Experimental Study And Numerical Simulation Of Seepage Characteristics Of Loess Joint

Loess joint is widely distributed in the Loess Region,it is the weak channel of loess and the dominant channel of water infiltration.Its existence affects the stability of tunnel,cave and highway slope in loess area.At present,there are few studies on the seepage characteristics of loess joints.Based on field investigation,laboratory test,theoretical analysis and numerical simulation,the seepage characteristics of loess joints and their influence on the deformation of tunnel entrance section are studied.Firstly,the hydraulic parameters of loess joints are studied by laboratory tests.By designing contrast experiment,the permeability coefficient of each sample is measured by making use of Global Digital System(GDS)triaxial osmotic instrument.The influence of joint aperture and the soil properties around the joint on the seepage are studied,and the difference between natural and artificial joints is compared.The experiment results are shown as follows: The permeability coefficient of homogeneous loess samples is far less than that of jointed loess samples,the existence of joint greatly enhances the seepage ability of loess mass.When the nature loess joint shares same average opening,its permeability coefficient is slightly lower than the artificial joint.The surrounding loess has a certain influence on the seepage flow,which improves the whole penetration ability;Lower dry density and porous soil make the soil particles on the wall fall off easily when the seepage occurs.In order to obtain the law of infiltration of the joint loess,the model test started by using the simulation test device for seepage of natural loess joints,the influence of joint and its aperture on water infiltration scope and moisture distribution are obtained.The wetting front of the specimen with a joint is a curve along the joint,the equivalent line of water content is bend down with a joint as a symmetric axis;The wetting front and the equivalent line of water content of the homogeneous loess specimen are horizontal lines.Infiltration depth of the specimen with a joint is about 2.6 times and the area of humid area is about 2.2 times of the homogeneous loess specimen.The existence of joints increases the infiltration depth and infiltration quantity,and the greater the joint aperture,the deeper the depth of infiltration is and the greater the infiltration quantity is.Through further analysis of model test data,the relationship between the width and the depth of the humid area with the variation of the joint aperture and the infiltration time is established.The method of determining the range of the humid area is obtained,the calculation method is programmed by the FISH language.According to the humid area,the speed of the joint infiltration is studied,which increases with the increase of joint aperture and decreases with the increase of time.Then the equivalent permeability coefficient of unsaturated joint is obtained by software,and the influence of joint space and number of joints on the wetting front is studied.By using the numerical analysis software,the deformation law of the tunnel entrance section with joint loess is studied.The roof settlement of tunnel increases with the increase of joint apeture and infiltration time.The greater the joint apeture and the longer the infiltration time,the greater the possibility of tunnel failure.