that liquefaction evaluation technique is developed in the ground having wide range of particle size and stress history. Various effects of gravel shape (pebble and rubble), gravel content and overconsolidation ratio (OCR) on liquefaction were examined with taking attention to wave propagation, small deformation properties, and the relevance of each properties.

The results obtained in the study are follows:
(a) Liquefaction characteristic of gravel-mixed sand
1. When gravel content exceeds 20% in mass, liquefaction strength greatly increases.
2. If gravel content is less than 10%, liquefaction strength is independent on gravel shape. When gravel content reaches 20%, the liquefaction strength in the rubble-mixed sand becomes larger than that in the pebble-mixed sand.
3. The liquefaction strength increases by overconsolidation. This effect is stronger in order of the rubble-mixed sand, the pebble-mixed sand and only sand specimens.

(b) Wave propagation and small deformation properties related to liquefaction strength
1. The liquefaction strength has close relation with a secant shear modulus of about 0.02% shear strain. Therefore, there is a possibility that liquefaction strength can be precisely estimated using this kind of secant shear modulus.
2. The shape of normalized G-ƒÃs relations, where a secant shear modulus is normalized by an initial shear modulus, only depends on the OCR. A technique to estimate liquefaction strength in the ground was proposed using this relation and Vs obtained from in-situ investigation.