Concrete plug replacement is a treatment method for faults in concrete gravity dam foundations. To investigate the effects of concrete plug installation on the shear strength of dam foundations, direct shear tests are carried out for rock models made of plaster. From the experimental results, it is confirmed that under constant and uniform normal confining pressure, the shear strength of plug-installed rock models is about twice that of no-plug rock models and that the strength does not depend upon the location of the plug. It is also confirmed that a failure plane can appear in the area on which the confining normal pressure is acting. A sliding analysis is carried out for the experimental conditions. From a comparison of the experimental and the calculated results, the applicability of the sliding analysis is discussed.


When a concrete gravity dam has to be built on a fault, its stability against sliding or overturning decreases. Therefore, an effective fault treatment is needed. Concrete plug replacement is a fault treatment method in which a part of the fault is replaced with concrete. The Shasta Formula, proposed from the research conducted during the construction of the Shasta Dam, has been used for a long time to determine the appropriate depth for the plug. The largest problem with the formula, however, is that it was made in consideration of the stability of dam bodies and does not take into account the stability of the foundations. The final aim of this study is to clarify the effects of plug installation on the stability of dam foundations and to present a rational design method for concrete plug replacement. In previous studies, the authors developed a gravity dam modeling tester and carried out several tests which considered the location of the fault and the depth of the plug (Nakashima et al. 2000; Adachi et al. 2000; Nakashima, et al. 2002). From the test results, it was confirmed that a plug has two effects on the stability of dam foundations, as shown in Figure I. One effect is that it restricts the harmful deformation of healthy rock. When vertical force acts on a fault, the healthy rock deforms toward the fault due to its softness, as shown in Figure I (a), and a harmful concentration of stress appears around the fault. However, if a plug with high stiffness is inserted into the fault, the harmful deformation and the stress concentration of the healthy rock are restricted. The other effect is the increments in shear strength on the foundation. Due to the working of water pressure against the dam structure, the shear strength may be insufficient to carry the shear force, as shown in Figure I (b). If a plug is installed, however, the deficiency of the shear strength due to the fault is covered by the projection of the plug. In this paper, direst shear tests on plaster rock models are carried out in order to investigate the shear strength of plug-installed foundations.

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