Mechanical behaviour of scaled-down unsupported tunnel walls in hard rock under high stress

•A number of empirical relationships have been determined for the onset of spalling and pillar crushing for scaled-down tunnels drilled into three different material types.•A distinctive ratio of compressive strength to induced stress can be determined for the different stages of progressive failure.•Contrary to previously published work, the stress to strength ratio for initial spalling is not a constant and appears to be a linear function of the compressive strength of the materials.•Ejection velocities determined from the scaled-down experiments are in a similar range to those back calculated from full scale underground failures.

A large number of scaled-down tunnel experiments were undertaken to investigate the response of unsupported walls to an increased stress field. The experiments were undertaken in 200 mm diameter tunnels that were drilled into intact rock blocks of sandstone and granite ranging in strength from moderately strong to very strong. The tunnels were loaded by a servo-controlled, 450 tonne capacity INSTRON compression testing machine. As the ratio of intact rock strength to induced stress decreased, the unsupported tunnel walls became increasingly unstable. Critical ratios of compressive strength to induced stress were determined for critical instability stages such as tunnel spalling and also pillar crushing adjacent to the tunnels. The physical models have been simulated using three-dimensional finite element modelling. The values of the critical ratios correlate well with underground observations of full scale tunnels with similar Uniaxial Compressive Strength materials. Dynamic ejection velocities similar to those calculated from back analysis of actual failures have been determined. In addition, the seismic responses prior and during key failure stages have been established as a function of the increased loadings.