Study On Mechanical Characteristics Of Super Wide And Deep Foundation Pit Support Structure Under The Action Of Excavation And Temperature

With the rapid development of urban underground space,the number of foundation pit works is increasing day by day,and gradually develops towards the direction of deep excavation depth,large excavation width,complex foundation pit type and sensitive surrounding environment.The control requirements for foundation pit deformation are becoming more and more strict.Therefore,mastering the deformation and structural stress characteristics of foundation pit during construction is particularly important to ensure the safety of foundation pit engineering.At the same time,with the increase of the width and depth of the foundation pit,the stress of the foundation pit structure is more complex,and the influence of the change of ambient temperature on the foundation pit support structure can not be ignored.However,at present,there are few studies on the influence of temperature effect on the foundation pit support structure,and the accurate calculation method of the change of the support structure affected by temperature is not proposed in the specification.Based on the above background,relying on the deep and large foundation pit of the sunken tunnel section of the comprehensive transportation reconstruction project of Binhai Avenue in Shenzhen,this paper makes an in-depth study on the mechanical characteristics of the supporting structure of the ultra wide and deep foundation pit under the effect of excavation and temperature by means of on-site monitoring,numerical simulation and theoretical analysis.The main research results are as follows:(1)The deformation data of the retaining structure and the measured data of the temperature and axial force of the supporting structure are analyzed.The deformation data of the retaining structure shows that the lateral displacement of the retaining structure caused by the excavation of the soil in the silt layer accounts for a large proportion of the overall deformation of the retaining structure.At the same time,the spatial effect of the foundation pit is obvious,the maximum lateral displacement of the retaining structure near the external corner is smaller,and the maximum lateral displacement position is shallower.The maximum lateral displacement of the retaining structure is about 0.067% ～0.113%(the excavation depth of the foundation pit);The measured data of support internal temperature and axial force show that the daily temperature difference in the second and third supports is lower than that in the atmosphere,and there is an approximate linear relationship between the axial force increment of the support structure and the temperature change.The longer the support length,the greater the buried depth,and the greater the variation of axial force under unit temperature.The variation of axial force of foundation pit support structure in Guanshen intercity station section under unit temperature is 199.94 kn / ℃,and the variation of axial force caused by the maximum daily temperature difference accounts for more than 40% of the maximum axial force value of that day.(2)Using the method of numerical simulation,the behavior characteristics of supporting structure under the effect of deep and large foundation pit excavation and temperature are analyzed.In the process of foundation pit excavation,the soil excavation in the silt layer and the soil excavation in the pit are the controlling processes for the stability of the retaining structure in the process of foundation pit excavation.The surface settlement caused by the soil excavation in the pit accounts for 62.25% of the total value.At the same time,due to the deepest pit in the pit on the north side of the foundation pit,the deformation of the retaining structure and the surface settlement are greater than the rest.The third support of the main foundation pit is most affected by the excavation of the pit in the pit,and the axial force increases by about 124.23%.The influence law of temperature effect on the supporting structure of foundation pit shows that the heating has a greater impact on the supporting axial force,and the cooling has a greater impact on the deformation of retaining structure and the surface settlement outside the pit.The deformation of the retaining structure near the surface is most affected by the temperature,and the axial force of the retaining structure at the bottom of the pit is most affected by the temperature.Cooling is more unfavorable to the bending of the retaining structure,and the stress of the supporting structure has a linear relationship with the temperature.The third support of the main foundation pit is most obviously affected by the temperature change.When the unit temperature is increased,its axial force value will increase by 280.17 kn,and when the temperature is increased by 20 ℃,its axial force value will increase to 3.03 times of the initial state.(3)Based on the two parameter elastic foundation beam theory and Euler Bernoulli beam theory,the theoretical relationship expressions between the additional axial force of the support structure,the additional displacement of the retaining structure and the temperature change are derived,and the MATLAB difference solution program is compiled.At the same time,combined with the field measured data,the accuracy of the calculation formula and solution program is verified,which provides a powerful reference for the design of foundation pit support structure considering the influence of temperature.Based on the simplified calculation method proposed,the influence of various factors on the mechanical properties of foundation pit support structure under the action of temperature effect is discussed.Among them,under different excavation depth of foundation pit,the value of additional axial force caused by temperature change on the same support structure is different;When the support length reaches 40 m,the temperature rises by 10 ℃,and the additional axial force of the support structure accounts for more than 70%,which is higher than the value recommended in the specification.Increasing the compressive stiffness of the supporting structure or increasing the thickness of the retaining structure can not improve the influence of temperature effect on the stress of the supporting structure.Increasing the horizontal spacing of supporting structures can improve the influence of temperature effect on supporting structures,but the effect is weakening.