Formation mechanism of the structural ring for tunnels in horizontal layered rock with high geostress

Tunnelling in layered rock with high geostress can cause large deformation disasters, and the reasonable countermeasures rely on a full understanding of the self-bearing capacity of the surrounding rock. In this article, the structural ring concept was introduced to represent the load-bearing capacity of the horizontal layered surrounding rock, whose formation mechanism and determination method were analyzed. Firstly, the mechanical response characteristics of the horizontal layered surrounding rock due to excavation were analyzed. Based on the stress transfer mechanism, the new concept of the structural ring which is a closed structure with a certain thickness was presented. Taking the stress element as the basic analytical model, the maximum increase ratio of the compressive stress was adopted to characterize the structural ring. Then the determination method and its implementation algorithm of the structural ring boundaries were proposed, based on which the beam-arch property of the layered rock was investigated. A series of model tests were carried out to validate the proposed structural ring concept and its determination method. Parametric studies were conducted to illustrate the effect of geological conditions and tunnel geometry on the position and shape of structural rings. Furthermore, the application of the structural ring concept in support design was discussed. It was found that the structural ring was usually oval-shaped with the major axis direction consistent with the dominant in-situ stress. Rock layers had a significant effect on the structural ring, and the beam-arch property was affected by the interlayers and bedding spacing. The support system was beneficial for the formation of the structural ring, which should be designed to balance the support capacity and the stability of the structural ring.