Study On Seepage Failure Mechanism And Calculation Method Of Surrounding Rock Pressure Of Tunnel With Weak Interlayer

With the rapid growth of the tunnel engineering in our country,the large-scale tunnel construction is inevitably encountering challenges from complex geological formation,especially some unfavorable geological conditions,such as groundwater and weak interlayer,which have become an essential and difficult aspect faced in the tunnel construction.The majority of existing researches mainly focus on the impact of a single factor on the tunnel failure mechanism,such as groundwater or weak interlayer,but in the engineering practices,the deformation-failure mechanism of the tunnel surrounding rock is much more complicated as the tunnel excavation in mountainous areas is commonly faced with both groundwater and weak interlayer conditions,and there is lack of relevant research currently.Therefore,in view of this engineering-geology condition,it's particularly necessary to reveal the deformation-failure mechanism of the tunnel surrounding rock and come up with a pressure calculating method for the surrounding rock with reasonable consideration of the actions of both seepage and weak strata.In the study,the interior similar model test method is used to find out the effect of the inclination angle,location and number of weak interlayers on the failure mechanism of the surrounding rock with and without the seepage action;based on the test results,the numerical simulation method is used to lead out the influence rule of different permeability ratios of the weak interlayer,as well as different inclinations and locations of weak interlayer under the seepage action on the failure mode of the surrounding rock;at last,Platts Theory is modified in the aspects of the seepage,inclination angle of the weak interlayer,and inclination location of the weak interlayer,and a pressure calculating method for the surrounding rock is obtained considering the mutual actions of seepage and weak interlayer occurrence under certain conditions.The main contents and outcomes of the study are as follows:(1)To study the impact of the mutual action of seepage and weak interlayer on the failure mode of the surrounding rock,three groups of model tests are designed under different conditions,including with and without seepage action,at 40-degree and70-degree angles of the weak interlayer,with the weak interlayer passing through the tunnel vault,and at the location of 0.3 times the span between tunnel walls.The comparison analysis is performed for the influences of seepage action,seepage-free action,and different occurrences of weak interlayer(angles and locations of the interlayer)on the stress field,displacement field,strain field,and crack extension of the surrounding rock.It is found through research that the seepage action can decrease the pressure of the surrounding rock of the loosened failure zone,increase deformation of the surrounding rock,and change the crack extension line after the tunnel is excavated;if the inclination of the weak interlayer is 40°?70°,the overall change tendency of the stress field of both models is basically consistent;the weak interlayer passing through the tunnel vault takes more impact on the distribution rule of both stress field and displacement field of the surrounding rock than the distance between weak interlayer and tunnel being 0.3 times the span between tunnel walls..(2)To further analyze the impact of the seepage on the failure mode of the weak interlayer,the numerical simulation method is conducted for the failure mode of the surrounding rock with the tunnel rock in the real size corresponding to the scaled model test to work out the failure mode of the surrounding rock respectively under the conditions of permeable layer,the same seepage as the surrounding rock,and water-resisting weak interlayer.As it turns out,if the weak interlayer is water-resisting or permeable layer,the permeability coefficient of the weak interlayer and surrounding rock has 4 and more magnitudes of difference,with relatively large impact on the stress field,displacement field,and seepage field of the surrounding rock.(3)The influence rule of the occurrences of weak interlayer on the failure mode of the surrounding rock is studied with the numerical simulation method,and the simulated calculation is performed for the failure rule of the tunnel surrounding rock under different conditions,including the inclination of the weak interlayer being from 0° to90°,and the distance to the tunnel wall equal to 0.1D?0.5D.The calculation result shows that,when the inclination of the weak interlayer is 0?25 degrees,the stress field,displacement field,and seepage field of the surrounding rock show the most obvious change as the inclination angle changes,and with increase of the inclination angle,such impact gradually decreases.When the inclination is greater than 55 degrees,the impact is almost unchanged.When the distance between weak interlayer and tunnel wall is within 0?0.3D,the impact on the stress field,displacement field,and seepage field of the surrounding rock gradually decreases with increase of the distance,but when the distance is greater than 0.3D,the impact of the interlayer on the surrounding rock shows a significant reduction,the stress on the left side of the interlayer increases,the left vault deflects towards the left under the tension shear action of the left arch foot,and asymmetrical distribution of the stress,displacement,and seepage gradually disappears.(4)Considering that the multiple-interlayer conditions are rather complicated,the failure mechanism of the surrounding rock containing two weak interlayers without seepage is only discussed in the study.Through the model test and numerical simulation,the comparison analysis is conducted for the evolution process of the fracture damage of the tunnel surrounding rock respectively with existence of one and two interlayers under the seepage-free condition,the differences of the stress field,strain field,plastic area and displacement field.The result shows that the failure process,failure mode and failure mechanism of the surrounding rock containing two weak interlayers are significantly different from those of the surrounding rock containing only one weak interlayer.Therefore,the supporting scheme designed only considering one weak interlayer of the surrounding rock can indicate great safety hazard.(5)Due to limitation of the model test,to do further quantitative research on the impact of the inclination angle of the interlayer,spatial location,and seepage on the supporting structure of the surrounding rock,firstly,the FLAC3 D is used to analyze the influence rule of the inclination of the interlayer,spatial location,and seepage on the surrounding rock pressure of the tunnel supporting structure.Then the Platts Theory is modified by fitting,and the modified Platts Theory is used to figure out the pressure of the surrounding rock of the supporting structure in the practical works.Through comparison with the measured data,the modified Platts Theory proves more feasible than the other two methods,which can be adopted for calculating the supporting load of the tunnel surrounding rock containing weak interlayer in certain mountainous area in Chongqing City considering the seepage action,in order to provide the corresponding theoretic basis for the design of the supporting scheme.