| With the rapid economic growth in China,the process of urbanization is rapidly advancing.In the process,a large number of subway deep foundation pit projects have been produced.Due to the particularity of the construction site for deep foundation pits in subways,the excavation environment is often very complex.Especially in areas where groundwater is abundant,engineering accidents such as piping surges and drift sands caused by groundwater control failures can cause foundation pit collapse even when the conditions are serious.,bringing huge economic losses.More than 90% of the foundation pit engineering accidents in Wuhan's dual structure stratum are caused by precipitation seepage.Therefore,it is particularly important to study the safety and stability of deep foundation pits in this area.Because the formation medium usually has obvious anisotropy and non-uniformity,the parameters often change due to the influence of geological structure and construction disturbance,and this change is nonlinear.If you continue to use test values or empirical values in the stability calculation analysis,it will inevitably produce a large error with the actual situation,or even an erroneous result.Therefore,this paper is based on the displacement back analysis method and aims to invert the soil parameters that are more in line with the actual conditions of the site,and further conducts the seepage-stress coupling study on the dewatering and excavation process of the foundation pit.This paper taking the Huangpu Road Station of Wuhan Metro Line 8 as its research background,using on-site monitoring data,theoretical research,and indoor test analysis as a means to combine the optimization algorithm with BP neural network.The intelligent displacement back analysis method uses Midas GTS NX three-dimensional numerical simulation software to study the deformation characteristics of the foundation pit considering the seepage-stress coupling.The main research content is as follows:(1)Based on the monitoring data of the foundation pit of Huangpu Road Station,the characteristics of the horizontal displacement,supporting axial force,surrounding soil settlement and lateral displacement of the soil behind the wall were analyzed,and the deformation law of the foundation pit in the dual structure of Wuhan was summarized.(2)According to the equation of motion of fluid mechanics,combined with the law of conservation of energy,the law of conservation of mass and Newton's law of motion,the differential equation of the seepage field is deduced.The interaction mechanism between the seepage field and the stress field is analyzed,and the seepage differential equation considering the coupling of the stress field is deduced.Considering the influence of the anisotropic seepage field in practical engineering,a three-dimensional anisotropic mathematical model of unsteady seepage-stress coupling is established on the basis of a mathematical model of two-dimensional isotropic steady-state fluid-stress coupling.And gives the finite element solution steps of the mathematical model.(3)The indoor precipitation test was carried out by establishing the dual structure stratigraphic model.The permeability coefficient of the confined aquifer and aquitard was determined by the permeability test.The influence of the pore ratio on the permeability coefficient was further analyzed.The water stop curtain model was used.The effects of water-retaining effect of the water stop curtain and the effect of the insertion ratio on the water barrier effect were analyzed.The change law of the permeation water pressure with time was also studied,and the seepage movement characteristics of precipitation in the dual structure soil layer were summarized.(4)Considering the nonlinearity and heterogeneity of soil,based on the back analysis method of intelligent displacement combined with the optimization algorithm and BP neural network,the parameters of the nonlinear constitutive model of deep foundation pit in soft soil are retrieved.Finally,the parameters obtained from the back analysis and the survey suggestion values are respectively substituted into the numerical model for forward calculation,and the calculated values are compared with the measured values of the corresponding points to verify the accuracy and rationality of the inverse analysis results,so that the inversion The soil parameters are closer to the actual situation.(5)Using the inverse soil parameters,the finite element software Midas GTS NX was used to establish a foundation pit model that does not consider the seepage effect,a foundation pit model that only considers the influence of seepage,and a foundation pit model that takes into account the seepage-stress coupling.Considering the coupling analysis and irrespective of coupling analysis of the deformation characteristics of the foundation pit.Based on this,the influence of the change of groundwater level on the deformation of the foundation pit was further studied,and the deformation law of the deep foundation pit of a dual-structure subway in Wuhan under the coupling of seepage and stress was summarized.According to the forward calculation analysis of the displacement back analysis results,the calculation results without considering the seepage-stress coupling have some errors with the actual monitoring situation.After considering the coupling effect,the error is reduced,and it is closer to the actual value.The calculation results of the performance values are all superior to the calculation results of the survey recommendations.Within the accuracy of the engineering errors,the accuracy and rationality of the back analysis results are verified.At the same time,the back analysis results are used to predict the excavation deformation of the excavation in the later period,and it is concluded that the predicted value of the seepage-stress coupling effect is in good agreement with the actual value,which indicates that the displacement inversion analysis can provide guidance for the information construction of the foundation pit. |
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