Analytical Study On Consolidation Of Multiple Wells On Composite Foundation

With the continuous expansion of the scale of infrastructure construction in China,the geological conditions encountered in the project are becoming more and more complex.The high water content and low bearing capacity of soft clay soil genes must be reinforced before normal use in the project.As a common technical means,composite foundation is usually used in combination with heap load preloading and vacuum preloading.At present,the consolidation theory of composite foundation has achieved some abundant achievements,but most of the calculation models are based on a "pile-soil" unit,which is slightly different from the actual situation of the project.In order to enrich the development of consolidation theory of composite foundation,based on the traditional consolidation model of composite foundation,a calculation model with "pile soil pile" as the element is established,and the consolidation model of composite foundation group well is studied.The main work and contents are as follows:(1)In view of the consolidation of composite foundation,considering the vertical seepage and well resistance of soil,a calculation model of multiple consolidation of composite foundation with pile soil pile as the element is established.Based on the assumption that the flow of central pile and side pile is equal,the consolidation equation of soil mass is established.According to the basic assumption,the boundary condition and initial condition are determined.By using the analytical solution,the analytical solution of consolidation of composite foundation group well under the action of instantaneous load is given.The difference of consolidation curves between the two models is analyzed by compiling calculation program.The results show that when the well diameter ratio is greater than 5,it is reasonable to assume that the flow rate of center pile and side pile is equal.At this time,the degree of consolidation under the traditional consolidation model and the group well consolidation model is almost the same.(2)On the basis of instantaneous load,considering the change of external load with time,the effect of well resistance and the effect of radial and vertical seepage of soil,the analytic solution of well consolidation of composite foundation group under variable load is derived.Based on this solution,the consolidation curve is drawn,and the influence of various parameters on the consolidation of composite foundation group well and the variation law of pore pressure are analyzed.The results show that the effect of loading time on consolidation rate of composite foundation is great.And the shorter loading time is,the faster consolidation is.When the loading is not completed,the loading duration is the main factor affecting the consolidation speed.When the load reaches the maximum value,the impact of the loadingduration on the consolidation speed is very small.At this time,the greater the permeability coefficient of pile,vertical permeability coefficient of soil and horizontal permeability coefficient of disturbed area are,the faster the consolidation of foundation is.(3)Based on the calculation model of multiple wells on composite foundation,considering the linear attenuation of vacuum negative pressure with depth,the analytical solution of consolidation of multiple wells on composite foundation considering the vertical seepage and well resistance of soil diameter is derived.The influence of vacuum degree on the consolidation rate is analyzed by drawing the consolidation curve,and the correctness of the solution is verified by the degradation analysis of the solution.The results show that vacuum surcharge preloading can be regarded as the superposition of vacuum negative pressure and surcharge.The pore pressure dissipation rate of composite foundation under vacuum surcharge preloading is faster than the consolidation rate of composite foundation only under surcharge or vacuum negative pressure.The faster the vacuum degree decreases with the depth is,the slower the consolidation of composite foundation is.(4)Based on the assumption that the flow rate of center pile and side pile is equal,considering the combined action of vacuum negative pressure and surcharge,and considering the load change with time and depth,the common solution of consolidation of composite foundation group with vacuum surcharge preloading under variable additional stress is given.On the basis of the general solution,the consolidation solutions in some special cases are given,and the average consolidation degree is calculated according to the stress and deformation respectively.Based on the general solution,the change chart of consolidation curve is drawn,and the influence of various parameters on the consolidation characteristics and the change of excess pore pressure is analyzed.The results show that the higher the additional stress on the top and the lower the additional stress on the bottom of composite foundation are,the faster the consolidation rate of composite foundation is.And the shorter the loading time is,the faster the consolidation of foundation is.In the early stage of consolidation,the excess pore pressure increases with the application of load.When the load is stable,the excess pore pressure dissipates gradually.The dissipation rate of pore pressure under vacuum surcharge preloading is higher than that under surcharge preloading alone.And when the well diameter ratio is greater than 5,it is reasonable to assume that the flow of center pile and side pile is equal.