ABSTRACT:

Despite recent progress, there are still fundamental deficiencies in recognition, prediction, and mitigation of underground construction hazards associated with complex geological conditions in weak rock and soil works. These fundamental deficiencies that exist in regards to design, could be addressed by improving the geomechanical analytical models. This improvement would provide better insight into underground infrastructure performance. Additionally, one issue that is unclear with regards to behaviour, are the guidelines associated with the installation of forepole temporary structural support members. This paper focuses on 3 critical design parameters of forepole supports: spacing between forepoles, positioning, and size of the forepoles. Two-dimensional (2D) and Three-dimensional (3D) numerical analysis have been used in order to determine the influence of these design parameters. The results summarize commonly used design values while also providing practical ranges associated with the design parameters investigated. Furthermore, the results confirm that the industry norm 2D numerical analyses are inaccurate and can lead to under/over design

1. INTRODUCTION

The optimization of the underground pre-support structural members, with respect to cost and required support measures, needs to be addressed along with the requirement for improving the geo-mechanical and numerical models associated with the material surrounding them. Forepoles [1,2] are long tubular steel members inserted sub-horizontally ahead and above the tunnel face in the longitudinal direction (i.e. along the axis of tunnel advance) as shown in Figure 1. Similar support structures are also cited in literature as pipe roofing [3], pipe roof support [4], pipe roof umbrella, steel pipe umbrella, umbrella arch method [5], long span steel pipe fore-piling [6], steel pile canopy [7], and spiles [8]. Forepoles are installed in groups in an ‘umbrella’ arrangement above the crown of the tunnel face, the effect of which provides improved stabilization of the tunnel face.

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