Design And Example Analysis Of Deep Foundation Pit Support Structure In Suzhou Area

Limited urban land limits further construction and development of the city.Today’s urban construction mostly revolves around the two themes of high altitude and underground,which shows that the development and utilization of urban underground space is one of the important measures to alleviate urban land tension and is also an inevitable requirement to promote sustainable urban development.High-rise buildings and all kinds of underground projects can not be carried out without the excavation and support of deep foundation pits.With the increasing density of urban buildings,the difficulty of deep foundation pit support has been greatly increased.Selecting the right type of support is the first issue of deep foundation pit project,which is directly related to the safety of the whole project,project budget and construction period.Deep foundation pit project has distinctive locality,and the selection and design of foundation pit support structure is influenced by geological conditions and environmental conditions.Suzhou is located in the plain area of Yangtze River Delta with well-developed water system,which is a typical soft soil area.Marked by the start of construction of Suzhou Metro Line One in 2007,Suzhou has entered a new stage in the history of underground space development and utilization.It is of both social and economic significance to study the typical cases of deep foundation pit projects in Suzhou and to explore the type and design method of deep foundation pit support applicable to the region.The main research and results of this paper are as follows.(1)The unique geological characteristics of the Suzhou area were studied,and the distribution of subsurface pressurized water layers and major weak soils in the area were compiled based on geological data and relevant literature,based on which the geological conditions of each area in Suzhou were evaluated.(2)Based on a large number of deep foundation pit projects in Suzhou,the general process of selection and design of deep foundation pit support structure in Suzhou is summarized,and the structural principles,applicability and stability calculation methods of several common support types in the area are systematically described.(3)Taking a new project in Suzhou Industrial Park as an example,the hierarchical analysis method was introduced to make a decision on the foundation pit support scheme for the project.According to the actual situation of the pit,12 indicators were selected from four aspects: safety,economy,construction impact and technical feasibility,and the weights of each indicator were determined by reviewing literature and expert survey method,and a hierarchical structure model was established to comprehensively evaluate the three alternative support solutions.The final decision result was found to be in line with the actual situation,indicating that bored piles combined with water stop curtain as the enclosure structure is the most suitable.The decision-making process can provide a reference for the application of hierarchical analysis in the selection of deep foundation pit support scheme.(4)The finite element analysis software MIDAS GTS NX is used to simulate the excavation and support of a new project in Suzhou Industrial Park.The soil settlement caused by the pit precipitation and excavation,the displacement of the support piles and its change during the excavation of the pit,the axial force on the internal support,and the vertical displacement of the columns and column piles are analyzed.The simulation results show that all values are within a reasonable range,and compared with the calculated values,there are some errors but they are also within a reasonable range.Finally,according to the simulation results,the impact of the foundation excavation on the surrounding environment and the support effect of the support structure were evaluated.(5)On the basis of the above numerical simulation,the change law of horizontal displacement of the supporting pile is studied and its causes are analyzed by two methods:changing the diameter of the supporting pile(thickness of the enclosure wall)and the size of the inner support section.Finally,it was found that the variation of the pile diameter of the support pile and the cross-sectional dimensions of the internal support had a specific influence on the overall stability of the support system,and the conclusions obtained could be used to guide the actual engineering design.