Construction Risk And Support Scheme Optimization Of Deep Foundation Pit Excavation In Existing Buildings

With the rapid development of economic globalization,the construction of domestic cities is in full swing.The tall buildings are mushroomed and the frequency of engineering practices near the existing buildings rises year by year.Deep excavation in high-rise buildings in the area with high density faces two major difficulties:how to ensure the safety of deep foundation pit under the influence of the load of existing buildings and the hydro-geological conditions;how to reduce the disturbance during deep foundation pit construction As for the impact of the normal use of existing buildings safely.The exploration of the above difficulties has become one of the major topics in the engineering field,and the current theoretical research on the subject still lags behind the engineering practice and there is no standard enough to express the complexity of such problems.Based on this consideration,based on the example of a deep foundation pit project of a high-rise building in Changsha City,Hunan Province,the combination of numerical simulation and on-site testing is used to study the risk of excavation of deep foundation pit near existing buildings and the optimization of support schemes.The main research results are as follows:(1)The basic physical and mechanical properties of red clay were analyzed with in-situ test and indoor geotechnical test.Red clay has high water content,natural density,liquid limit and saturation.It is rich in elements such as Si,Al,Fe,Ca,K and Na.The free expansion rate is 30.7%,far less than that of expansive soil.,Is a low-compression soil.(2)Construct Hierarchy Model to analyze the influencing factors of construction safety of deep foundation pit.The factors that affect the safety of the excavation near the existing buildings are:the properties of the stratum,the influence of the adjacent construction load,the influence of groundwater,the influence of excavation and the impact of natural disasters.The numerical model was established by ABAQUS to identify the risk of foundation pit elevation,surface settlement and support structure failure,and to refine the risk of each construction step.Finally,the risk of pit excavation damage and failure of supporting structure was determined Large,need to focus on strengthening prevention and control.(3)The advantages and disadvantages of the "Diaphragm Wall and Artificial Digging Pile + Anchor Cable" support scheme are compared and analyzed;the maximum lateral displacement of the support structure 1.5mm(top),only 7.89%of the original support program.At the same time,there was no obvious cracks in the soil around the top of the foundation pit,and the quality of the connecting nodes between the cable and the double-framed I-beam was good.The compressive strength of the core-beam concrete core sample was 33.1-35.4 MPa and the tension of the anchor cable was 876-879 kN Value of 1.11 times).Verify the construction of "manual digging pile + bolt" support program to meet the design requirements.