ABSTRACT

ABSTRACT

The roughness of rock joint surfaces has a marked influence on the shear strength of rock masses. The joint roughness angle is generally estimated either from direct asperity angle measurements or analysis of joint surface profiles. Fractal is shown to be applicable to the characterization of irregular joint surface profiles. According to this theory, rock joint surface roughness is represented by a fractional number called a fractal dimension (D). Additionally, the self similarity concept of fractals suggests that as measuring step length (E) decreases, the total joint profile length (L) increases, as shown by the fundamental relation: Log (L) = (l-D) Log (E). The theoretical relation established between the fractal dimension is given. Fractal dimensions of the ISRM suggested roughness profiles and Silurian sandstone joint profiles were determined and the measured roughness of angles of sandstone surfaces are compared with those estimated. Finally, the limitations of the fractal method of joint roughness determination are discussed.

1 INTRODUCTION

Rock mass is a jointed medium. The engineering properties of rock masses are influenced by joints which break up the rock mass into small rock blocks. While the joints increase permeability and deformation of a rock mass, they tend to decrease its strength and bearing capacity. However, the influence of joints on rock mass behaviour is controlled by joint properties such as roughness, weathering grade, dip and dip direction, presence of infilling, openings. Of these properties, rock joint roughness has attracted the attention of several researchers over the last 20 years (Patton, 1966; Barton,1973; Fecher and Renger, 1971; Mogilevsakya, 1974; Schneider, 1976; Barton and Chouby, 1977; Wu and Ali, 1978; Bandis et al, 1983; Lamb and Johnston 1985; Turk and Dearman, 1985; Reeves, 1985, and Gerrard, 1986) because of its important influence on the shear strength of rock joints.

Studies have shown that roughness angle increases the apparent shear strength of rock joints (Patton, 1966).

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The effect of the roughness angle of a rock joint surface is dependent upon the normal stress acting across the joint surface. As the normal stress increases, the effect of the roughness angle on the shear strength decreases, but the apparent cohesive strength of the rock joint increases. Schneider (1976) proposed the following relation to estimate the effective roughness angle, under any normal stress, based on experimental findings:

(mathematical equation)(available in full paper)

io can be determined either by direct measurements of the rock joint surface asperity inclinations in the field or by analysing joint profile traces.

Barton and Chouby (1977) proposed an alternative relation for estimating shear strength of rock joints:

(mathematical equation)(available in full paper)

While rock joint compressive strength is determined by using a Schmidt hammer on the rock joint surfaces, the rock joint roughness coefficient is determined by comparing the rock surface roughness with 10 sets of standardized rock roughness profiles ranging from 0 to 20, in steps of 2. This method of roughness determination has also been supported by ISRM (Brown, 1981).

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