Abstract

Rock joints morphological characteristics strongly influence the shear strength dilatancy response of the discontinuity. Morphological features of rock joints are commonly identified with a roughness descriptor along standard 100 mm-length profile detected by instrumentation such as profilometers. This work extends a method proposed for fractal analysis of particle contours to describe rock joint profiles in terms of quantitative descriptors of their roughness. It is well-established that natural surfaces have a fractal nature, self similar over a wide range of scales. This implies that the measured length of their outline is a function of the measurement scale: the smaller the measurement scale, the longer the profile length. Based on the interpretation of the fractal analysis of rock joint profiles, relating the length of the profile to the measurement scale, descriptors identifying the roughness and its characteristic scale are proposed. The method is first applied to some artificial profiles, and later to real rock joint profiles.

Introduction

The morphological features of natural rock discontinuities strongly influence the hydro-mechanical behaviour of a jointed rock mass. The former is strongly affected by the discontinuity condition, in terms of opening, roughness, filling and alteration [1]. This work focuses the attention on the characterisation of joint roughness.

Roughness generally describes the morphology of a joint surface by means of two distinct components [2, 3]: a small scale component named "unevenness" related to the textural surface irregularities and a large scale component named "waviness" related to the surface curvature. The first is thought to affect mostly the shear strength of the discontinuity in accordance to the following criterion [4, 5, 6]:

(equation)

where τ and σ are the peak shear strength and the normal stress respectively, ϕr is the residual friction angle, while ieff is a dilatancy angle related to the discontinuity features, equal to ieff = JRC log (JCS/σ) for σJCS and ieff = 0 for σ > JCS, at which point asperities are "flatten out" and ϕ = ϕr. The failure surface in equation (1) is non linear, as its slope depends on σ. The larger the value of JRC (Joint Roughness Coefficient), that ranges between 0 and 20, the more pronounced is the curvature of the failure envelope for σ < JCS (Joint Compression Strength). The JRC gives a qualitative indication of the roughness of a rock joint profile. It is generally estimated by a visual comparison between the rock joint profile, obtained experimentally from e.g. profilometers, and the 10 Barton’s paradigmatic profiles related to JRC values ranging from 0 to 20 [5]. However, this procedure is affected by an element of subjectivity and provides at best an indication of the profile roughness. In recent years, several quantitative methods are proposed for the determination of roughness [7, 8]. The fractal dimension, Df, ranging between between 1 for a perfectly smooth profile and a maximum value of less than 2 for an extremely rough profile, is considered one of the most promising [9, 10] and several empirical correlations exist to estimate JRC from Df [8].

This content is only available via PDF.
You can access this article if you purchase or spend a download.