Calculation Method for the Loose Circle of Tunnel-Surrounding Rock Considering the Strain-Softening Effect

Abstract

During tunnel excavation, stress redistribution is caused by disturbance of the surrounding rock; when the stress exceeds the strength of rock mass, the surrounding rock enters the postpeak deformation stage and then produces the strain-softening effect. This effect will lead to a decrease in rock mass strength, the expansion of the fracture zone, and a decrease in stability. To further study the influence of strain-softening effect on the loose circle of the surrounding rock, the strain-softening factor is introduced into the unified strength criterion. On this basis, the triaxial test with elastic–plastic theoretical analysis is combined, and the strain-softening model based on the unified strength criterion is proposed. Based on this strain-softening model, the stress state of the postpeak plastic zone of the surrounding rock after excavation is studied and analyzed, and the theoretical calculation formula of the loose circle radius considering the strain-softening effect is obtained. Through theoretical calculations and engineering examples, the influence of strain-softening effect and support reaction force on the radius of the surrounding rock loose circle is compared and analyzed. The results show that: (1) the strain-softening effect further deteriorates the mechanical properties of the surrounding rock in the plastic zone, the stability of the surrounding rock is reduced, and the radius of the loose circle is increased, but closer to the measured values; and (2) with the increase of supporting reaction force, the constraint ability of surrounding rock deformation increases and the radius of the loose circle gradually reduces. Therefore, by enhancing the residual strength of the surrounding rock, controlling the radius of the loose circle of the surrounding rock by improving the strength of the supporting structure, and performing primary support on time, the stability of the surrounding rock can be increased. During tunnel excavation, the process disrupts the surrounding rock, causing stress redistribution. When the local stress surpasses the strength of the rock, it results in damage, forming a loose circle and triggering strain softening. At the same time, both contribute to the expansion of the loose circle, further compromising the stability of the surrounding rock. Analyzing the distribution pattern of loose circles in the tunnel-surrounding rock considering the strain-softening effect, enhancing the residual strength of the surrounding rock, and improving the strength of the supporting structure while implementing the initial support in time can effectively control the expansion of the loose circle and improve the stability of the surrounding rock. This research provides an important theoretical basis and engineering experience for tunnel engineering to ensure construction safety and reduce construction costs.