Tunnel Single Shell Lining Numerical Simulation Study

In the numerical simulation analysis, the continuous medium theory is usually adopted, namely, the geotechnical material is treated as continuous. However, the nature rock mass is usually divided into several parts by the potential joints and structure surface. So, the rock mass couldn't be treated as continuous under some condition. Especially in the vicinity of the excavated surface, the rock mass appears significantly discontinuous characters, and it's difficult to analysis that with traditional method, such as continuous medium method and boundary element method. It's known that the discrete element method well considers the discontinuity character of rock mass, and can be used to analyze the joint rock mass.Single shell lining is a reflection of the mechanical concept, namely, the lining without isolation layer which can well bare the external applied load together. In the tunnel supporting technology, the single shell lining method appears later, and is a new supporting method. Although the single shell method has lots of merits, such as good integrality, reducing the excavation quantities and lining masonry quantities and economizing funds, there are still many problems and uncertain factors in the single shell lining method, such as the scope of application, mechanical properties and the key technology and so on. So its engineering application is still limited.This paper analyzes the surrounding rock characteristics of single shell lining only from the mechanical mechanism aspects using the discrete element software of UDEC. And the influence of stress release ratio, joint spacing, joint dip angle and the depth of tunnel are considered respectively. The main conclusions are drawn:(1) for the broken rock, high stress release rate is easier to induce the cavity instability, and after excavation, the lining structure should be applied in time. With the increase of stress release rate, the force of the supporting structure gradually transfers from the vault to the wall; (2) with the increase of joint interval, the mechanical properties of rock mass increase, which is benefited to the self load bearing. But there is a sensitive joint interval, when the joint spacing is larger or smaller than that, the interval has little effect on the internal force of the single shell lining structure; (3) joint dip angle has significant influence on the rock mass mechanical property. When the joint sets are level, they are likely to keep the rock mass stability relatively.(4) Forâ…£,â…¤class loose rock, when the cavity depth is more than 120 meters or less than 60 meters,lower rock stress release ratio brings well support effect. Whereas, the higher stress release ratio is benefit to keep the rock mass stability when the cavity depth is between 60 meters and 120 meters.Finally, an example is analyzed, namely the 2nd shaft between X2DK0+110 and X2DK0+130 in the Dabie Mountain tunnel of HeWu railway. The feasibility of single shell lining structure is verified from the mechanical mechanism aspects.This article is only an exploratory study, the correctness of the results remains to be evaluated. And it's hope that the results will provide a reference to the engineering.