Study On Stability Evaluation For Excavation Face Of River-crossing Shield Tunnel Across Water-rich Soft Soil

Shield technology is widely used in the construction of river-crossing tunnels because of the advantages of high construction efficiency and small environmental disturbance.At the same time,construction risk accidents caused by improper operation and unknown geological investigation also occur frequently,among which excavation face instability is the most serious disaster accident.Once the excavation face of the shield tunnel occurs instability,the mud and water in the soil will quickly flood into the narrow construction space,making it difficult to carry out temporary remedial work on the site.Based on a river-crossing shield tunnel engineering in China,the geological survey report and the construction monitoring data were collected by field investigation and test.By means of theoretical analysis,numerical calculation and data mining,the stability evaluation of the excavation face of the shield across through the water-rich soft soil is investigated.The main conclusions are as follows:(1)Through the shield tunnel excavation face balance theory research,the mechanical mechanism of excavation face instability is studied from the conditions and critical criteria of excavation face instability.Based on the limit theory,the equation of the minimum support pressure to maintain the excavation face stability is derived,which provides important mechanical criterion and theoretical basis for evaluating the excavation face stability.(2)Through the numerical model,the instability evolution low and failure characteristics of the excavation face under different conditions are studied.The conclusion shows that the soil deformation increases sharply when excavation face instability,and the maximum deformation occurs within 2-3m above the center of the excavation face.When the internal friction angle or cohesion is increased,the limit support pressure and the ultimate deformation of the excavation face are reduced.While increase the buried depth and the overlying water depth,the ultimate displacement and the limit support pressure of the excavation face are increased.(3)It is found that there is a certain degree of negative correlation between the segment lagging deformation and the excavation face stability.The mutual feed mechanism between excavation face stability and the lagging deformation of the segment is studied through data mining,the results show that during the excavation face instability,the segment lagging deformation increases,especially the segment within 12m behind the excavation face.Similarly,the supporting pressure is gradually increased,the excavation face stability is gradually improved,and the lagging deformation of the segment is reduced.Taking the segment lagging deformation as the characteristic parameter for the excavation face stability,7 indexes for predicting the segment deformation were selected,and the segment lagging deformation prediction model based on BP neural network was established.The engineering application showed that the model can predict the segment lagging deformation effectively and accurately.(4)The excavation face stability evaluation under the safe state is investigating by comprehensive empowerment method and TOPSIS.Tunnel buried depth,coverage ratio,supporting safety coefficient,penetration of soil,groundwater condition,internal friction angle and cohesive force are selected for establishing the stability evaluation model of shield tunnel excavation face.Finally,the Visual Basic 6.0 is used to realize the intelligence and visualization of the evaluation model,so as to ensure the high efficiency and real-time performance of the evaluation model in engineering application.