Microseismic Monitoring and Experimental Study on Mechanism of Delayed Rockburst in Deep-Buried Tunnels

Delayed rockburst occurs after the stress adjustment of surrounding rocks with discontinuous microseismicity, which is difficult to predict accurately. To predict and prevent the occurrence of the delayed rockbursts, study on time behavior of the surrounding rock failure and the mechanism behind the delayed rockbursts is necessary. In this article, based on the rockburst data from Jinping II Hydropower Station headrace tunnels, China, the characteristics of the delayed rockbursts are summarized. Considering the microseismic monitoring results of two typical delayed rockbursts, the formation process of delayed rockbursts is analyzed. Subsequently, the time behavior of rock failure is revealed by a uniaxial constant loading test. The results from our study suggest the surrounding rocks are in a relatively stable state, where the delayed rockburst occurs. Geo-stress, geological conditions and excavation are the key factors affecting the delayed rockbursts. Within the rockburst-affected area, the microseismic activities were frequent during the early stage of excavation, then the stress adjustment of the surrounding rocks gradually stabilized. As a result, the surrounding rocks were in a dormant stage, under the long-term high stress. Supported by laboratory experiments, our results suggest an obvious time behavior in the rock failure which corresponds to the time behavior of the crack propagation, under the high stress. Under the condition of long-term high stress, when the damage is accumulated to a certain extent, the rock mass is destabilized and then the rockburst is triggered, accordingly. The results from this study can provide a reference for the prediction and prevention of the delayed rockbursts, in similar situations, e.g. deep-buried tunnels, around the world.