Hideyoshi Shimizu

A study of the measurement of fracture toughness in cohesive soil – relationship between the size of initial crack and diameter of specimen

The phenomenon of hydraulic fracturing is considered to be one of the causes of leakage in fill-type dams. In recent years, it has been found that an estimate of the fracture toughness of a given type of soil could be used as an indicator of the soil’s resistance to hydraulic fracturing. One of the problems encountered in its estimation is the assumption that fracture toughness has been theoretically defined for samples of infinite sizes. In this study, laboratory tests were conducted to determine the fracture toughness of samples (of the same size) prepared with initial cracks of various lengths. The main objective was to investigate the relationship of the initial crack length to fracture toughness. The stress distribution around the crack tip for each laboratory test sample was estimated by FEM analysis and by a theoretical equation. These analytical results corresponded reasonably well to results from laboratory tests to determine the appropriate length of the initial crack for the test specimens.

ENGINEERING SIGNIFICANCE OF WATER CONTENT OF FAULT CLAY OF TERTIARY MUDSTONE WITH SPECIAL REFERENCE TO SQUEEZING TUNNEL

Taking advantage of the opportunity of excavation of Nabetachiyama Tunnel driven through techtonically disturbed mountain consisting of Tertiary mudstone of Shiiya formation, samples of severely faulted mudstone (fault clay) were taken and tested mainly from the viewpoint of critical state soil mechanics. The conclusions are; (1) The water contents of fault clay in the sections suffered from very heavy squeezing pressure were in the range of critical state corresponding to the effective confining pressure at their depths.(2) The competence factor Fc. decreases as the depth of tunnel h(m) increases obeying the formulae fc=0.79-0.13•`1.13h-0.13.