ABSTRACT
A methodology for characterizing the hydraulic properties of fractured rock masses is presented. The methodology utilizes geological and geophysical investigations and conventional single-hole packer tests to provide background information. A recently-developed "cross-hole" test is used to examine anisotropy in hydraulic conductivity, which may be caused by presence of fractures. Because the cross-hole test can investigate a relatively large volume of rock mass, it can serve as a valuable focal point for integrating information obtained from geological, geophysical, and small-scale hydraulic investigations. Experience gained from application of the methodology at two crystalline rock sites is discussed.
1 INTRODUCTION
The key hydraulic properties of a rock mass are the hydraulic conductivity and the specific storage. In geotechnical engineering, the single-hole packer test has served as the traditional field method for determining these properties. Although single-hole tests are simple to conduct, they can investigate the rock mass only in the immediate vicinity of the test interval, and computed values of specific storage are generally unreliable. Furthermore, fractures often render the hydraulic conductivity of the rock mass anisotropic, but single-hole tests are not very suitable for investigating anisotropy. Consequently, there is a need to develop additional field methodologies that (i) can determine hydraulic properties on larger scales and (ii) are suitable for investigating the relationship between fracturing and anisotropy. The methodology presented in this paper employs geological and geophysical investigations and conventional single-hole packer tests to provide background information. The key element, however, is a recently developed "cross-hole" test. This test involves injecting fluid into a packed-off interval in one borehole and monitoring hydraulic-head response in packed-off intervals in neighboring boreholes. If the rock mass can be treated as a uniform, anisotropic porous medium, then the test yields the complete, three-dimensional, hydraulic conductivity tensor and the specific storage of the rock mass on a scale that is much larger than that of a single-hole test. The variation in directional hydraulic conductivity as determined from the test also serves as a basis for examining the effects of fracturing on the hydraulic properties of the rock.
2 BACKGROUND INVESTIGATIONS
Fracture mapping, geophysical logging, and single-hole packer tests are the principal means of gaining background information. Fractures observed along outcrops can be characterized by methods such as scanline surveys (La Pointe and Hudson, 1985). Mapping fractures intersected by boreholes will require the availability of oriented cores or borehole- image logs such as acoustic televiewer logs. Conventional geophysical logs can also be of help in locating fractures in boreholes (see, e.g., Davison et al., 1982). The presence of fluid and/or altered minerals associated with fractures can often be detected by electric logs (e.g., resistivity) and nuclear logs (e.g., neutron). Single-hole packer tests, which include constant-pressure injection test (Zeigler, 1976) and pressure-slug test (Bredehoeft and Papadopulos, 1980) yield hydraulic-conductivity profiles of the boreholes. These profiles are extremely helpful in selecting packer intervals for cross-hole tests.
3 CROSS-HOLE TEST
Cross-hole tests utilize packers to isolate boreholes into separate intervals. During the test, fluid is injected into the "injection interval" while the hydraulic-head response is recorded at "monitoring intervals" (Figur
