ABSTRACT:

In this paper, a series of physical model tests of jointed rock masses with several skew joint set is performed to investigate the interaction between intact rock and joints. Through this study, it is found that:

1. The failure modes of rock masses can be simply divided into three types:

• fracture mode;

• sliding mode;

• mixed fracture-sliding mode of (a) and (b).

2. Superposition of joint sets on rock mass strength is only correct for the fracture and the sliding failure modes which the interaction of joints can be ignored.

For the mixed failure mode, the strength of rock masses with two skew joint sets might reduce to 50% of those with one single joint set, mainly due to the strong interaction of joints for different sets. The strength of the rock mass with two skew joint sets is dominated by the weakest set of joint sets.

1
INTRODUCTION

The presence of discontinuities makes the evaluation of the strength of rock masses a difficult matter. The strength of a regularly jointed rock mass is directional. The theory of single weakness plane was proposed by Jaeger(1960) to predict the strength of a rock mass cut by a joint or a joint set and slip failure along the single joint. Bray(1966) extended this strength criterion, based on the superposition principle, to suit for the rock mass with two or more joint sets. However, an assumption is made that the slip only takes place on one of sets of planes and the other set of planes has no slip. The principle of superposition has been widely used to predict the anisotropic strength of a rock mass with several joint sets by superimposing the effect of individual joint set. It means that the interaction between the different joint sets is negligible. Amadei(1988) also states that the superposition of the effect of several joint sets on rock mass strength is not mechanically correct. In this paper, the deformation characteristic of rock masses with multiple skew joint sets is studied by experimental approaches. A series of three-dimensional physical models of rock mass with specified joint sets are produced. Then the anisotropic strength of these models under uniaxial compressive loads can be studied.

2.2 Artificial extension joint In order to rapidly produce parallel sets of artificial extension joints with identical surface characteristics for jointed rock mass models, a large controllable double-blade guillotine with 60°-wedge blades similar to the Brazilian test device has been developed in this research (Huang & Yang 1991). By carefully changing the position and the direction of the blades with a l mm/min cutting speed, a physical jointed slab with multiple skew joint sets can be generated. Based on the study of the surface roughness measurement and direct shear tests on different specimens, the provided procedure for enerating artificial joints gives consistent and satisfactory results. The joint roughness coefficient (Barton 1973) of the extension joints calculated by Tse and Cruden's equation (Tse & Cruden 1979) is 14.4 and the basic friction angle of the smooth joint is 31°.

3
TEST PROGRAM OF JOINTED ROCK MASSES UNDER UNIAXIAL COMPRESSION
3.1
Preparation of physical models

The mechanical behavior of a jointed rock mass depends on several key factors (Muller & Pacher 1965), such as the angles made by the joints with the principal stress directions, principal stress ratio, degree of joint separation, number of joints, and mechanical properties of the joint and the rock material. In the process of preparing physical models, several slabs of the model materials with specified dimensions has been cast first. Then the double -blade guillotine is used to generate extension joint sets with specified orientation and 5cm joint spacing on these slabs. Thereafter a series of small jointed blocks can be obtained. By carefully assembling the blocks with rough joint surfaces, three-dimensional physical models with various joint patterns are produced.

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