ABSTRACT

Both hydraulic and mechanical properties of fractured rock masses are related to the geometry of fractures. From the perspective of hydro-mechanical coupling, the nonlinear decrease in transmissivity with depth can be utilized to calculate fracture normal stiffness of large scale rock masses. In the current study, the degree of weathering and the non-linear decrease in transmissivity with depth are considered simultaneously to estimate fracture normal stiffness of a granite rock mass. The equations of transmissivity-depth correlation in each zone with different degree of weathering are employed to calculate the corresponding fracture normal stiffness; then the equivalent continuum model is utilized to calculate deformation modulus of the rock masses. In the highly weathered zone, the deformation modulus ranges between 2 and 4 GPa; in the moderately weathered zone, the deformation modulus ranges from 15 to 19 GPa; in the slightly weathered zone, the deformation modulus ranges between 24 to 26 GPa. Compared with the values of deformation modulus obtained from measurements or engineering analogy method, which were determined by others, the results in the present study are reasonable.

1 INTRODUCTION

The deformation modulus is the most representative parameter describing the pre-failure mechanical behavior of a rock mass. Numerous researchers had reported that deformation modulus, or compressibility, which is defined as the reciprocal of deformation modulus, of rocks is stress dependent (Adams and Williamson, 1923; Fatt, 1958; Zimmerman, 1991).

Unfortunately, in situ measurements of the deformation modulus involve difficult test procedures, and are expensive and time-consuming. Moreover, even such in situ tests are still not able to obtain parameters that can represent large scale rock masses.

Both hydraulic and mechanical properties of rock masses are related to the geometry of fractures (Chen, 1990), which suggests a new way to estimate mechanical properties from hydraulic information. Rutqvist (1995) utilized hydraulic jacking test to determine normal stiffness of fractures in hard rocks. Jiang et al. (2008) estimated the stress-dependent fracture normal stiffness of large scale rock masses using the permeability data from packer test in a wide range of depths.

In Jiang et al. (2008), the most important and sensitive parameter for calculation of normal stiffness is the permeability-depth correlation. It was assumed that the depth dependency of permeability was caused by the nonlinear normal stress-aperture relationship of fractures (Goodman, 1976), which had been observed by Snow (1968) using field permeability measurements. However, the depth-dependent permeability is also influenced by weathering of the rock mass (Rutqvist and Stephansson, 2003). Moreover, it is well known that the mechanical properties, including deformability, differ greatly in rock masses with different intensity of weathering. Therefore, the degree of weathering should be considered while estimating the deformation modulus of large scale rock masses.

In this paper, the theory for estimation of deformation modulus using permeability data is presented. This method is then applied to a study area which is composed of weathered granite.

2 THEORY

2.1 The relationship among fracture normal stiffness, transmissivity and stress.

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