Study On Instability Mechanism And Pre-Reinforcement Measures Of Tunnel Face In Sandified Dolomite Strata

Dolomite is a very common rock mass,which is widely distributed in my country.Due to the sandification of dolomite during the tunnel excavation process,water inrush and sand inrush,instability of the face,and falling blocks of the vault occur.It is necessary to conduct an in-depth study on the instability mechanism of the face of the sandy dolomite formation and how to deal with it.In this paper,relying on the Jixin tunnel of the Chengdu-Kunming double-line expansion project,using the methods of laboratory test,field investigation,theoretical analysis,discrete-continuous coupled numerical analysis,the sandification dolomite is analyzed from the sandification mechanism to the failure mechanism of the face,and then to the A series of studies have been carried out on the pre-reinforcement measures of the face,trying to reveal the causes and consequences of the instability of the face,and what kind of pre-reinforcement methods to adopt.The current research results are as follows:(1)According to the results of laboratory tests and the chemical properties of dolomite,the general process of dolomite sandification was deduced.After analyzing the results of onsite geophysical exploration of Jixin Tunnel and the correspondence with the degree of sandification,it was concluded that dolomite was Sandification is a process in which dolomite is continuously dissolved under the erosion of water,and then disintegrated into gravel.The geophysical results also show that the relationship between water content and the degree of sandification is the most closely,and the transient electromagnetic method has a significant effect on the degree of sandification.detection is the most accurate.(2)According to the upper bound theorem of the limit analysis method,the mechanical model of the face instability is established,and the calculation formula for calculating the stability of the tunnel face is deduced,and the formula is applied to dolomite with different degrees of sandification.In the case of weak sandification and slight sandification,the tunnel face does not become unstable,while under the condition of moderate sandification and strong sandification,the tunnel face is unstable,which is consistent with the actual situation on site.It is also confirmed by the discrete-continuous coupled numerical simulation results,which shows that the method is applicable.At the same time,the discrete-continuous coupled numerical simulation also found three stages of face instability,and the study shows that the core surrounding rock of the tunnel face is Bonding is the key to the stability of the tunnel face.(3)According to the actual situation of Jixin Tunnel,a discrete-continuous coupled numerical model was established to study the applicability of different pre-reinforcement schemes under different sandification degrees,and finally found a pre-reinforcement scheme suitable for each sandification degree.And a set of evaluation system for pre-reinforcement scheme was established.The research shows that the combination of pipe shed and advanced small conduit may be the most suitable and reinforcement scheme for sandy dolomite strata,while face grouting is not suitable for dry surrounding rock,but The applicability in waterbearing surrounding rock may be better.