ABSTRACT

The only feasible method for dealing with extremely squeezing ground is providing sufficient space for accomodating rock deformations and installing a so-called yielding support, i.e. a support which can deform considerably without damage. This paper provides a critical overview of the flexible support systems proposed and applied in the past, and presents design nomograms for estimating the amount of convergence required in order to reduce rock load to a technically manageable level.

1 INTRODUCTION

The term "squeezing" refers to the phenomenon of large long-term rock deformations triggered by tunnel excavation. Squeezing may lead to the destruction of a temporary lining or even to a complete closure of the tunnel cross section. Two basic concepts exist for dealing with squeezing conditions (Kovari 1998). According to the so-called "resistance principle", a practically rigid lining is adopted, which is dimensioned for the expected rock pressure. In the case of high rock pressures this solution is not feasible. The so-called "yielding principle" is based upon the observation that rock pressure decreases with increasing deformation. By installing a flexible lining, rock pressure is reduced to a value that is structurally manageable. An adequate overprofile and suitable detailing of the temporary lining will permit the non-damaging occurrence of rock deformations, thereby maintaining the desired clearance from the minimum line of excavation. The rock load reducing effect of flexible supports, as well as various technical solutions, have been known – at least in principle – since the first decades of the 20th century (Fig. 1). Major progress was made in 1932 with the introduction of sliding connections by Toussaint-Heintzmann. Typical design issues concern the feasibility of a rigid support in a given geotechnical situation (rock strength and deformability, depth of cover and magnitude of pore pressure), the amount of deformation required in order to reduce rock load to a technically manageable level and the structural detailing of a flexible support. The present paper addresses these questions by outlining and discussing the yielding support systems proposed and applied in the past (Section 2), and Figure 1.

  • Layer of wood between rock and U section steel sets;

  • Concrete with wood interlayers (from Heise & Herbst 1913).

by presenting design nomograms for estimating the amount of deformation required to reduce loading (Section 3).

2 TYPOLOGY OF FLEXIBLE TUNNEL SUPPORTS

There are basically two technical options for accommodating deformation without damage to the lining (Fig. 2):

  • Arranging a compressible layer between the extrados of a stiff lining and the excavation boundary;

  • Installation of a yielding lining in contact with the rock face.

In the first case, the rock may experience considerable convergence, while the clearance profile remains practically constant as the lining's stiffness limits deformations. Such a solution is therefore advantageous particularly in cases with slow and prolonged deformations during the service period of a tunnel. It is a standard solution for the final support of tunnels crossing highly swelling rock (Kovari et al 1988).

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