Strain Softening Model Of Rock Mass And Its Application Research On Surrounding Rock Stability Analysis Of Extra-Large Section Bifurcation Tunnels

With the continuous deepening of urbanization process and the rapid rise of emerging transportation industries,traffic flow has shown explosive growth in China.The highway capacity in the initial stage of construction has long been unable to meet the people’s increasing travel demand,which brings the construction peak in the extra-large section highway tunnel projects.As the most difficult construction among extra-large section tunnels,the section size and the construction process of extra-large section bifurcated tunnel is larger and more complicated.During its excavation,the obvious strain softening characteristics exist in the surrounding rock,and it is more prone to instability and failure due to frequent disturbances.Based on the Liantang tunnel extra-large section bifurcation project of the Shenzhen Eastern Transit Expressway,theoretical analysis,program development,numerical simulation and on-site monitoring verification methods were applied to carry out the strain softening model of rock mass and its application in the stability analysis of surrounding rock of large section bifurcation tunnel in this thesis.Based on the construction of the rock mass strain softening model,the following aspects are focused on: calculation method of surrounding rock pressure of super-large section,optimization of net distance of asymmetric small-distance tunnel,timing of support of super-large section,and construction mechanical behavior of super-large section bifurcated tunnel.The main results are as follows:(1)Based on the post-peak characteristics of the rock mass and the Hoek-Brown yield criterion,a rock mass strain softening model(CS＿SS model)considering the influence of confining pressure is constructed,which can reflect the dynamic change of the critical plastic softening coefficient in the plastic softening stage of the rock mass and the nonlinear dilatancy behavior of the rock mass.According to the incremental plasticity theory,the constitutive model built in FLAC3 D was developed based on Visual Studio 2010 software development platform.The CS＿SS model was compiled into a dynamic link library file DLL and embedded in FLAC3 D,and the correctness and applicability of the model was verified by comparing with the results of the existing theoretical model and the indoor test.(2)On the basis of considering the characteristics of the strain softening of the surrounding rock,a combination of CS＿SS model and orthogonal experiment is used to study the factors affecting the range of the loosening zone of the surrounding rock,and the functional expression of the contour curve of the loosening zone is fitted.A limit analysis model of surrounding rock loosening pressure under the nonlinear failure criterion is established and the calculation formula of surrounding rock loosening pressure is deduced by using the boundary function of the loosening area and the upper limit analysis theorem.The rationality and universal applicability of this method was verified by the comparative analysis between the calculated value and the monitored value of the surrounding rock pressure of the largest section in the long-span section interval of the Liantang Tunnel.(3)The calculation program of the element safety factor method considering the strain softening behavior of the surrounding rock was compiled through the FLAC3 D secondary development platform.The criterion for the reasonable clear distance of asymmetric small clear tunnels was established by combining the results of small-distance tunnels.Based on the small-distance section of the Shenzhen Liantang tunnel,the close construction law of extra-large section asymmetric small clear distance tunnel under different surrounding rock quality and excavation sequence is systematically studied by applying CS＿SS model in this paper,and the reasonable value of the minimum net-distance under different construction conditions is concluded.Finally,a reasonable reinforcement plan and technical measures for the middle rock are put forward according to the quality of the surrounding rock.(4)The CS＿SS strain softening model was applied to investigate the change law of the longitudinal deformation curve and the characteristic curve of surrounding rock in the extra-large section tunnel.A predictive method for determining the optimal time of supporting construction and the estimation formula for construction position of initial support at the largest section of the branch of Liantang tunnel were proposed based on catastrophe theory and the coupling relationship between LDP and GRC.On this basis,the second-stage surrounding rock-support interaction in the convergence-constraint method was numerically simulated,and the interval range of the lag distance of the initial support of the largest section under different surrounding rock conditions was determined.The relationship curve between the reserved deformation of surrounding rock of the super-large section tunnel and the ultimate load of the supporting structure was established.(5)Based on the CS＿SS model,the construction mechanics behavior of the Liantang tunnel extra-large section bifurcated tunnel from the small clear distance section to the large span section was investigated and the construction optimization plan of the large-span section and the start and end mileage of the lateral expansion section are determined according to the analysis results.The safety of the construction plan and the rationality of the application of the rock strain softening model were verified by the on-site monitoring results.Related research results can provide useful reference for the design and construction of similar projects.