Theoretical Analysis And Model Test Of Siphon Drainage In Soft Soil Foundation

Soft soil foundation treatment has always been a technical problem in geotechnical engineering.The meanBased on the advantages of well point dewatering technology and the traditional sand well foundation method,this paper creatively puts forward a method of siphon drainage combined with surcharge to treat soft soil,and carries out theoretical analysis and model test on related problems.(1)In view of the characteristics of small permeability coefficient and strong siphon drainage capacity of soft soil,the seepage problem of vertical drainage penetrating soft soil layer is treated as unsteady flow problem of pumping water in integral well with constant drop depth.The approximate solution which is convenient for application is derived.The comparison shows that the approximate solution is in good agreement with the strict analytical solution.In the practical application of constant drawdown pumping project,the variation of drawdown and velocity obtained by the approximate solution is also consistent with the measured data.(2)On the basis of unsteady well flow theory and in view of the technical characteristics of siphon drainage in soft soil,an unsteady well flow model with fixed drawdown depth for diving well without considering overflow is proposed.By using Boltzmann transformation,the analytical solution of the mathematical model is obtained.The analytical solution can describe the change of groundwater level and siphon discharge in soft soil during siphon drainage.Compared with the results of classical analytical solutions and test results,the proposed analytical solutions have smaller error and can meet the needs of engineering applications.(3)By means of physical simulation,the variation characteristics of pore water pressure,surface subsidence and drainage discharge in soft soil after siphon drainage combined with surcharge loading are studied.The variation of pore water pressure and settlement calculated by traditional consolidation theory of sand well foundation is compared.It shows that radial seepage consolidation is dominant in the soil when siphon drainage combined with surcharge preloading is used to treat foundation.By reducing the water level in plastic drainage panels and promoting the seepage of groundwater in soft soil,the drainage rate of soil can be increased.And the seepage direction is from top to bottom,which is consistent with the direction of soil self-weight,and can accelerate the consolidation settlement of soil.(4)By comparing and analyzing the pore water pressure,consolidation degree and flow change in vertical drainage under siphon drainage combined with surcharge consolidation method,the model test of surcharge consolidation without siphon drainage and siphon drainage consolidation model test without surcharge,it is shown that the proposed method of siphon drainage combined with surcharge consolidation can improve soil strength more quickly.At laboratory scale,siphon drainage can shorten the consolidation time of soil by about 20%.Simultaneously,the results show that the final surface settlement and settlement difference with surcharge are larger than those of siphon drainage consolidation test without surcharge.However,the settling time and pore water pressure stability time are similar.Surcharge can not improve the drainage efficiency of soil,but it makes the soil more compact.It can be concluded that siphon drainage combined with surcharge technology has obvious advantages.(5)The model test of siphon drainage group wells was carried out.Model tests of single well,double well and three wells were carried out respectively.By analyzing the changes of pore water pressure at the bottom of soil,surface settlement of soil and siphon flow in drainage plate,it is shown that the time for pore water pressure and settlement to reach stability under group wells is significantly shorter than that under single wells.(6)Based on hydrodynamic theory and particle settlement model,the motion characteristics of soil column under the action of siphon water flow are analyzed.It shows that the soil column is easy to form silting in the curved arc section of the siphon.The silting can be reduced by reducing the radius of the curved arc section of the siphon,the height of the siphon outlet and the length of the siphon.