Research On Ground Settlement Caused By Dewatering Considering The Initial Hydraulic Gradient And Interlayer Recharge

In order to ensure the construction of foundation pit without groundwater seepage above the pit surface,the most commonly used groundwater control measure is dewatering, which may cause ground settlement under some geological conditions and the surrounding environmental conditions, and may cause damage to the adjacent building pipelines, causing serious consequences. Prediction of surface subsidence caused by pumping groundwater has become an important subject of engineering. In engineering soils are layered soils, often containing aquifer and aquitard interaction, so the calculation method of dewatering based on Dupuit assumption can not reflect the real seepage of groundwater in layered strata. In additional, the initial hydraulic gradient may exist in cohesive soil will change the extent and depth of pore pressure dissipation after pumping, and then reduce the value of compression deformation of clayed soil. At present, calculation methods of ground settlement caused by dewatering of foundation pit have not considered the above factors.In this paper, seepage test in layered aquifer with different weak permeable layer are carried out through the independent development seepage model box to study the effects of interlayer permeability, head difference and aquifer thickness on Seepage field of layered aquifer. The two-dimensional finite element analysis program of the layered aquifer is developed by using VB.net language, and the calculation results are compared with the results of classical calculation method and the test results. The initial hydraulic gradient of different soils is studied by GDS consolidation apparatus. Calculation method of the ground settlement based on the analysis of the seepage field and considering the initial hydraulic gradient is proposed. The main results are as follows:(1) Free water surface line calculated by finite element method considering interlayer seepage and by classical method are compared with the measured result. It shows that the finite element method considering interlayer seepage is in agreement with experimental result, and classical methods has sufficient accuracy only under certain conditions. The finite element method considering the seepage flow between layers is more suitable for calculation of dewatering in practical engineering.(2) The calculation method of the ground settlement based on the analysis of the seepage field and considering initial hydraulic gradient is proposed. The calculation of an engineering example shows that the cohesive soil settlement calculated by considering the initial hydraulic gradient method is significantly smaller than that of the classical method. When initial hydraulic gradient is much bigger, and the thickness of clay layer is greater, the computing result of settlement is reduced more significantly.(3) Results of GDS consolidometer permeability test prove the existence of initial hydraulic gradient in clay soil, and its value is mainly affected by soil types and pore pressure.(4)The permeability of the aquitard has influence on both the line water surface line of adjacent upper aquifer and the water head line of lower aquifer. The greater the ratio of permeability coefficient of aquifer and aquitard, the higher the surface line of upper aquifer is, and the lower the water head line of lower aquifer is.(5) The smaller the thickness of the upper aquifer, the more likely to appear " drying up " phenomenon the water surface line. When the water surface line of the upper aquifer appears " drying up" phenomenon, the water surface lines with different thickness of the upper layer are approximately parallel lines. The greater the thickness of the aquifer,the more likely to appear the problem of inexhaustible pumping.(6) When the water level is higher than the top of aquitard, the type of aquitard has little effect on seepage field, and the form of seepage field under various soil layers is close to that of homogeneous soil. When the water level falls below the bottom of aquitard, and the water head difference is bigger, the influence of the type of aquitard on the seepage field is greater.