Strength Behaviour of Persistent and Non-Persistent Jointed Synthetic Rock

Abstract

Several constructions in the field of civil and mining engineering quite often need to deal with rocks intersected by a discontinuity (joint) or a set of discontinuities. The engineers associated with the analysis and design of gravity dams, bridge foundations and high-rise buildings on rocky strata should have an adequate understanding of the strength behaviour of intact rocks, discontinuities, and rocks with discontinuities as a whole. A discontinuity can be thoroughgoing across the rock volume of interest or made up of intact rock bridges interspaced with joint segments. The joint attributes that mainly influence the strength behaviour of the rock are its orientation with principal stress direction, persistency level, frequency, number of the joints in a given direction, degree of joint separation, type of foreign discontinuity material and surface roughness characteristics of the joints. In the literature, the studies were conducted to analyse the strength behaviour of jointed rocks having a persistent discontinuity and the material forming the discontinuity was the same as that of adjoining rock.While in the field, the discontinuity is generally non-persistent and comprises of the foreign material. Therefore, the objective of the present study is to understand the engineering behaviour of persistent and non-persistent jointed rock having foreign material as a discontinuity. To accomplish the objective, an experimental study has been conducted on modelled prismatic rock specimens having closed non-persistent joint. Uniaxial compressive and triaxial strength tests were conducted on intact and jointed specimens. The parameters varied in the study are (i) orientation (θ) of the discontinuity, (ii) number of joint segments, (iii) persistency level of discontinuity (expressed in terms of ‘discontinuity length ratio’ (DLR)) and (iv) confining pressure range. Two different types of foreign materials having zero (chart paper, Type-1 discontinuity) and finite tensile strength (textile, Type-2 discontinuity) were employed to induce the discontinuity in the jointed specimens. Based on discontinuity configurations and type of discontinuity material, the jointed specimens are classified into three categories, designated as Types-A, B and C. Types-A and B category specimens are made up of Type-1 discontinuity with one and more than one joint segments, respectively. Types-B1 and B2 specimens have two joint segments with the configuration as ‘I-J-I-J-I’ and ‘J-I-J’, respectively, where ‘I’ stands for an intact rock bridge and ‘J’ for the jointsegment. Type-B3 (I-J-I-J-I-J-I) specimens have three joint segments. Type-C category specimens have one joint-segment; however, the discontinuity material used is of Type-2.By analysing UCS and triaxial strength tests results, it is observed that non-persistency and discontinuity orientation jointly affect the failure mode and strength of jointed specimens. For specimens having Type-1 discontinuity (Types-A and B) and for the same discontinuity orientation, specimens with higher DLR values have lower strength as compared to those with lower DLR values. DLR has a dominating influence near 𝜃 = 60°, whereas near 𝜃 = 15° and 90°, the effect of DLR is minimum. When discontinuity orientation (θ) and DLR are kept constant, and the number of joint segments is varied from ‘2’ to ‘3’, then the specimen with the greater number of joint segments exhibit greater strength as compared with the specimen having lesser number of joint segments. For specimens having Type-2 discontinuity (Type-C), it is observed that the strength decreases approximately to DLR = 60%. After this threshold DLR value, specimens have higher strength as compared to the strength of specimens at lower DLR values. The strength behaviour of the rock having discontinuity comprising of foreign material is quite different as compared to rocks comprising fresh joints without any foreign material. In the case of fresh joints, the strength envelopes of jointed and intact rocks tend to merge with each other at sufficiently high confining pressure (Brown, 1970; Singh and Singh, 2012). However, for joints filled up with foreign material, the gap between strength envelopes of intact and jointed rock widens with the increase in confining pressure. This gap depends on the strength behaviour of foreign material. For modelling triaxial strength behaviour of jointed rock specimens (Type-1 discontinuity), the applicability of strength criteria is evaluated. It is observed that Single plane of weakness theory (SPWT) (Jaeger, 1960; Jennings, 1970) can only be applied for a narrow range of discontinuity orientation (θ = 45° to 60 and DLR ≥ 60%), i.e. in the cases where pure sliding occurs along discontinuity plane and the strength behaviour is completely governed by discontinuity alone. It is further observed that the Modified Mohr-Coulomb (MMC) criterion (Singh and Singh, 2012) can be confidently used for predicting the strength of rock specimens. For application in the field, MMC criterion parameters can be assessed as discussed in the present study. A real-life problem from Indian Garhwal Himalayas is considered, and the approach suggested in the study is utilised to obtain the bearing capacity (by Bell’s approach) of a shallow bridge foundation placed near the crown of a rock slope.