Analysis On The Stability Of Tunnels In Soft Rocks And Measures To Collapses

Case histories of tunnel and underground structure constructions have indicated shat the stability of their surrounding rocks have a significant influence on a project cost, time schedule and safety. Thanks to the blooming of economy and technology in our country, the utilization of tunnel and underground space are increasingly necessary. We will have more chance of building tunnels or underground structures in soft rocks. And therefore, more attention should be paid to the stability of surrounding rocks.This paper analyzes the factors, which affect the stability of the surrounding soft rock of a tunnel, and three criteria of wall rock stability for tunnel construction and design. Of the three criteria, it seems that Hoek's criterion is more practical than the two others.The results of the analysis on the deformation characteristics, damage shapes and collapse height of soft rock tunnels indicate that the Hoek-Brown's brittleness parameter method is a practical and reliable approach of predicting the collapse height of a tunnel in soft rocks. However, as the ratio of the intensity of surrounding rocks and in situ stress of a tunnel is lower than 0.2, the initial ground stress has dominate influence on the stability of the surrounding rocks.Base on the statistic analysis on the causes, shapes, types and treatment measures from more than 70 case histories of soft rock tunnels, with collapse during construction, as well as the collapse and treatment measures for Shaziya tunnel, the collapses of the tunnels can be mainly attributed to the shortage of a proper evaluation of the surrounding rock during site investigation and an effective cooperation of the involved groups during tunnel construction. The collapse shapes of the soft rock tunnels mainly belong to three types, i.e., V-type, day-lighted type and arch-type. For each type of the collapses, a few accustomed treatment measures are widely accepted in practice.Both the case histories and related numerical analysis indicate that it is practical and economical for using the method of "hollow out" to treat V-type collapse, which is more than 4 m in height, and it will be beneficial to apply this measure in similar cases.