ABSTRACT:

Double Shear Tests (DST) were conducted to understand the performance of rockbolts under shear loading with different pretensions. During testing, five pairs of strain gauges were installed along a rockbolt to measure the variation in tensile strain. Two load cells were also used to measure the normal load generated at either end on the protruding rock bolt. The results showed that the normal load measured with load cell is close to the result calculated from the strain gauges data. In the vicinity of the intersection of joint and bolt, the shear load applied on the rockbolt was converted to the resistance of axial elongation and bending. It was also found that the shear performance is dependent on the tensile elongation capacity to some extent. Furthermore, the interaction of rock and rockbolt with varying pretensions created different magnitude of tension and bend.

1 INTRODUCTION

There has been much theoretical and experimental research undertaken examining the effectiveness of fully grouted rockbolt since the 1970s (Farmer 1975, Hagan 2004, Mahony & Hagan 2006). Some of this work has been directed at understanding the axial and shear behaviour between the rock and grout when tension is applied on the rockbolt (Farmer 1975, Li & Stillborg 1999). Signer et al. (1997) measured the in situ loading of rockbolts to assist the evaluation and selection of roof bolt. Both axial and bending forces were measured by strain gauges at multiple locations along the length of fully grouted bolts during different stages of mining.

However, underground investigations showed that a high proportion of reinforcing elements can fail in shear (Haile 1999). Researchers tried to use the strain gauges to measure the behaviour of axial and bending when the rockbolt was subjected to shearing load. McHugh & Signer (1999) conducted the single shear tests to measure the distribution of axial and bending strain along the rockbolt with different axial load. In their tests, strain gauges were installed in a groove machined along the length of a rockbolt. However, it was demonstrated that the slot reduced axial bolt strength by approximately 10% (McHugh & Signer 1999). Another problem with this test is the strain gauge signal is often terminated before reaching the peak load (McHugh & Signer 1999). Double shear tests were carried out by Jalalifar (2006) to study the contribution of rock bolt to the bolted joint shear strength. Numerical modelling was also adopted to investigate the axial stress distribution. However, the results were not verified by strain gauges.

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