ABSTRACT

ABSTRACT

Design of structures in rock based on ultimate limit states or strength criteria is unrealistic. The fracture process which inevitably occurs in rock around an underground structure is an energy release phenomenon. The development of fractures and associated displacements around a structure may therefore be more logically described using strain energy concepts and design may sensibly be based on serviceability limit states.

INTRODUCTION

In the past, various methods have been used to design tunnels in rock. Because of the complexity of rock behavior, both in small specimens and in a naturally occurring massive state, these have involved compromises. The current most common approaches are based on:

  • empirical relations between case histories in familiar types of rock, which are usually defined by some form of characterization of rock mass properties.

  • relations between stress distributions and ultimate limit states, such as "strength',through analytical or numerical models.

While acceptable in engineering practice, neither of these approaches is entirely satisfactory. For instance, case history and empiricism rely on a presumption that all rock behavior can be predicted from previous experience. In a typically heterogeneous rock mass, this is not always the case. Similarly, ultimate limit states are based on an assumption of idealized behavior, usually precluding or describing only poorly, rock fracture mechanics. The behavior of rocks around an underground structure may be described relatively simply. The initial redistribution of stress - or storage of excess strain energy - at the surface of the structure, commonly satisfies the requirements for fracture initiation along at least a portion of the tunnel boundary. This results in rock fracture or movement along existing discontinuities and subsequent dilation. The amount of dilation will depend on the energy release characteristics of the rock mass. The success or failure of a tunnel design may therefore be demonstrated through the deformation behavior of the surrounding rocks. Inherent in this concept is the idea of performance. Common design procedure is to evaluate tunnel performance based on the rock's strength characteristics. An alternative approach would be to consider performance standards for serviceability limit states based on rock deformation in addition to an ultimate limit state based on strength.

LIMIT STATE DESIGN FOR TUNNELS

A successful tunnel design is one in which the tunnel remains serviceable for the purpose for which it was built. Performance is a measure of that serviceability and it can reasonably be assumed to take on a range of values. Performance may be bounded by complete catastrophic failure and zero deformation as the two extreme cases. Failure, as it is defined here, would be purpose dependent in that some underground structures would be more sensitive to deformation than others, and, as such, would require a much stricter performance standard. Limit state design, a concept taken from reinforced concrete design, is ideally suited to the design of structures in rock. In reinforced concrete design the performance of structural concrete is evaluated in terms of the attainment of certain limit states, the most important of which are the ultimate limit state, based on strength exceedance, and serviceability limit states, based on tolerable deformations.

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