Study On Mechanism Of The High Vacuum Dewatering In Low Permeability Soil

There are widespread low permeability muddy clay layers in the southeastern coastal areas of China.There is a problem of low dewatering efficiency when applying foundation pit dewatering in these areas.Based on the background of this project,a high vacuum double-tube dewatering well structure is proposed,which has the tightness structure that allows the vacuum in the well to be maintained at a high level during the dewatering process,effectively improving the effluent efficiency in the low permeability soil layer.In this paper,based on the project of foundation pit dewatering in Ningbo Metro,using the finite difference software FLAC3 D numerical simulation method,combined with field test and theoretical analysis,the mechanism of high vacuum dewatering in low permeability soil is systematically studied as follows:(1)The paper proposes the structure of double-tube high vacuum dewatering well and compares the high vacuum tube well with the conventional well at the foundation pit dewatering site of Ningbo metro station.The comparison of dewatering effect verifies that high vacuum tube well can improve the amount of water in the low permeability soil.In addition,the water level,pore water pressure and surface subsidence distribution rules are explained.(2)The FLAC3 D finite difference software is used to establish the numerical model of the vacuum well.The fluid-solid coupling method is used to simulate the field test of Ningbo Metro and the case of vacuum tube well dewatering given in [5],[7] and [37].Comparing the calculated result with the field test results,it is found that the coincidence of the results of water discharge,depth reduction and negative pressure distribution is good,which shows that the numerical model adopted in this paper can reflect the complex seepage problem in vacuum dewatering process.(3)The FLAC3 D numerical model was used to analyze the single-well and double-well dewatering under different conditions.The transmission law of water discharge and negative pore water pressure in high vacuum tube well was studied emphatically.Through parameter analysis,it was found that the water discharge is difficult in low permeability clay,and the greater the degree of vacuum in the tube well is,the greater the amount of water is discharged.The influence range of vacuum negative pressure is related to vacuum degree,permeability coefficient of soil layer,groundwater level and well depth,etc.The larger the vacuum is,the higher the soil permeability coefficient is,the greater the influence range of vacuum negative pressure is.The vacuum negative pressure in the low permeability muddy clay is faster above the water level,and it is limited below the water level.On the basis of numerical simulation,the vacuum coefficient was introduced to propose the formula of well water discharge in vacuum tube.(4)Analysis of settlement consolidation of foundation soil caused by high vacuum tube well dewatering in low permeability soil layer shows that low permeability soil layer has low drainage rate and long consolidation time.For a single vacuum well,the maximum settlement of dewatering in 30 days is 1 mm,and the settlement is up to 2.2 mm after 90 days of dewatering.The vacuum suction has a great influence on the consolidation of soil within the negative pressure area.The range of surface subsidence and its influence caused by dewatering increases with the increase of vacuum degree,well depth and permeability coefficient of foundation soil.Under the condition of Ningbo formation,the influence range of single well settlement is about 30 m,but the settlement value is small.Based on the stratified summation method,considering the influence of vacuum degree,the reference formula for settlement calculation of vacuum tube well under dewatering is proposed.(5)In order to study the interaction of tube-well dewatering,this paper analyzes the distribution of double-well simultaneous vacuum,vacuum negative pressure and surface subsidence by double-well dewatering model.When the spacing between wells is small,the vacuum negative pressure zone in the soil layer is superimposed to accelerate the seepage of groundwater,but the discharge rate of single well decreases.With the increase of well spacing,single well water output increased.According to single well daily average water discharge and pore pressure distribution,high vacuum double well spacing is recommended to be 8 m under the soil conditions given in this paper.