Abstract

The paper introduces design principles of ground support. The topics include underground loading conditions, the natural pressure arch in the rock mass, design methodologies, determination of the factor of safety and compatibility between support elements. A natural pressure arch is formed in the rock mass in a certain distance behind the tunnel wall. The methodology of ground support in an underground opening is dependent on the size of the failure zone and the boundary depth of the natural pressure arch. In the case of a small failure zone, rockbolts should be long enough to reach the natural pressure arch. In the case of a vast failure zone, an artificial pressure arch could be established in the failure zone with tightly spaced rockbolts and the artificial pressure arch is stabilised with long cables anchored on the natural pressure arch and/or by external support elements like shotcrete liners, girdles, steel arches and shotcrete arches. In addition to the factor of safety, the maximum allowable displacement in the tunnel and the ultimate displacement capacity of support elements should be also taken into account in the design. Finally, the support elements in a ground support system should be compatible in terms of displacement and energy absorption.

1. Introduction

Ground support design is associated with the rock mass quality, the in situ stresses and the size and geometry of the underground opening. Knowledge of the in situ loading condition is crucial for the design of ground support. The methodology and design principles of a ground support program are determined by the potential failure mode and failure extent of the rock mass as well as the engineering requirements to the maximum allowable displacement. In this paper, some key parameters for ground support design are presented which include the natural pressure arch, the artificial pressure arch established in the failure zone, support layers, the factor of safety, and the compatibility between support elements.

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