Effectiveness of Matched and Mismatched Natural Rock Joints Using Experimental Direct Shear Tests (original) (raw)

EVALUATION OF SHEAR STRENGTH OF MODEL ROCK JOINTS BY EXPERIMENTAL STUDY

In this paper, variation of the shear strength of artificial rock joints under constant normal loading condition is studied. Idealised joint surfaces were prepared using a developed molding method with special mortar and shear tests were performed on these samples under CNL conditions. Different levels of normal load and shear displacement were applied on the samples to study joint behaviour before and during considerable relative shear displacement. Nine types of saw-tooth joints have been selected for simplicity of modelling to quantify the effect of CNL conditions on joint shear behaviour. It was found that the shear strength of joints is related to rate of shear displacement, joint roughness (varying joint asperity angles) and applied normal stress condition. Finally, based on the experimental results and observations made of sheared joint samples, a new peak shear strength envelope is proposed to model sawtooth type joints tested under CNL conditions.

Shear Behaviour of Rock Joints

2000

This title covers the fundamental properties of rock joints, the method of laboratory testing of rock joints, and shear strength assessment under different loading conditions. This work is intended as a reference text for students and practitioners in mining and rock engineering.

LABORATORY MODELLING OF ROCK JOINTS UNDER SHEAR AND CONSTANT NORMAL LOADING

In this paper, the shear behaviour of artificial rock joints under constant normal loading conditions is studied. Idealised joint surfaces were prepared using a developed molding method with special mortar and shear tests were performed on these samples under CNL conditions. Different levels of normal load and shear displacement were applied on the samples to study the shear behaviour of sawtooth shaped joints before and during considerable relative shear displacement. Nine types of saw-tooth joints have been selected for simplicity of modelling to quantify the effect of CNL conditions on joint shear behaviour. It was found that the shear strength of joints is related to rate of shear displacement, joint roughness and applied normal stress condition. Finally, based on the experimental investigations and observations made of sheared joint samples, four conceptual models of shear stress-shear displacements have been developed. These four models summarize the entire experimental results based on elastic, dilation and residual zone defined along the shear displacement axis. The findings of this study expand current state of knowledge of joint shear strength which may be of significance for further research and for understanding the shear behaviour of rock joints for a stability analysis of the designed structures in surface and underground rock engineering.

Assessing the Shear Behavior of Oriented Rock Joints under Constant Normal Loading Conditions

Geo-Congress 2014 Technical Papers, 2014

This paper presents results from a series of constant normal load direct shear tests on artificially created rock joints, in an attempt to quantify the effects of different factors that contribute to the shear strength along "ideal" rock joints. Particular focus is given to the effect of asperity orientation with respect to the direction of shearing. In many historical rock joint shear strength criteria, the effect of a joint's roughness on the joint shear strength is described based on analysis of only a single profile in the direction of shearing. More recent studies have observed that the distribution of sheared area on a joint surface during shear is significantly affected by the location and distribution of the three-dimensional contact area of the joint surfaces, and results can vary with changes in the asperity inclination angle, the direction of shear, and the applied normal stresses. Results from the current study indicate that the asperity orientation angle has a significant influence on the relative contribution of the contact surface area to the shear strength of a rock joint. In joints with oriented asperities, a combination of vertical displacement (dilatancy) and lateral displacement during shearing was observed. The lateral displacement resulted in a reduction in the magnitude of observed shear-induced dilation of the joint, as well as a reduction in the strength of the joints against the induced shear load. Finally, an alternate approach for describing the shear behavior of oriented rock joints was used to characterize the impact of asperity orientation with respect to the direction of shear on the shear behavior of the rock joint.

Experimental study of shear behavior of rock joints under constant normal stiffness conditions

International Journal of Rock Mechanics and Mining Sciences, 1997

The shear behavior of synthetic soft rock joints (regular saw-tooth) was investigated in the laboratory under constant normal stiffness condition (CNS). A large-scale shear apparatus was designed and constructed which can test joints under both constant normal load (CNL) and CNS conditions. It is observed that CNL condition overestimates joint dilation compared to CNS condition and thereby, underestimates the peak shear stress of joints. Plot of shear stress against normal stress shows that a bilinear shear strength envelope is suitable for soft rock joints subjected to CNL conditions, while linear or bilinear envelopes are acceptable for CNS testing depending on the asperity angles. The shear behavior of infilled joints was also investigated under CNS conditions, and it was found that a very small thickness of bentonite infill reduced the shear strength significantly. The shear strength of joints almost approached that of pure infill, when the infill thickness to asperity height ratio reached 1.60.

Shear strength of rock joints under constant normal loading conditions

2019

The variation of shear strength of rock joints under constant normal loading conditions was studied. Three dimensional printing technology was incorporated to produce moulds of rock joints. Rock joints samples with three different roughness values were cast using concrete with uniaxial compressive strength of 20 MPa. Samples were sheared using a direct shear testing machine for normal stress values ranging from 0.25 to 0.7 MPa. In addition, effects of shear rate on shear strength properties of rock joints were experimentally investigated. It was found that the shear strength of rock joints is a function of normal stress, joint roughness and shear rate values. In addition, it was shown that three dimensional printing technology is a useful tool to replicate real rock joints.

Shear strength criteria for rock, rock joints, rockfill and rock masses: Problems and some solutions

Although many intact rock types can be very strong, a critical confining pressure can eventually be reached in triaxial testing, such that the Mohr shear strength envelope becomes horizontal. This critical state has recently been better defined, and correct curvature or correct deviation from linear Mohr-Coulomb (M-C) has finally been found. Standard shear testing procedures for rock joints, using multiple testing of the same sample, in case of insufficient samples, can be shown to exaggerate apparent cohesion. Even rough joints do not have any cohesion, but instead have very high friction angles at low stress, due to strong dilation. Rock masses, implying problems of large-scale interaction with engineering structures, may have both cohesive and frictional strength components. However, it is not correct to add these, following linear M-C or nonlinear Hoek-Brown (H-B) standard routines. Cohesion is broken at small strain, while friction is mobilized at larger strain and remains to the end of the shear deformation. The criterion 'c then n tan ϕ' should replace 'c plus n tan ϕ' for improved fit to reality. Transformation of principal stresses to a shear plane seems to ignore mobilized dilation, and caused great experimental difficulties until understood. There seems to be plenty of room for continued research, so that errors of judgement of the last 50 years can be corrected.

Laboratory Study of the Shear Behaviour of Natural Rough Rock Joints Infilled by Different Soils

Periodica Polytechnica Civil Engineering, 2015

Natural rock joints infilled with soil materials may show a reduced shear strength, which influences rock mass stability. The aim of this paper is to experimentally investigate the shear behaviour of infilled rock joints, taking into account joint surface characteristics and the properties of the joint and infill materials. A new model for predicting the shear strength of infilled joints is presented, on the basis of a series of tests carried out on natural rock joints with same surface roughness, with clay, sand and sandy-clay used as infill materials. All tests were carried out in a shear box apparatus under constant normal load (CNL) conditions. The empirical model was finally validated based on the experimental data from the literature. The results showed an acceptable confidence level for the model and reported that the new model successfully describes the observed shear behaviour of natural infilled rock joints.

Characterization of the parameters that govern the peak shear strength of rock joints

In Switzerland, there is concem that sliding along joints under dams could lead to stability problems. As part of a research project funded by the Swiss Federal Office for Water and Geology, more than fifty constant-normal-load direct-shear tests have been performed on induced tensile fractures for seven rock types. Damage zones are evident on all of the sheared surfaces. There is evidence of both crushing and breaking of surface asperities. Damage is relatively sparse, and the location of the damaged zones is strongly related to geometrical features. However, the relationships between surface roughness, stress distribution, and damage are complicated and difficult to study, in part, because the boundary conditions goreming the mechanical behavior change continuously during shearing. One of the primary objectives of this work is to better understand the micromechanical behavior of joints under shear loads, including the creation of damage zones. This requires understanding the relationships between material properties, surface geometry, contact area, stress distribution, and the creation of damage during shearing. A methodology for predicting damage during shearing has been developed based on analysis of maps of the joint surfaces obtained before and after shearing using a three-dimensional optical system. The surface data is analyzed to identify the areas on the joint surfaces most likely to be in contact during shearing; i.e. areas with positive slope with respect to the shear direction. Ix)cal gradients are also taken into account in predicting those areas of the joint surfaces most likely to be damaged during shearing. The damage predicted is compared to the damage mapped on laboratory test specimens.

Modelling the shear behaviour of sedimentary rock joints under constant normal stiffness conditions

The typical shear behaviour of rock joints has been studied under a constant normal load (CNL) or zero normal stiffness condition, but recent studies have shown that this boundary condition may not replicate more practical situations, and that constant normal stiffness (CNS) is a more appropriate boundary condition to describe the stress-strain response of field joints. In addition to the effect of boundary conditions, the shear behaviour of a rough joint also depends on its surface properties and the initial stress acting on its interface. Despite this, exactly how these parameters affect the shear behaviour of joints is not fully understood because the stress-strain response of joints is governed by non-uniform asperity damage and the resulting gouge that accumulates on their interfaces. Therefore, an attempt has been made in this study to predict the complete shear behaviour of rough joints incorporating the asperity deformation under CNS conditions. In order to validate this analytical model, a series of CNS shear tests were conducted on rough tensile (natural) joints and their replicas at a range of initial normal stresses that varied from 0.4 to 1.6 MPa. Comparisons between the predicted shear behaviour and the experimental results show close agreement.