Abstract
Considering the possibility that hydraulic fracturing is induced in a dam body or foundation by the cracks which occurred during construction, caused by an earthquake or shrinkage due to drying, it is important to obtain the mechanical properties of the material of a dam body and foundation. To measure small strains and avoid bedding errors, gap sensors were used in laboratory test on foundation clay soil of Oyachi reservoir.
It is well known that the ratio of tensile strength to compressive strength of a hard material such as rock is 1/10-4/15. However, the test to obtain tensile strength of soft clay of pyroclastic ash loam has been scarcely carried out. In order to investigate the growth of tensile cracks in a dam body or foundation, it is important to evaluate the relationship between tensile strength and strain. A series of tensile tests on foundation clay of pyroclastic ash loam were performed using thick wall cylindrical specimens. Based on these tests, the characteristics of foundation soil in tensile region were compared with those in compressive region for disturbed and undisturbed soils. When the leakage in Oyachi reservoir occurred, ground water was observed only in the dam foundation (undisturbed soil) and no seepage water was observed in the dam body (disturbed soil), hence it was concluded that the leakage was through the foundation.
Based on the test results mentioned above, it was found that tensile and compressive deformation moduli are nearly equal in the level of small strain and the ratio of tensile strength to compressive strength is about 1/5-1/4. The dam foundation was confirmed to be over?consolidated and the tensile strain at failure for disturbed soil is 3-6 times as large as that for undisturbed soil. This suggests that the cracks can occur much more easily in the dam foundation than in the dam body.
