Research On The Evolution Characteristics Of Temperature Field And Stress Field Of Combined Frozen Support For Foundation Pit

With the acceleration of urbanization,the number and scale of foundation pit supporting projects are increasing rapidly.Groundwater is one of the important problems faced by foundation pit engineering.Precipitation method is generally used to control groundwater in water-rich strata,but the pumping and drainage of groundwater will cause water pollution and surface subsidence.Many cities also restrict the pumping and drainage of groundwater.Therefore,how to carry out foundation pit excavation without dewatering has become an urgent problem to be solved in foundation pit construction.Based on the design concept of foundation pit support of ’ combined freezing method ’,this thesis studies the design scheme of foundation pit support.By constructing a numerical calculation model of typical foundation pit shape,the interaction mechanism between freezing curtain and underground continuous wall of foundation pit reinforced by combined freezing method and the stress state of supporting structure in excavation process are analyzed,and the feasibility of combined freezing method in water-rich strata is discussed.The main conclusions are as follows:(1)The combined freezing method can effectively realize the dewatering-free construction of foundation pit engineering in water-rich strata.The calculation results show that the whole supporting system of foundation pit is safe and reliable after adopting the combined freezing method,which meets the requirements of relevant specifications.(2)In the case of this thesis,the freezing curtain completed the intersection after20 days of active freezing,and initially had the water sealing effect.After 50 days of active freezing,the freezing curtain met the design requirements.Because there is no groundwater problem in the early stage of foundation pit excavation,it is suggested that two processes of freezing and excavation should be arranged according to the formation conditions and the development of freezing curtain.It is not necessary to excavate after all the freezing curtains reach the standard according to the traditional freezing method.When the freezing pipe is kept at a certain distance from the excavation bottom surface(0.5m in the calculation),the excavation bottom surface will be in the state of high temperature frozen soil.At this time,the bottom surface has certain bearing capacity and is convenient for operation,and will not produce obvious excessive freezing.It can be considered that the freezing curtain has no effect on the excavation of the foundation pit.(3)Due to the radial arrangement of the freezing pipe,the thickness of the freezing curtain is continuously weakened along the drilling direction.In this case,the thickness of the freezing curtain near the underground continuous wall is about 75 % of the thickness of the freezing curtain near the temporary shaft,and the temperature field has obvious weakening area,which should be paid attention to in construction,and the area with the largest freezing hole spacing is used as the key node for the evaluation of the freezing effect.(4)Affected by the freezing curtain,the deformation of the overlying soil on the underground continuous wall and the foundation pit occurs.The maximum normal frost heave force on the underground continuous wall is 47.2 k Pa,and the diffusion trend of the underground continuous wall under the normal frost heave force is cylindrical diffusion along the contact position between the freezing pipe and the underground continuous wall.The growth trend of horizontal displacement increases rapidly and then tends to be stable.The maximum displacement of underground continuous wall under normal frost heaving force is 1.54 mm.The freezing method causes the vertical displacement of the soil around the foundation pit,and the maximum frost heave vertical displacements of the inner and outer soil of the underground continuous wall are 5.7 cm and 2.86 cm,respectively.(5)Under the influence of foundation pit excavation,the area with the largest compressive stress on the underground continuous wall is the contact position of the two sides of the underground continuous wall,with a value of 1850 k Pa,and the area with the largest tensile stress is the frozen curtain area and the upper end area of the underground continuous wall,with a tensile stress value of 750 k Pa.The horizontal displacement of the diaphragm wall decreases gradually along the longitudinal direction,and the maximum displacement appears at the top of the diaphragm wall,which is 1.26 mm.With the increase of confined water head,the tensile and compressive stress of freezing curtain increases linearly.For every 1 m increase in the confined water head,the tensile stress and compressive stress on the upper and lower surfaces of the freezing curtain increase by 1.1 k Pa and 0.9 k Pa,respectively.