Unloading experiment and rock strength failure of rock thick-walled cylinders under triaxial compression

In order to study the excavation effect in underground engineering,multiaxial tests were adopted on rock thick-walled cylinders.First,high axial pressure,confining pressure and inner pressure were applied on the cylinder samples.Then,inner and confining pressures were unloaded in time order.The experimental results show that a main ringlike failure surface forms in the thick wall of the cylinders.This kind of ringlike failure phenomenon,which is different from the result of failure initiated at the inner surface for the cylinders under high pressure based on general elastic-plastic analysis,has not been reported in published literatures so far.This experimental result is similar to intermittent zonal fracturing characteristics of deep underground roadways after excavation.