Support design for tunnels with deep overburden in weak rock

Tunnelling in weak rock under high stresses often results in damage to the lining and failure of bolts. Large displacements can lead to violation of the clearance profile, requiring costly, dangerous, and time-consuming repairs. The main problem is that conventional supports have a rather limited strain tolerance. Shotcrete, for example, can tolerate a maximum of 0.6% to 0.8% strain without failure. Any imposed strains beyond this level will lead to destruction of the lining and consequently a more or less complete loss of support resistance, causing additional displacements and disintegration of the rock mass. Over the last two decades, ductile elements have been developed and successfully used on a considerable number of tunnels. The aim of these elements is optimal utilization of the capacity of the lining, while preventing damage of the lining by limiting the resistance. Although this sounds simple, the task of designing such a ductile lining system is not at all trivial. Many factors need to be considered, like development of the displacements in relation to face advance and time. The ground structure and the relative orientation between the tunnel axis and dominant geological features, like foliation or bedding, strongly influence the longitudinal displacement profile. The development of the displacements over time also depends on the advance rate, influencing the degree of utilization of the shotcrete, which changes its properties over time. Currently used ductile element systems are presented and their characteristics as well as pros and cons discussed. More or less sophisticated tools for a preliminary design of ductile linings exist. Examples of applications are presented. Another issue is the performance of grouted rock bolts under large strains. Installation of bolts at the time of the largest strain rates in the rock mass close to the face results in considerable shear strains in the grout while it is setting. This may lead to a loss of bond between bolt and ground, making the bolts more or less ineffective. Special rib designs of the bolts can reduce this effect.