Model test study of failure modes of surrounding rock for circular caverns

Abstract

For studying the deformation and failure modes of tunnel surrounding rock systematically under different continuum media, model tests have been performed on artificial cohesive material and artificial sandy materials. Barite powder, quartz sand and vaseline are the key ingredients of the cohesive similar material. Two types of sandy material made of quartz, and quartz+barite powder are tested respectively. Pressure cell, displacement gauge and non-contact precision measurement are used to monitor the surrounding rock response after excavation, the stress and deformation of the surrounding rock induced by tunnelling and further loading. For cohesive material in the process of step loading, failure first occurs on both sides of the tunnel and then at tunnel crown. According to the displacement and stress monitoring results, in response to the increase in load, the size of plastic zone increases as well. It also reveals that once the plastic zone develops, the rate of deformation in the tunnel surrounding rock will increase. Tunnel excavation in the sandy quartz material does not possess self-stability. However, for sandy material made of quartz and barite, the tunnel remains stable after excavation. For this mixture material, with the increase in applied pressure, failure arch first appears at the tunnel crown which follows the shape of a parabola curve. The arch foot then moves towards the sides of the tunnel. Stability is achieved when it reaches the central axis of the tunnel; at this point, the collapse arch sliding surface is similar to the Platts theory. This series of model tests, which allow the observation of the deformation failure modes for tunnel under different continuum media, provide significant information for the determination of tunnel supporting parameters.