A new perspective for support design of shallow tunnels in mudstone formation: basic philosophy and engineering practice

Abstract

The traditional support design of rock tunnels mainly relies on the comparison of the geological condition and tunnel characteristics with those prescribed in rock mass classification schemes. The supporting patterns and supporting parameters can then be empirically determined in accordance with the classified grades of rock masses. However, a variety of rock mass classification schemes are used worldwide, which require different input indices. Different classification schemes may lead to varied design suggestions for the same project. Thus, determining the most reliable suggestion presents a challenge. In addition, most of the traditional rock mass classification schemes neglect the intrinsic and distinct properties of rock masses and often either underestimate or overestimate the necessary support requirements. In this study, a new design philosophy is proposed by emphasizing the importance of considering the intrinsic properties of rock mass. Based on a tunneling project in Pakistan, we first determined the basic mechanical properties of mudstone, which is the major formation in the tunneling region. Then, a series of model tests was conducted to analyze the tunnel stability under different scenarios, from which a reduced supporting system that was weaker and more flexible than those determined from the classic rock mass classification schemes was derived. Practical application showed that the proposed supporting system was adequate and reliable. Apart from referring to the rock mass classification scheme, the current study highlights the importance of considering the mechanical properties of rock masses for tunnel support design.