| Based on the research project on the influence of formation head pressure on the shield construction and chamber stability of saturated loess in the Hanjiwei Phase II project,this paper carried out the site investigation and data collection of the loess tunnel project in the South trunk line of the Hanjiwei Phase II project,and studied the permeability characteristics of saturated Q2intact loess and reshaped loess based on laboratory tests.Furthermore,based on the model test,the influence of the limited drainage rate,formation permeability coefficient and water level height on the external water pressure reduction coefficient of the lining unit was further explored.The model test was numerically simulated by the finite difference software FLAC3D,and the results of the model test and the numerical test were compared and analyzed to verify the correctness of the numerical analysis method.Furthermore,based on the principle of fluid-solid coupling,the influences of loess formation permeability coefficient,shield tunneling velocity,disturbance range of surrounding rock,grouting thickness behind wall,grouting level and other factors on the water pressure reduction coefficient outside the tunnel and the mechanical response of lining were studied.Sensitivity analysis was carried out for each influencing factor to explore the influence degree of each factor on the water pressure reduction coefficient outside the tunnel.The main results achieved are as follows:（1）The permeability coefficient of Q2loess obtained in the field test is much higher than that in the laboratory test,and the difference between the two is 1-2 orders of magnitude.It is difficult for the indoor test to reflect the node of the old yellow soil body.It is suggested that the field test results should be used to design the external water pressure in the project.（2）When the permeability coefficient of the soil layer is small（0.04～0.46m/d）,the external water pressure reduction coefficientβof the lining unit increases rapidly with the increase of the permeability of the formation.When the permeability coefficient is greater than0.46m/d,the change rate of the external water pressure reduction coefficientβdecreases significantly with the increase of the permeability coefficient of the formation.When the drainage rate of the lining unit is 0.16%～5%,the external water pressure reduction coefficientβis larger,ranging from 0.04～0.74;when the drainage rate of the lining unit is more than 8%,the external water pressure reduction coefficientβis smaller,ranging from 0.06～0.25.Under different drainage rates and formation conditions,the external water pressure reduction coefficientβincreases linearly with the increase of water level.（3）The results of model test are compared with those of numerical simulation.The error of the reduction coefficient of external water pressure obtained by the two research methods was within 10%,indicating that the numerical analysis method is basically feasible in simulating the external water pressure characteristics of underground structures,and can be used to simulate the actual external water pressure characteristics of the tunnel.（4）The numerical analysis results show that during the construction period,the external water pressure reduction coefficientβ+limited emission rateξ≈1.0;When the permeability Coefficient kl=0.01～3.0m/d,shield tunneling velocity V=1.2～30.0m/d,disturbance range ws=0～2.0m,grouting thickness behind wall h=20～100cm,and grouting level N=0.2～300.0,The external water pressure reduction coefficientβwas 0.22～0.99,with an average value of 0.50.For this project,it is suggested that the shield tunneling speed V=6～9m/d,the grouting thickness h≥20cm,the grouting level N=20～50,the distribution range of the external water pressure reduction coefficient isβ=0.27～0.67,the averageβ=0.42.The sensitivity coefficient of the external water pressure reduction coefficient of the above influencing factors is ranked from high to low:Mζ>Mkl<0.1m/d>Mh>MV<9m/d>Mws>MN>MH,it is suggested that during shield construction,appropriate drainage measures should be taken to reduce the disturbance to the stratum,and grouting behind the wall should be strictly controlled,so as to effectively reduce the external water pressure load. |
