| Highway tunnels are currently at the peak of construction in China,confronting complex geological environments and high safety risks,of which soft rock tunnels are the most typical case.Soft rock tunnels are usually constructed by partial excavation and advance support to ensure the stability of the surrounding rock,but there are problems such as frequent change of support system,frequent disturbance of the surrounding rock and low construction efficiency.The use of micro bench excavation method can improve construction efficiency,but can easily cause the tunnel face to collapse.In order to achieve the goal of large cross-section excavation in soft rock,the critical distance of face instability was selected as the stability evaluation index,based on practical engineering,and a study on the application of the micro bench method near the fault zone,the tunnel face stability and the optimization of advance support parameters was conducted using theoretical analysis,numerical simulation,orthogonal test and monitoring analysis.The main results are as follows:(1)The stability of the soft rock tunnel face was investigated by limit equilibrium analysis.Based on the limit equilibrium analysis method,a mechanical model for the critical distance of instability was established,which found that the critical distance in practical engineering is 11.05 m,considering the self-stability of the surrounding rock only.According to the variation of the critical distance of instability with the geological condition and construction geometry parameters,the influence law of cohesion,internal friction angle,unit weight,depth-height ratio and cross-height ratio on the stability of tunnel face is obtained.(2)The factors influencing the stability of the tunnel face in soft rock tunnelling was studied.A numerical model,verified by monitoring data and theoretical results,was developed to analysis the practical project.The model was used to discuss the influence of different excavation methods on face stability.The plastic zone distribution of the surrounding rock,the axial displacement law of the tunnel face were then obtained for the three bench method,the micro bench method and the full face method,as well as the failure mechanism of the tunnel face in large-section excavation;the influence of different advanced support parameters on stability of tunnel face was further discussed and it was found that the pipe shed length,the outer diameter of pipe shed and the grouting length were the main factors affecting the stability of tunnel face and their influence laws were analysed.(3)Optimisation and regression analysis of advance support parameters for the micro bench method was carried out.The main advance support influencing factors were selected for orthogonal simulation tests,and the optimum parameters of pipe shed outer diameter,pipe shed length and grouting length for micro bench construction were obtained as 130 mm,25m and 40 m respectively.Regression analysis was performed on the test results to obtain a regression model for the critical distance of instability.The engineering application results showed that the model has strong applicability under different working conditions,and can provide a reference for tunnel face stability evaluation and advance support design in similar engineering. |
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