Study On Tunnel Comprehensive Prediction And Surrounding Rock Stability Under Complex Geological Conditions

Due to the unpredictability of unexcavated geological conditions in tunnel engineering,independent sealing and complexity,fracture zone and other factors have great influence on the strength of rock mass,integrity and stability of surrounding rock of tunnel,it is easy to cause roof fall and collapse of tunnel.It also increases the difficulty of rock classification and prediction.It is an urgent problem to study the classification,prediction and stability of tunnel surrounding rock under complex geological conditions.In view of the surrounding rock classification of shallow-buried tunnels under complex geological conditions,the traditional classification of surrounding rock is often limited and uncertain,and the influence factors taken into account are not comprehensive enough,so it is difficult to classify the surrounding rock accurately.Based on the principles of rock mechanics and geomechanics,this paper grasps the key points of rock mass anti-shear and slip resistance,collapse instability mode and surrounding rock stability,analyzes the difficulty of surrounding rock classification,and considers the dispersion and randomness of quantitative classification index.Based on the analysis of the influence of weak structural plane and broken zone on the classification of surrounding rock,a modified BQ classification formula is put forward.Combined with the engineering example,the three dimensional wedge limit equilibrium method and finite element model are used in this paper.The stability of surrounding rock of tunnel is analyzed by pseudo-method,and the relationship between stability of surrounding rock and classification of surrounding rock is discussed.As a result of the study,the following new results have been achieved:(1)the division of the rock mass on the palm face and the high attention to the influence of the rock mass fracture zone,the weak layer and the rock mass structure plane are put forward.A modified BQ classification formula is proposed by introducing three adjustment coefficients?,?,?,and so on.The flow chart of surrounding rock classification in shallow tunnel is given and an example is given.The corresponding relations between the classification of surrounding rock and the ability of self-stabilization of surrounding rock,the driving scale,and the way of reinforcement and support are discussed.The classification of surrounding rock should be compared with the results of follow-up geological survey,advanced geological prediction and monitoring measurement,and all aspects of comprehensive analysis,mutual verification,and improve the accuracy of classification.(2)the stability of surrounding rock of tunnel is analyzed by means of three-dimensional wedge limit equilibrium method.Combined with"structural cybernetics"in rock engineering,the stability of possible unstable blocks composed of fault fracture zone and weak interlayer or joint crack is analyzed emphatically.The region where the unstable block is located is the key to tunnel excavation and support.It is also the key to the classification of surrounding rock.It is necessary to take into account the discounting of the parameters of the structural surface region which has great influence on the stability,and even to downgrade the classification results of the surrounding rock in the possible unstable block region which has a great influence on the stability.The accuracy and practicability of the modified BQ classification method are further emphasized.(3)the finite element simulation of shallow-buried LH tunnel is carried out with Midas/GTS software,and the effects of deflection stress and arch hole on deformation and stress are analyzed.It can be seen from the calculation that for different buried depths,the greater the inclination angle(0°~45°)of the overlying layer above the tunnel,the greater the deflection stress,the vertical displacement increases from 12mm to 33mm,and the horizontal displacement increases from 4mm to 10mm.The stress increased from 500kN/m~2 to 800kN/m~2.That is,with the increase of deflection stress,the stress and displacement of tunnel increase.The ratio of displacement increment to span is 1.4‰.When the size of the tunnel vault is different,When the radius increases from 2m to 4m,the vertical displacement increases from 15mm to 35mm and the horizontal displacement from 8mm to 12mm.The stress increased from 600kN/m~2 to 800kN/m~2.That is,with the increase of collapse range,the stress and displacement of tunnel also increase,and the ratio of displacement increase to span is 1.3‰.In the process of tunnel excavation,the surrounding rock should be strengthened and strengthened in time.This further explains the importance of surrounding rock classification and stability analysis of shallow buried tunnel under complex geological conditions.