Concrete dams which are constructed in narrow gorges on rock formations should be able to sustain the high pressure imposed from the water behind the dam. This pressure is transmitted to the rocks at the abutment and foundation through the concrete dam body. In double curvature concrete dams in which the pressure resulted from impounding is imposed to the abutments, choosing the dam site (from rock mechanics point of view) and recognizing rock properties at the abutments are of paramount importance. One of the most important parameters which have to be considered is the shear strength of contact surface between rocks and concrete in the abutments. In this paper that is resulted from a series of in-situ direct shear tests on four concrete blocks prepared on the rocks inside an excavated gallery at the abutments, the shear strength parameters and peak and residual strengths of the contact surface.


The aim of performing in-situ direct shear test is measurement peak and residual shear strength parameters of a discontinuity in rock as a function of normal load. The discontinuity is generally a main and important joint which may be filled with soft materials. Shear strength parameters, C and φ, in design of rocky structures, particularly stability analysis of rock slopes and also stability of dam foundation are of paramount important. In-situ direct shear test is the simplest method in determining the shear parameters, C and φ, and due to large dimensions of test blocks in compare to laboratory samples, in-situ tests lead to more precise and accurate results. For determining shear strength of concrete to rock contact surface of the abutments of a concrete dam, four series of in-situ direct shear tests have been conducted inside the galleries which have been excavated at the abutments of a concrete dam.

General Description of Test System

The required equipment for cutting and preparing of test blocks consist of electrical saw, handy cutting tools such as hammer and chisel, solid casts in suitable dimensions and concrete additives. The equipment used for conducting the tests consist of a 200 tones hydraulic jack and two 100 tones jacks for applying normal and shear loads, respectively. These jacks are fed with handy and/or electrical pumps. In order to prevent rotation of blocks under the tests, shear load is applied with a degree of 15 regarding to shear surface so that the load effect line is passing through the center of test block (figure 1). Normal load is also applied regularly and kept exactly normal to shear surface throughout the tests using rollers with low friction. Shear and normal loads are controlled using hydraulic gauges with the accuracy of 0.5 bars. In order to measure normal displacement four mechanical or electrical gauges are used with the accuracy of 0.01 mm or 1 micron, respectively. For measuring lateral and shear deformation 4 gauges of the same type are used. General lay out of the test may be seen in figure 2.

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