Study On Mechanical Characteristics Of Water-rich Weak Surrounding Rock Tunnel

During the process of tunnel construction,engineering problems such as excessive deformation,invasion,cracking,and even collapse may happen when the tunnel passes through the water-rich weak surrounding rock part.Therefore,it is necessary to research the mechanical characteristics of the water-rich weak surrounding rock tunnel.Based on the mechanical characteristics and deformation mechanism of weak surrounding rock,some railway tunnel is selected as the research engineering.Then,the mechanical characteristics and stability of the water-rich weak surrounding rock tunnel is analyzed,the softening characteristics of the weak surrounding rock is revealed when the laboratory experiment,numerical simulation and field monitoring test are combined.The specific conclusions are as follows:(1)On the basis of analyzing the meaning and classification of weak surrounding rock,the deformation and strength characteristics,the plastic deformation mechanism,and the expansive extrusion deformation mechanism of weak surrounding rock are summarized.(2)Rock samples are selected in the weak surrounding rock section of some railway tunnel.The density test,water absorption test,P-wave velocity test,nuclear magnetic resonance test and uniaxial compression deformation test of rock samples were respectively carried out under natural conditions,free soaking for 1,3,6 and 9 months.From the test process,the water absorption,the porosity,the peak strain and the Poisson’s ratio of rock samples gradually increase with the increase of free soaking time,and the P-wave velocity,the ultimate uniaxial compressive strength and the elastic modulus gradually decrease with the increase of free soaking time.When the rock samples are soaked freely for 6 to 9 months,the ultimate uniaxial compressive strength of rock samples is 8.433 MPa.It is 70.08% lower than that of 28.187 MPa in natural condition.This reveals the softening characteristics of the weak surrounding rock when exposed to water.(3)The numerical models of both side drift method and three bench method with reserved core soil are analyzed by finite element software MIDAS/GTS under different soaking time conditions respectively.The deformation laws and distribution of vertical displacement,horizontal displacement are separately summarized when the tunnel is constructed by two construction methods.At the same time,mechanical characteristics and distribution of surrounding rock stress,shotcrete stress,steel frame stress,secondary lining concrete stress are separately analyzed.From the numerical simulation results,the surrounding rock stability of the tunnel is better with three bench method with reserved core soil than the result with the both side drift method.(4)The monitoring measurement of this railway tunnel were carried out,the deformation laws and distribution of crown settlement and convergence deformation in key parts of the tunnel were summarized when the tunnel deformation changed with the time and space change.And the mechanical characteristics and distribution of surrounding rock pressure in key parts of the tunnel with time were analyzed.Then,the numerical simulation results are compared with the monitoring measurement data.From the results,the tunnel deformation and stress trend of the numerical simulation and the monitoring measurement are approximately the same,and it is safer and more effective to use three bench method with reserved core soil in excavating the weak surrounding rock section.At the same time,results of the monitoring measurement test verify the correctness of the analysis method and main conclusions of the paper.