Shear Strength Of Rock Discontinuities Research Articles

A Shear Device with Controlled Boundary Conditions for Very Large Nonplanar Rock Discontinuities

Abstract The shear strength of rock discontinuities is an important design parameter that is noted to be scale dependent. Historically, discontinuity shear strength and scale dependency studies have used small to medium size laboratory shear devices that accommodate specimens with a relatively small area (0.01 to 1 m2) compared to real discontinuities. In the literature, there is a limited amount of shear strength studies available on discontinuity surfaces with areas of several square meters. This is inherently linked to the challenging nature of performing large shear tests. As shear strength scale dependency is still a timely topic, there is still a need for conducting large scale direct shear tests to support research. To facilitate investigations into the shear response of large discontinuities, a very large shear device was designed and built at the University of Newcastle, Australia. The device is designed to accommodate 2 m by 2 m specimens and restrict rotation and translations of the top surface during shearing. This article presents the design and characteristics of the shear device, the process undertaken to create 2 m by 2 m mortar discontinuity replicas and the experimental results of the first series of direct shear tests conducted. The experimental results confirm the systems design for restricting undesired translations and rotations of the tested specimen surfaces and test repeatability.

Modelling the Constant Normal Stiffness Shear Strength of Rock Discontinuities Filled with Unsaturated Materials

Modelling the Constant Normal Stiffness Shear Strength of Rock Discontinuities Filled with Unsaturated Materials

Theoretical model for the shear strength of rock discontinuities with non-associated flow laws

In an earlier publication (Serrano et al. 2014), the theoretical basis for evaluating the shear strength in rock joints was presented and used to derive an equation that governs the relationship between tangential and normal stresses on the joint during slippage between the joint faces. In this paper, the theoretical equation is applied to two non-linear failure criteria by using non-associated flow laws, including the modified Hoek and Brown and modified Mohr-Coulomb equations. The theoretical model considers the geometric dilatancy, the instantaneous friction angle, and a parameter that considers joint surface roughness as dependent variables. This model uses a similar equation structure to the empirical law that was proposed by Barton in 1973. However, a good correlation with the empirical values and, therefore, Barton's equation is necessary to incorporate a non-associated flow law that governs breakage processes in rock masses and becomes more significant in highly fractured media, which can be induced in a rock joint. A linear law of dilatancy is used to assess the importance of the non-associated flow to obtain very close values for different roughness states, so the best results are obtained for null material dilatancy, which considers significant changes that correspond to soft rock masses or altered zones of weakness.

Experimental Study of Factors Controlling Tilt-Test Results Performed on Saw-Cut Rock Joints

Abstract One of the main inputs when estimating the shear strength of rock discontinuities, according to most existing strength criteria, is what is known as the basic friction angle, which is a parameter that responds to the frictional resistance of a planar, unfilled, and nonweathered rock joint. The basic friction angle can be estimated according to various procedures, although the tilt test is the simplest and possibly the most popular approach. With a view to standardizing tilt-test procedures, the present study provides experimental results regarding the influence of factors such as tilting rate, machine vibrations, and rock surface wear and roughness on sliding angle values. The tilting rate has been statistically and experimentally proved to scantily affect tilt test results when using motorized tilting tables and, by observing certain cutting guidelines, it is possible to expect reasonably representative and reproducible results. Nevertheless, the effect of wear associated with repeated testing and of microroughness significantly affects the resulting basic friction angle values. General recommendations are provided regarding better control of the studied factors as well as test results.

Experimental Studies on the Basic Friction Angle of Planar Rock Surfaces by Tilt Test

Abstract The basic friction angle is a basic parameter for estimating the shear strength of rock discontinuities. The value of the basic friction angle can be determined using the tilt test. There are still no standard methods for such a test. Thus, the values from different researchers usually differ significantly for the same rock type. In this work, a number of tilt tests were conducted on core-based samples under different conditions. In this study, 33 different rock types and 53 groups of samples were collected, and more than 6,600 valid tilt tests were conducted. Based on the test results, some issues that may affect the value of the basic friction angle are discussed. This work may contribute to the present studies on the basic friction angle by tilt test.

A step towards the end of the scale effect conundrum when predicting the shear strength of large in situ discontinuities

The shear strength of rock discontinuities is known to be scale dependent, and past research has revealed that both positive and negative scale effects could be observed. It is far from trivial to predict the occurrence of the scale effect and, to date, there is still no consensus on how to satisfactorily predict the shear strength of large discontinuities. A new stochastic approach was proposed and validated at laboratory scale by the authors. The approach consists of (1) using the information available from visible traces to create synthetic surfaces via a random field model and (2) estimating the shear strength of each one of the synthetic surfaces in order to obtain a distribution of shear strength and a mean shear strength. This paper presents the first application of this new approach to a large discontinuity that was surveyed with a resolution of 1 mm and an accuracy in the order of 150–230 µm in the Pilkington reserve of Newcastle, Australia. The paper first confirms that a scale effect does exist for the surface tested, before demonstrating that the new stochastic approach produces a strength envelope that is very close to the deterministic failure criterion of the whole surface. The key conclusion of this research is that there is enough information on visible traces, if surveyed accurately, to obtain an estimate of the shear strength of the discontinuity.

A Method for Estimating the Surface Roughness of Rock Discontinuities

The primary objective of this study is to develop a parameter with a clear physical meaning to estimate the surface roughness of rock discontinuities. This parameter must be closely related to the shear strength of rock discontinuities. The first part of this study focuses on defining and computing this parameter. The estimation formula for the shear strength of a triangle within a discontinuity surface is derived based on Patton’s model. The parameter, namely, the index of roughness (IR), is then proposed to quantitatively estimate discontinuity roughness. Based on laser scanning techniques, digital models of discontinuities and discontinuity profiles are constructed, and then their corresponding IR values are computed. In the second part of this study, the computational processes and estimated effects of the two-dimensional (2D) and three-dimensional (3D) IR values of the discontinuities are illustrated through several applications. Results show that the 2D and 3D IR values of these discontinuities indicate anisotropy and sampling interval effects. In addition, a strong linear correlation is detected between IR and the joint roughness coefficient (JRC) for seventy-four profiles and eleven discontinuity specimens, respectively. Finally, the proposed method, back analysis method, root mean square (Z2) method, and Grasselli’s method are compared to study the use of the parameter IR.

Shear strength of discontinuities in sedimentary rock masses based on direct shear tests

Accurate estimation of the shear strength of rock discontinuities is of great importance in many rock engineering practices, especially for the stability assessment of dam foundations and rock slopes. This study was conducted through both in-situ and laboratory direct shear tests on rock discontinuities, under the constant normal load (CNL) boundary condition, focusing on the influence of the length of discontinuities on the shear strength. The results of more than 109 direct shear tests carried out on rock discontinuities at the Bakhtiary dam site were analyzed to consider the size effect on rock discontinuities strength. The surface roughness of discontinuities was described by fractal dimension using a modified divider method. Moreover, numerical simulations of direct shear tests were carried out on discontinuities with different lengths in order to establish an empirical relationship that can link the shear strength of discontinuities of laboratory scale to in-situ scale. From the experimental and numerical results, it was confirmed that the peak shear strength of the discontinuity decreases as a function of the discontinuity length.

Assessing the Shear Strength of Rock Discontinuities at Laboratory and Field Scales

This paper sets out an approach to assessing shear strength of rock joints at project scale based on measurement and analysis rather than empiricism. The role of direct shear testing in this process is discussed in detail and the need for dilation measurement and correction emphasised. Dilation-corrected basic friction angles are presented for various rock types. The characterisation of first and second order roughness features and their contribution to shear strength at project scale are discussed with reference to possible scale effects. The paper is illustrated by a case example of a spillway slope for a dam in the Himalayas.

Micromechanical basis for shear strength of rock discontinuities

The theoretical basis for evaluating shear strength in rock joints is presented and used to derive an equation that governs the relationship between tangential and normal stress on the joint during situations of slippage between the joint faces. The dependent variables include geometric dilatancy, the instantaneous friction angle, and a parameter that considers joint surface roughness. The effect roughness is studied, and the aforementioned formula is used to analyse joints under different conditions. A mathematical expression is deduced that explains Barton׳s value for the joint roughness coefficient (JRC) according to the roughness geometry. In particular, when the Hoek and Brown failure criterion is used for a rock in the contact with the surface roughness plane, it is possible to determine the shear strength of the joint as a function of the relationship between the uniaxial compressive strength of the wall with the normal stress acting on the wall. Finally, theoretical results obtained for the geometry of a three-dimensional joint are compared with those of the Barton׳s formulation.

The effects of discontinuity surface roughness on the shear strength of weathered granite joints

Surface roughness is one of the most important parameters governing the shear strength of rock discontinuities. Roughness types may vary based on genesis, physico-mechanical, and mineralogical properties of rocks. In this study, granite samples representing three different weathering degrees were selected to evaluate the effects of surface roughness and weathering degree on shear strength. To this aim, we determined the profile roughness coefficient (PRC), profile roughness angle (PRA), and joint roughness coefficient (JRC) for the selected fresh and weathered granite joint samples. Values of PRC were in the range of about 1.043–1.073, and PRA and JRC varied in the ranges of 16.67–21.45 and 12–18, respectively. Weathering led to the increment of joint surface roughness of the selected granitic joints due to the higher resistance of quartz crystals in the weathered matrix. However, the increment in surface roughness did not result in an increase in the shear strength. On the contrary, the shear strength of discontinuities dramatically decreased.

Influence of humidity conditions on shear strength of clay rock discontinuities

The shear strength of rock discontinuities strongly depends on the water content especially when the rocks contain clay materials. To assess the decrease in the mechanical properties of clay-infilled discontinuities due to water saturation, a series of direct shear tests was performed using an advanced shear box that allows the injection of water into the discontinuity. Results show that both the friction coefficient and the cohesion decrease when the discontinuity is saturated. Overall, the shear strength of the discontinuity is considerably reduced to approximately 50% of its original value. This reduction has to be accounted for when conducting stability analyses of rock slopes, dam foundations or underground openings.

Multiscale Modelling of Stick-Slip Transition of Rough (Fractal) Surfaces

The apparent shear strength of rock discontinuities is lower than that of small scale samples. At the same time, the sliding behavior is characterized, in situ, by marked instabilities. Numerical algorithms permit to calculate contact forces at any point, and to describe the stick-slip transition. On the other hand, the critical aspects are not captured by classical theories. Multiscale simulations show that the contact domain between rough surfaces is a lacunar set. This explains the size-dependence of the apparent friction coefficient. By applying an increasing tangential force, the regime of partial-slip comes into play. However, the continuous and smooth transition to fullsliding predicted by the Cattaneo-Mindlin theory is not occurring in real situations. We implement a numerical renormalization group technique, taking into account the redistribution of stress consequent to partial-slip. This permits the critical value of the tangential force to be found. The critical force is less than the one predicted by Coulomb’s theory, and depends on the specimen size and on the topology of the interface.

The Effect of Scale and Criticality in Rock Slope Stability

Summary The apparent shear strength of rock discontinuities is considerably smaller than that of small scale samples. At the same time, the sliding behavior is characterized, in situ, by marked instabilities, with the typical features of Critical Phenomena. Contact mechanics permits to calculate normal and tangential forces at any point, and to follow the stick-slip transition for arbitrary loading histories. On the other hand, the above aspects are not captured by the classical theories, including those based on roughness indices. We argue that the multiscale topology of contact domains plays a fundamental role in determining the behavior of rock joints. In particular, experiments and numerical simulations show that these domains are lacunar sets with fractal dimension smaller than 2.0. This provides peculiar scaling of normal and tangential pressures at the interface, and the consequent size-dependence of the apparent friction coefficient. Moreover, we implement Renormalization Group to determine the critical point (e.g. the critical shear force) when rock sliding occurs. We show that the critical force is less than the one predicted by the classical Coulomb’s theory, and that it depends on the specimen size and on the topology of the interface. The same reasoning can be extended to other phenomena, e.g., to the rupture of brittle materials.

A probabilistic model for the assessment of uncertainties in the shear strength of rock discontinuities

Discontinuity shear strength plays a critical role in many problems encountered in rock engineering, especially in the design of rock slopes. Since its precise estimation is generally not possible, it is crucial that the errors and uncertainties associated with its estimate be quantified and reflected in the design procedure. In this study, the uncertainties underlying discontinuity shear strength are thoroughly examined and an uncertainty analysis model is developed for the estimation of in situ discontinuity shear strength with a special emphasis on rock slopes. An extensive literature survey on shear behavior of unfilled rock discontinuities has been carried out and the necessary data for the quantification of uncertainties are extracted from this survey. These uncertainties stem from the discrepancies between laboratory-measured and in situ discontinuity shear strength values, as well as from the inherent variability of shear strength within a rock medium. The main causes of discrepancies, namely, scale, anisotropy and water saturation are considered. For each source of discrepancy a correction factor, treated as a random variable, is assigned and guidelines for the quantification of the statistical parameters of these correction factors are presented within the framework of the proposed uncertainty analysis model. The proposed uncertainty model provides an analytical tool for the systematic treatment of uncertainties involved in the estimation of the in situ value of peak friction angle from the laboratory test results.

The effect of frictional fill thickness on the shear strength of rock discontinuities

The results of a series of direct shear tests on models of a sandstone discontinuity of mean roughness amplitude 7 mm and filled with different thicknesses of dry pulverised fuel ash (PFA) are reported. Peak shear strength is shown to decrease with increasing fill thickness ( f), expressed as a ratio to discontinuity mean roughness amplitude ( a), and approaches a minimum when f a is between 1.25 and 1.5, depending upon the normal stress. Residual strength decreases less markedly with increasing f a ratio and tends towards a constant value when f a > 1.0 . For layers of fill thicker than the discontinuity mean roughness amplitude, failure planes may develop partly through the fill and partly along the interface. When f a > 0.8 , the peak shear strength of the discontinuity is less than that of the fill but higher than that of the rock-fill interface. Expressions for prediction of peak and residual friction coefficients for a filled discontinuity are derived from the experimental results: μ = μ min + (μ max − μ min) n and n = [1 − 1/ c( f a )] m where μ = frictional coefficient (%) μ max is that for the clean discontinuity, μ min is the minimum for the system, f a is the ratio of fill thickness to asperity height and c and m are experimentally derived constants.