Name: Shiori DAMBARA

Thesis title: Effects of gravel grading on shear modulus and liquefaction strength in gravel-mixed sand

Supervisor: 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 on gravelly ground during the 1995 Kobe Earthquake. Nevertheless, few studies have been conducted on liquefaction of gravelly soil. By the way, although a pre-loading method has been also reported to be effective as countermeasures against liquefaction, the mechanism of preventing 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. Various effects of gravel size, 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. In normally consolidated soil, when gravel content exceeds 20% in mass, liquefaction strength greatly increases. In over consolidated soil, liquefaction strength sensitively increases even if gravel content is only 20%.
2. If gravel content is less than 20%, liquefaction strength is independent on gravel size. When gravel content reaches 40%, the liquefaction strength in larger size of gravel becomes greater than that in smaller one.
3. An influence line between gravel content and OCR was obtained considering the results above mentioned.
(b) Wave propagation and small deformation properties related to liquefaction strength
1. The liquefaction strength has close relation with a secant shear modulus of 0.008% - 0.010% shear strain. Therefore, there is a possibility that liquefaction strength can be estimated using this kind of secant shear modulus.
2. Normalized G-ƒÃs relations where a secant shear modulus is normalized by an initial shear modulus are shown in each case of OCR. A technique to estimate liquefaction strength was proposed using these relations and Vs obtained from in-situ investigation.