A Depression Curve Computing Method Based On The Principle Of Minimum Energy Loss Rate

The decrease of groundwater level has an important influence on land subsidence: on the one hand,the decrease of groundwater level will change the stress state of soil within the radius of influence;on the other hand,the change of water head pressure will cause the seepage pressure within the radius of influence.The depression curve can directly reflect the change of water level around the foundation pit after dewatering,and the accurate location of the depression curve is the basis of analyzing the land subsidence.At present,scholars at home and abroad are generally based on the basic equation of Dupuit stable well flow and the basic equation of theis.The main methods to solve the depression curve according to Theis equation include the straight line diagram method,the wiring method and the analytical method.In view of the disadvantages of complex solution and large human error of graphic method and wiring method,and insufficient consideration of the basic equation of Dupuit stable well flow to the factors influencing the hydraulic jump phenomenon,this paper proposes two methods to solve the depression curve of the phreatic aquifer based on the finite element method.The main research results include:(1)A depression curve computing method based on the principle of minimum energy loss rate.The calculation model of precipitation depression curve is established in homogeneous phreatic aquifer: when the foundation pit dewatering reaches the control water level,it can be regarded as a stable seepage domain with known upstream water level(groundwater level before dewatering),downstream water level(water level in dewatering well after dewatering)and escape boundary(water jump section).In this stable seepage domain,the real depression curve will make the energy loss rate of the whole seepage domain minimum.The finite element division of the seepage domain is carried out.Based on the principle of the minimum energy loss rate,the finite element calculation method for solving the depression curve is deduced.The problem of solving the depression curve is transformed into the problem of solving the minimum energy loss rate of the seepage basin.(2)A depression curve computing method based on the principle of total potential energy in the real domain.When the precipitation reaches the water level of the control point,the minimum principle of the total potential energy in the real domain is used to solve the water level point of the groundwater entering the well(the height between the seepageescape point,the escape point of the well wall and the water level in the well is the hydraulic jump section).The transition elements cut by the depression curve are selected,and the triangle similarity is used to solve the transition elements.The intersection of each element and the depression curve can be directly obtained,and the depression curve to be calculated can be obtained by connecting each point.(3)Fortran program for solving the depression curve of the above two methods is compiled respectively,and the two programs are applied to solve the actual case.The calculated results are compared with the theoretical calculation results of jubuy formula and the measured values of the project.