Static testing of large scale ground support panels

Abstract

The Western Australian School of Mines (WASM) developed a large area static test facility to enable the evaluation of three forms of surface support; namely, mesh, shotcrete and membranes. The purpose of this thesis is to document the test conditions and procedures under which these tests were conducted and to document the outcomes from testing.A review of previous mesh testing established that the most common method used for the evaluation of mesh types was two-dimensional linear elastic analyses, often using catenary principles. These analysis techniques have been used to estimate the tension in the wires of mesh and the strength characteristics of the mesh. These methods assume that forces are only transferred along directly loaded wires and that failure of the mesh is only related to the tensile strength of the wire.The force – displacement response from fully restrained mesh tests conducted at the WASM test facility have been characterised into distinct phases, clearly demonstrating the non-linear behaviour of mesh. The initial force response to displacement is slow; however the force response increases dramatically with further displacement. This behaviour has been shown to follow a cubic relationship.The force – displacement results have also been used to develop load transfer concepts for both weld mesh and chain link mesh. These concepts suggest that forces are transferred away from the directly loaded wires through adjoining wires, distributing forces over a greater area of the mesh. The force capacity of the weld mesh is not only dependent on the tensile strength of the wire but also the quality of the welding process. Likewise, the force capacity of chain link mesh is not only dependent on the tensile strength of the wire but also diamond configuration which allows load to be shared across a greater area of mesh.Several other test conditions were evaluated as part of the test program, including mesh sheet overlaps and wire orientation. The results have demonstrated that the boundary conditions and mesh orientation alters the force – displacement response of mesh.Most current shotcrete testing techniques focus on quality assurance and quality control. Shotcrete support mechanism and failure mechanisms are complex and not well understood. The WASM punch test method was developed to evaluate shotcrete using realistic shotcrete failure mechanisms such as shear and flexural failure and adhesion loss.The behaviour of shotcrete is characterised by an initial stiff reaction followed by rupture of the cement matrix. Rupture generally occurs at displacements of less than 5mm. The rupture force of fibre reinforced shotcrete is dependent only on the cement content of the shotcrete mix and the thickness of the layer, and not on the fibre type. The post rupture reaction of shotcrete is dependent on the reinforcing material; namely fibres or mesh. Mesh reinforced shotcrete had much greater force and displacement capacity compared with fibre reinforced shotcrete.Membranes have two theoretical support models (Norcroft, 2006); namely, the membrane support model and the beam support model. A total of 6 tests were undertaken to investigate the behaviour of a particular membrane product under the two theoretical support models. These tests were aimed at determining a suitable test method that could determine the capacity of the membrane and the behaviour of the membrane under realistic loading conditions.The results from both test programs demonstrated that the membrane has limited force and displacement capacity and cannot be compared with conventional mesh and shotcrete as suggested in the product data sheet. The failure mechanism was shear failure with minimal adhesion loss observed.The development of the WASM test facility has enabled the evaluation and comparison of various surface support elements. The results of this testing have provide a valuable insight into the performance of each of the individual products.