Study On Frost Heaving Deformation Of Support Structure For Overwintering Deep Foundation Pits

The distribution of frozen soil in China is extremely extensive.The seasonal frozen soil accounts for approximately 53.5% of the country's total area.The foundation pit projects for large-scale projects such as subway stations and high-rise buildings in these areas generally have a long duration,and inevitably,the bare pit has entered the winter break period without constructing the main structure.The problem of stability of the support structure caused by the frost heave of the foundation pit during overwintering becomes increasingly prominent.Therefore,it is necessary to study the mechanism of frost heaving and the deformation law of the support structure in the deep foundation pit during overwintering,which provide a basis for the safe wintering of the foundation pit.Based on the deep foundation pit project of a station on Harbin Metro Line 3,theoretical analysis,empirical methods,numerical simulation and on-site monitoring methods were used to study the frost heave mechanism of soil and the wintering process of deep foundation pit.The relationship between the two research contents lies in the micro-establishment and macro-application of frost heave model.The main research contents of this paper are as follows:(1)At the mesoscopic level,based on the mechanism of soil frost heaving,the governing equations of temperature field and moisture field in the frozen soil were derived.The coupled equations for water and heat of the frozen soils were established.The distribution of temperature and moisture distribution during soil freezing was analyzed based on related examples.(2)By designing a series of numerical simulation experiments,the change of ice content and total water content after freezing of soil were obtained.The relationship between soil frost heave coefficient and temperature in open system was deduced and fitted into correlation function.(3)At the macro level,an effective 3D numerical model was established using the universal finite element analysis software ABAQUS to simulate the excavation process of the deep excavation.The change of the axial force of the internal support and the horizontal displacement of pile were analyzed.The comparison of the results with the monitoring results verified the rationality of the model.The initial temperature field before winter and surface temperature model of the soil were established.Using the function of soil frost heave coefficient and temperature,the whole process of over-wintering of the foundation pit was simulated,and the causes of frost heave deformation of the foundation pit supporting structure were analyzed.Finally,several protective measures for the wintering of foundation pits are proposed from different perspectives,and the thermal insulation board method was analyzed in depth.The prediction model of frost heave and simulation of wintering of foundation pits established in this paper can not only provide reference for practical projects,but also lay a foundation for solving similar problems in the future,and has certain scientific value.