Discussion On The Instability Mechanism Of Steeply Inclined Strata Around Tunnels And Treatments For Collapse

The stability of the surrounding rocks of a tunnel or underground structure is one of the key problems during design and construction. Of the structure, built in the rock mass, whose strength and stability is controlled by the properties of rock mass structure, the stability of the surrounding rock has direct relevance with the occurrence and properties of the rock mass structure. This paper discusses the instability mechanism of steeply inclined strata around tunnels and treatments for collapse. A case study is presented to show the treatments for the collapse of Mohecun tunnel.After reviewing the factors, which have influence on the stability of tunnel wall rocks, this paper analysis the key factors of controlling the stability of steeply inclined strata around tunnels and the instability mechanism of the rocks. Three instability mechanisms are discussed and their related collapsing appearances are also presented, i.e., (1) the collapse of the gravity driving rock blocks surrounding by more than three sets of rock structures; (2) the instability at sidewall due to rock blocks sliding along rock structure; (3) the bending damage of the rock strata controlled by tensile strength.On the basis of the predecessors' work, this paper presents the analysis method for the the above mentioned three types of instability mechanisms and stress states for the three mechanisms. The measures of treating the three types of instabilities are discussed according to their instability mechanisms.A case study on the treatments for the collapse of Mohecun tunnel is used to show the dominating influence of the rock mass structures and their occurrences on both the stability and the appearances of the collapse area. The effect of the treatments is analyzed in terms of practical conditions.