A study on the shear behaviour of infilled rock joints under cyclic loading and constant normal stiffness conditions (original) (raw)
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The Effect of Asperity Inclination and Orientation on the Shear Behavior of Rock Joints
Geotechnical Testing Journal, 2013
This study investigates the effect of asperity inclination angle and asperity orientation on the shear behavior of rock joints under 3 constant normal loading conditions. The effects of these two rock joint characteristics were investigated by creating artificial rock joints having a 4 regular pattern of triangular asperities that were oriented at different angles in the plane of shear. Large-scale direct shear tests were conducted 5 over a range of normal stresses, on 0.30 Â 0.30 m gypsum blocks containing well-mated joints with different asperity orientation and inclination 6 angle characteristics. Experimental results illustrate the importance of considering both the asperity orientation with respect to the loading direction 7 and the applied normal stress when predicting the shear behavior of rock joints. In general, higher normal stresses increased the stiffness of the 8 rock joints in shearing, while a reduction in the shear strength of the rock joints was observed when increasing the asperity orientation angle. The 9 dilation curves indicated the occurrence of both dilation and lateral displacement during shearing. Two different techniques are used to quantify 10 the condition of the joint surfaces: the first approach utilizes the concept of fractal dimension, and the second utilizes the concept of potential con-11 tact area. These approaches can be applied in a useful fashion within the framework of existing shear failure criterion for oriented rock joints.
Effects of shearing direction on shear behaviour of rock joints
2014
Effects of shearing direction on shear behaviour of rock joints were studied. Artificial triangular asperities with initial asperity angles of 9.5° (Type I) and 18.5° (Type II), inclined at 0°, 30°, and 60° from the direction perpendicular to the shearing movement were cast using high strength gypsum plaster. Samples were tested at different initial normal stress ranging from 0.56 MPa to 2.4 MPa under constant normal stiffness of 8 kN/mm. The measured data were analysed and accompanied by a mathematical model to describe the effects of shearing direction on shear strength of rock joints. The proposed model simulated reasonably the reduction in the shear strength of rock joints with increase in the angle of shearing direction.
This paper describes the role of mobilized asperity angles in shear behaviour of weak rock joints based on the results of direct shear tests using roughness profiles in 1mm intervals. Matched joint sets of plaster casts, which simulate Barton’s typical joint roughness profiles, are created for the tests. In order to simulate accurate asperities with the same intervals, special moulds are produced by a 3D printing technique. Based on the measured compressive strength of the plaster casts, the direct shear tests are performed under low normal stress conditions. The interpretation of the test data demonstrates a parameter which is the relationship between the mean values of mobilized asperity angles in damaged areas to the asperity component of Barton’s shear strength criterion. In low normal stress conditions, Baton’s criterion, combined with the proposed parameter, shows high correlation with the test results. This indicates that the shear behaviour of joints is governed by the partly mobilized asperities in low normal stress conditions. As a result, the parameter has a linear relationship with the joint roughness coefficients according to the normal stresses and the compressive strength of the plaster materials. As the parameter is obtained from the asperity angles estimated by 1mm intervals, this can correlate with measured roughness profiles obtained by manual or remote sensing methods.
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.
Shear strength of rock joints influenced by compacted infill
International Journal of Rock Mechanics and Mining Sciences, 2014
Discontinuities such as fault planes, joints and bedding planes in a rock mass may be filled with different types of fine-grained material that are either transported or accumulated as gouge due to weathering or joint shearing. Previous laboratory studies have mainly examined the role of saturated infill that exhibits the minimum shear strength. However, in practice, the infill materials are often partially saturated generating matric suction within the joint that can contribute to increased shear strength. To the authors' knowledge this is the first study to examine the influence of compacted (unsaturated) infill on the joint shear strength. A series of laboratory triaxial tests on idealised model joints and imprinted natural joint profiles was carried out, with constant water contents of the infill being maintained. From the laboratory results, it is observed that the peak shear strength of infilled joints increased with the decrease of degree of saturation from 85% to 35% for both idealised joints and replicated natural joints. Based on the laboratory observations an empirical model for describing the infilled joint shear strength was developed.
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.
Shear Tra Sfer Alo G I Terfaces: Co Stitutive Laws
2014
This paper presents constitutive laws adequate for predicting the maximum shear load that can be transferred along reinforced concrete interfaces subjected to monotonically or cyclically imposed displacements. A previous empirical formula is modified with the purpose to reach reliable prediction of the maximum resistance of various types of reinforced concrete interfaces. The application of the modified formula to 580 experimental results from the literature proves the adequacy of the proposed laws.
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.
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.