Year

2016

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining and Environmental Engineering

Abstract

Bolting is currently the most commonly accepted rock reinforcing method of dealing with rock strata instability in both surface and underground mines as well as civil engineering. Bolts are installed in fractured rock mass to build composite beams in weak strata or to attach weak excavation surfaces to deeper and competent rock layers to achieve stability.

After opening, rock mass around the excavation space experiences stress redistribution and rock fracturing occurs in this process. Bolts installed in the fractured rock mass around excavations normally bear combined tensile and shear loads due to the opening and sliding of rock fractures. Direct shear restraint and high normal stress are applied by the bolt to the fracture surfaces to minimise their displacements. This mechanism allows the excavation surfaces to stabilise and makes them self-supporting. In the interactive process of rock mass and bolt, two load transfer mechanisms are involved: the axial tensioning (axial shearing at the bolt-grout interface, in essence) and the lateral shearing. Axial tensioning was the common focus in past studies, whereas lateral shearing is attracting more attention at present. The strength of the bolt when subjected to both shear and tensile loads is smaller than bolt strength when subjected to tensile load only. Thus, ignoring the shear load in bolting design probably leads to a higher safety factor. Hence, a deep understanding of bolt behaviour subjected to combined loads is essential in bolting system design and assessment.

In the past, a number of experimental and theoretical investigations were conducted on the shear behaviour of the rock discontinuity reinforced by rock bolts and a few preliminary conclusions were drawn. Yet, few studies were performed on the shear behaviour of cable bolted rock discontinuities. Thus, this research investigated the shear performance of cable bolted rock discontinuities both experimentally and theoretically to provide more knowledge in this area.

A series of experimental studies were undertaken to investigate the shear behaviour of cable bolted concrete joints (representing rock discontinuity) and relevant influencing factors. Double shear tests with joint friction were carried out and test results were analysed in respect of joint shear strength, joint shear displacement, joint shear stiffness, cable pre-tension effect, cable wire surface profile effect, cable failure mode and the shear force-axial force relationship. Double shear tests without joint friction were also carried out using a modified double shear test apparatus. By comparing the double shear tests with and without joint friction, the joint friction and the cable dowel effects on the shear behaviour of cabled concrete joints were studied. British standard single shear tests were also carried out on four cable bolts and were compared with double shear tests. The comparison showed that the joint shear strength from the British standard single shear tests was much smaller than the double shear test results and the reason was analysed.

Bolts used in underground rock engineering include Fibre Glass (FG) bolts, steel rebar bolts and cable bolts. Since these three types of bolts have different mechanical properties, such as tensile strength, shear strength and axial tensile modulus, their reinforcing effect on rock discontinuities are expected to be different. Thus in this study, cable bolts were compared with FG bolts and steel rebar bolts in terms of reinforcing concrete joints with consideration of their basic mechanical properties. Comparison showed that the influence of their basic mechanical properties on their reinforcing effect on concrete joints was evident. Since the mechanical properties of FG bolts was completely different from the other two types, the FG bolt reinforcing effect on the concrete joint was entirely different. The reinforcing effect of cable bolts and steel rebar bolts was more similar due to their similar mechanical properties.

Apart from the experimental investigations, an analytical study was also carried out based on the statically indeterminate beam theory and some conclusions drawn from existing test results, with consideration of most pertinent influencing factors. An analytical model was proposed and compared with the double shear test results, which showed close agreement. In the light of the proposed theoretical method, parametric investigations were performed on four influencing factors, including cable pre-tension, joint friction coefficient, concrete strength and cable installation angle. From the parametric investigations, it was known that the joint friction coefficient and the concrete strength influenced the cable bolted joint shear strength in a consistent manner. However, the cable pre-tension and the cable installation angle affected the cabled joint shear strength in an inconsistent manner. In addition, the cable pre-tension showed opposite influences on cabled joint shear strength when changing the cable failure modulus, which was consistent with the double shear tests on the plain wire cable and the indented wire cable.

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