In-situ experiments to determine the hydrologic and mechanical characteristics of large naturally occurring fractures have been conducted at the NAGRA test site in Grimsel, Switzerland. In addition to seismic measurements across a fracture zone in the FRI test area and flow measurements into the zone, deformation of the fracture resulting from pressurization of the zone was also measured. The deformation is modeled in three different ways: as a mathematical crack employing linear elastic fracture mechanics; as a mathematical crack with an additional restraining stiffness between the faces of the crack, and as a row of coplanar two-dimensional cracks.
In recent years with the increasing concern over the proper disposal of nuclear wastes, the modelling of potential nuclear waste disposal sites, and in particular of geologic discontinuities of these sites, has received increasing attention. The behavior of fractures and fracture zones as possible conduits for water into and out of repositories, and as locations for possible future deformation or failure needs to be accurately defined. Seismic methods have potential in providing a way to extrapolate from what is known from surface observations or borehole logging to what is present in the rock mass. Recent work (Pyrak-Nolte et al., 1987; Myer et al., 1990) has demonstrated on a laboratory scale that there are qualitative and quantitative relationships between seismic and hydrologic properties of fractured rock because of the dependence of both of these properties on the mechanical stiffness of the fractures. Theoretical and laboratory work (Schoenberg, 1980; Pyrak-Nolte, 1990) have shown that changes in seismic velocities and amplitudes can be related to the mechanical stiffness of individual fractures and joints in the rock. Transferring this knowledge to field applications is difficult in that little is known of the stiffness of fractures in-situ. To this end a fracture pressurization experiment was conducted at the Nagra Gdmsel Test Site on a naturally occurring fracture zone. Measurements of pressure and deformation have been used to develop three models to describe the deformation behavior of the fracture.
