SHEAR STRENGTH BEHAVIOUR OF JOINTED MODEL MATERIAL UNDER LOW CNL CONDITION

Abstract

Discontinuities are inevitable part of rock masses. The civil and mining engineering projects are usually located in or on rock mass. Rarely will any such project be located in or on intact rock. The shear strength behaviour of rock mass is greatly influenced by the presence of discontinuities particularly the joints. Orientation ofjoints, their roughness characteristics, and, the level of applied normal stress are some of the controlling factors in the assessment of shear strength of rock joints. Presence of more than one intersecting joint set provides blocky nature to the rock mass thereby introducing additional degree of freedom in the sense that the blocks can rotate in addition to sliding and shearing and the mass becomes more dialatant especially under low normal load conditions. The shear strength behaviour under such conditions is highly non linear. Considerable research has been conducted over the past four decades to investigate the behaviour of rock joints under the application of shear load. Most of these studies are available for the rock containing single joint only. Very few studies are reported in literature on the shear strength behaviour of jointed rock mass, where, the mass is ofblocky nature. In the present study, an attempt has been made to investigate the shear strength behaviour ofjointed blocky rock mass through direct shear tests under low normal load conditions. A model material was used to simulate the intact rock material. Specimens of rock mass, having various joint orientations and steppings, were assembled using model material blocks. These prepared specimens consisted of two joint sets. The joint set-1 was kept continuous and was inclined at various inclinations, 9, with the shearing direction. The tests were performed for joint inclination, 9, varying from 0° to 180° at an interval of 15°. The joint set- n was orthogonal to joint set-1 and was stepped. For the present study, mainly two sets ofsteppings viz; 0and 0.5 b, where b is the width of the elemental block, were used. Some preliminary tests were also performed on the specimens with single joint, to verify the applicability ofthe available shear strength theories to jointed rocks. Single joint specimens having joint interface in the form ofregular right angled triangular shaped asperities, with asperity angles of 7.5, 15 and 22.5° respectively, were prepared and tested. Aspecially designed and fabricated large size direct shear test apparatus was used to perform the direct shear tests. The tests were performed under low normal stresses (c„) of 0.1, 0.2, 0.3, 0.4 and 0.5 MPa respectively (0.0 11 < an / oci < 0.057). Sliding and shearing modes of failure were observed for single joint specimens, whereas, rotation ofblocks or struts was observed for the jointed blocky mass specimens consistently for all joint orientations and steppings. For a given normal stress level and stepping, the shear strength of blocky mass specimens was found to be dependant on joint orientation, 9. The ratio of maximum to minimum shear strength due to orientation, 9, was found to be as high as 4. The shear strength behaviour of the jointed blocky mass specimens was therefore observed to be highly anisotropic. To assess the applicability of shear strength theories available in the literature, the shear strength of single joint specimens was predicted through these theories. It was observed that most of these theories could be applied satisfactorily to predict the shear strength of single joint specimens. To analyze the shear strength vs. shear displacement response of single joint specimens, a slip based constitutive model was suggested. Joint stifnesses, which were found to be normal stress dependant, were used in the model to compute the shear displacement. The sheared area of failed single joint specimens was measured by using a carpenter's profilometer. The sheared area was used in some of the shear strength models as input parameter. Applicability of the available shear strength theories were found to give unsatisfactory results when applied to the jointed blocky mass specimens. The shear strength models for rock joints in vogue generally consider sliding and shearing modes in of failure only. Ajointed rock mass, with well developed joints and block system, while failing due to shear under low normal load will observe rotation of blocks in addition to sliding and shearing. Presently the shear strength models do not consider the rotational mode. To incorporate the rotation of blocks in ajointed mass, two approaches have been suggested i.e. an empirical approach and amechanistic approach. Many empirical strength criteria have been proposed in the past to predict the strength of jointed rock masses. Some of these include Ramamurthy (1993), Hoek and Brown (1997) and parabolic criterion (Singh and Rao, 2005a). Acritical evaluation of the applicability of these criteria to the results obtained under present study is carried out. The comparison of the results predicted through various strength criteria with the experimental results indicates, that, the parabolic criterion gives the best fitting values for the present study despite the fact that the least number of input data were required by this criterion. It is therefore suggested that the parabolic criterion may be used to predict the shear-strength of blocky rock mass with reasonable accuracy. To improve the results further, modification is suggested to compute the criterion parameter. The rock mass strength, Oq, is an important parameter in the application of strength criteria. An indirect method to obtain this parameter is to link the rock mass strength to the intact rock strength through joint properties. The Joint Factor concept suggested by Ramamurthy (1993) was a significant development in this direction. The Joint Factor, Jf, is aweakness coefficient and reflects the effect ofjointing on intact rock. Higher the value of Joint Factor, weaker is the rock mass in strength. In the present study, the applicability of the concept has been extended to jointed blocky rock mass subjected to direct shear and failing due to rotation ofblocks. Correlations for assessing rock mass strength under direct shear, with Joint Factor and intact rock strength have been suggested. Amethodology is also suggested to compute the shear strength of jointed blocky mass in the field by using Joint Factor concept. IV A mechanistic approach on lines similar to Ladanyi and Archmbault (1970) is also suggested for shear strength prediction of jointed blocky rock mass, by incorporating the effect of block rotation implicitly into the available shear strength model. The suggested approach was applied to the data from the present study and it was found that the approach may be applied satisfactorily to predict the shear strength of jointed blocky rock mass failing under rotation. It is however suggested that, further work is required for making a reasonable assessment of the parameters used in this approach. A UDEC model has been developed to assess the applicability of distinct element modelling to simulate the shear strength and deformation behaviour of jointed blocky mass specimens tested in the laboratory under CNL conditions. The UDEC model uses the simple joint and block models. In general, the shear strength predicted by UDEC model is slightly higher as compared to laboratory test results; however, the model is able to capture the failure mode as observed in the laboratory tests.