In Switzerland, there is concem that sliding along joints under dams could lead to stability problems. As part of a research project funded by the Swiss Federal Office for Water and Geology, more than fifty constant-normal-load direct-shear tests have been performed on induced tensile fractures for seven rock types. Damage zones are evident on all of the sheared surfaces. There is evidence of both crushing and breaking of surface asperities. Damage is relatively sparse, and the location of the damaged zones is strongly related to geometrical features. However, the relationships between surface roughness, stress distribution, and damage are complicated and difficult to study, in part, because the boundary conditions goreming the mechanical behavior change continuously during shearing. One of the primary objectives of this work is to better understand the micromechanical behavior of joints under shear loads, including the creation of damage zones. This requires understanding the relationships between material properties, surface geometry, contact area, stress distribution, and the creation of damage during shearing. A methodology for predicting damage during shearing has been developed based on analysis of maps of the joint surfaces obtained before and after shearing using a three-dimensional optical system. The surface data is analyzed to identify the areas on the joint surfaces most likely to be in contact during shearing; i.e. areas with positive slope with respect to the shear direction. Ix)cal gradients are also taken into account in predicting those areas of the joint surfaces most likely to be damaged during shearing. The damage predicted is compared to the damage mapped on laboratory test specimens.
Shearing of rock joints in-situ occurs under a variety of boundary conditions. However, it is possible to identify two characteristic behaviors. The first occurs under conditions where the joint can freely dilate, such as exist on rock slopes. This condition is duplicated in the laboratory by maintaining a constant normal load (CNL) during shear tests. The second characteristic behavior occurs when joints are constrained, so that any dilatancy increases the normal load acting on the joint; e.g., for joints in foundation piles or a block in a rock mass. This condition is simulated in the laboratory by keeping the normal stiffness constant (CNS) during shearing.
For studying joint behavior under the foundations of dams, it is reasonable to postulate that the high water pressure against the wall of the dam produces shearing along fractures under the foundation. Depending on the orientations of the joint sets and their depth, each joint is relatively free to dilate; normal loads are typically in the range between 0.2 and 5.0 MPa. To study these conditions, it has been judged that the most appropriate laboratory experimental set-up is the CNL shear test. Therefore, to study the frictional response of rock joints under conditions that simulate those in dam foundations, a series of direct-shear tests were performed on induced tensile fractures in seven rock types. Tests were also performed on replicas of tensile fractures. In addition to shear loads induced by water pressure acting on dams, the change in water level that occurs during the year can cause cyclic loading of joints under the dam. To study the effect of cyclic loading, tests were also conducted with up to five shear cycles on the same sample; i.e. after 5 mm of shear displacement, the samples were repositioned at the origin and sheared again.
The obvious advantage of using natural rock samples is that it allows us to test rock types commonly found underground and in foundations. We can thus investigate
