The Mechanism Of Occurrence And Of TBM Trapped Under High Geostress And Medium Hard Rockmass Environment

In recent years,with the continuous improvement of infrastructure in China,more and more tunnel projects are characterized by large embedment depth and large-span.In the process of tunnel construction,it is easy to encounter the environment of high ground stress.Tunneling in such environment is easy to cause large deformation for surrounding rock.So far,many studies have been made on the large deformation of the weak surrounding rock under the high stress environment,but the medium hard rock in the high stress environment will still have large deformation.The surrounding rock of the Tibet DXL highway tunnel belongs to the medium hard rock,which is constructed by Herrenknecht double shielded TBM.Serious surrounding rock deformation occurred during the crossing of DXL mountain,due to the influence of high ground stress,during the excavation of TBM,which resulted in the interruption of tunneling and TBM trapped at the stake number of K10+209.Firstly,this paper introduced the engineering geological conditions of the tunnel,and achieved an intuitive understanding for the geological environment of the project.Achieving the initial geostress of the surrounding rock through the estimation of in-situ stress theory,the hydraulic fracturing method,and the laboratory rock acoustic emission test.In view of the stress and strain characteristics of rock mass in tunnel excavation,the three axis compression and three axis unloading rock mechanics experiments are carried out,which obtained the physical and mechanical parameters of the surrounding rock,and providing data for the later research.Based on the previous research results,the theoretical analysis of deformation and stress of surrounding rock and the numerical simulation of excavation by FLAC3 D are carried out.The results of theoretical analysis and numerical simulation are combined with the spatial relationship between the TBM shield and the surrounding rock to get the friction between the surrounding rock and the shield.Then,taking the Max.total thrust force and the advance rate of the TBM into consideration to find the mechanism of TBM jamming in the DXL tunnel;Finally,according to the causes of TBM jamming,the author have put forward the measures for preventing TBM jamming and solving it.The main achievements of this article are as follows:1.On the basis of the research results of the geostress in the tunnel site,the strength stress ratio of rock is less than 2.5,the ratio of the maximum horizontal stress to the self weight stress is about 2 and the Maximum principal stress is greater than 20 MPa,thus judging DXL tunnel is in a high stress environment.2.The surrounding rock is mainly composed of banded granite gneiss.The uniaxial compressive strength of the specimen obtained from three axis tests of rock is 83 MPa,after reduction,the uniaxial compressive strength of rock mass is 30.2MPa.At the same time,according to the measured data,the wet compression strength of rock mass is between 40 and 80 MPa,and it is concluded that the surrounding rock of the tunnel belongs to medium hard rock.3.According to the stress strain relationship curve of surrounding rock excavation unloading,the deformation and stress' distribution of surrounding rock are studied by elastic theory,elastoplastic theory and strain softening theory,and the results of strain softening theory happened to coincide with the actual situation.At the same time,FLAC3 D numerical simulation results verify and supplement strain softening theory.4.Based on the theoretical analysis and numerical simulation results,combining the spatial relationship between TBM shield and surrounding rock,the friction between TBM shield and surrounding rock could be obtained.The friction force is greater than the Max.total thrust force provided by TBM,so the TBM shield will be stuck once the surrounding rock is squeezed it.5.Combined with the actual situation of TBM jamming in the K10+209 section of DXL tunnel and the statistical data of the advancing rate of TBM,we can get the deformation rate of the surrounding rock and the minimum advancing rate of TBM in this section should be greater than 0.4m/h,so that it could be passing through before the shield is squeezed by the surrounding rock to avoid the TBM jamming.6.The corresponding prevention and solution measures for the causes of TBM jamming are provided,and providing engineering experience for similar projects.