Doctor of Philosophy
Faculty of Engineering
Dey, Ashitava, Shear behaviour of fully grouted bolts under constant normal stiffness condition, Doctor of Philosophy thesis, Faculty of Engineering, University of Wollongong, 2001. http://ro.uow.edu.au/theses/1837
Research on rock bolting technology has been extensively pursued, in both civil and mining engineering, for several decades. The study of bolting technology embraced initially rigid re-bars and subsequently flexible cable bolts. Generally, the past studies were focused on bolt strength and their load bearing capacities in relation to physical size (length and diameter) and material properties. Very little work has been reported on the influence of bolt surface configuration on the load transfer mechanisms between the bolt and the host rock. Also, previous research on the influence of bolting on rock joint reinforcement and stabilisation has been carried out under Constant Normal Load (CNL) conditions, which does not reflect the actual deformation behaviour of reinforced joints. This matter has lately been recognised, and some recent publications have highlighted the shortcomings of the tests conducted under CNL conditions. Accordingly, this thesis is concerned with various aspects of bolt research related to the better understanding of the load transfer mechanisms between the bolt/resin/rock, and the impact of infill on the reinforced joint shear strength under Constant Normal Stiffness (CNS) condition. Two types of bolts, commonly used in Australian coal mines were used for both laboratory and field investigations, and referred to as type I and type II bolts, respecively.
The shear behaviour of bolt/resin interface was studied under CNS condition in the laboratory by testing cast resin/plaster samples in a specially constructed CNS apparatus. The samples were tested for various loading cycles at initial normal stress (σno) levels ranging from 0.1 to 7.5 MPa, representing the in-situ horizontal stress condition in the field. Laboratory test results indicated that, bolts with deeper rib profile and wider rib spacing (e.g. bolt type I) offers higher shear resistance at low confining pressures less than 6 MPa, whereas, bolts with shallower rib profile with narrower rib spacing (e.g. bolt type II) offers marginally higher shear resistance at confining pressures exceeding 6 MPa. The maximum dilation of the bolt/resin interface occurs at a shear displacement of about 60% of the bolt rib spacing.
The laboratory study on the shear behaviour of the bolt/resin interface of fully grouted bolts was supported with field investigations in two local coal mines. At West Cliff and Tower Collieries, in the Southern Coalfield of Sydney Basin, NSW, Australia, 18 instrumented bolts and 6 extensometer probes were installed at three different sites. Field investigations in those mines revealed that, the load transfer on the bolt is influenced by; a) the confining stress condition in the field, b) the strata deformation, and c) the surface profile of the bolts. The influence of front abutment pressure was observed by sharply increasing load build up on the bolts, when the approaching longwall face was 60m and 150m from the test sites, in the travelling and belt roads, respectively. The field study also showed that, under the influence of low horizontal stress (both in magnitude and the direction), type I bolt offered significantly higher shear resistance, whereas under high influence of high horizontal stress, type II bolt offered marginally greater shear resistance at the bolt/resin interface, which were in accordance with the findings of laboratory testing.
The shear behaviour of bolted and non-bolted joints containing infill material, up to 7.5 mm in thickness, was studied under various initial normal stress levels between 0.13 M P a and 3.25 MPa, at a constant strain rate of 0.5 mm/min and a constant stiffness of 8.5 kN/mm. Significant reduction in shear strength was observed when the joint contained a layer of clay infill of 1.5 mm. Bolting contributed to increasing the strength and stiffness of the joint composite, except at large normal stress levels and at high infill thickness. The dilation and overall friction angle for bolted and nonbolted joints were also compared along with stress profiles. At high infill thickness (t/a>1), the shear behaviour under both CNL and CNS conditions was found to be similar for both bolted and non-bolted joints, while at low infill thickness (t/a<=0.3),the CNL strength envelope plotted significantly above the CNS em-elope. Thus, for rough joints with little or no infill, the CNS behaviour is more realistic in practice, especially for underground mining conditions in bedded or jointed rock.
An analytical model is presented using Fourier transform and the hyperbolic stressstrain simulation, to predict the shear behaviour of both clean and infilled bolted joints. The values of shear stress, normal stress, and dilation predicted by the analytical model were tallied favourably with the laboratory results. The UDEC model presented in the thesis underestimated the shear strength when compared with laboratory tested values, whereas it overestimated both the normal stress and dilation, especially at high initial normal stress conditions. Identical shear behaviour was observed, in both CNL and CNS UDEC model predictions, at low shear displacements and at high initial normal stress conditions, whereas the shear stress predictions were different at low normal stress conditions.