Deformation And Influencing Factors Of Nanjing Metro Foundation Pit Considering Small Strain Characteristics Of Soil

In the urban construction,in order to protect the existing structures around the subway foundation pit,the deformation of the subway foundation pit is often controlled to a small extent.It is found from field measurement data that the soil around the subway foundation pit is generally in a small strain state under working load.The stiffness of soil is related to strain,and it has obvious nonlinearity.However,in engineering practice,little consideration is given to the influence of changes in the stiffness of the surrounding soil on the deformation of the foundation pit.The paper relies on the subject of"Standardization Design of the Foundation Pit Retaining Structure of Nanjing Metro in the"13th Five-Year Plan"Period of Nanjing Metro".A series of indoor experiments were carried out on the soft soil of the Yangtze River floodplain in Nanjing,combined with on-site monitoring and numerical simulation methods to study the deformation of the subway foundation pit and influencing factors considering the small strain characteristics of soil.The main research contents are as follows:（1）The Nanjing area was divided into engineering units.The typical soil layers and parameters of each area were counted,and the commonly used subway foundation pit retaining structures were summarized.The foundation pit project composed of the retaining wall and internal support in the soft soil of the Yangtze River floodplain was used as the research object in this paper.（2）The consolidation test and triaxial test of the soft soil of the Yangtze River floodplain in Nanjing were carried out,and the HSS model parameters of the soil were calibrated.The empirical formulas for the maximum shear modulus of the soils were fitted by experimental data.The proportional relationships of_1-2,₀（9）⁰1)),₅₀⁰1)),⁰1))and₀⁰1))were summarized.（3）The small strain tests of the soft soil of the Yangtze River floodplain in Nanjing were carried out by using the GDS stress path triaxial apparatus combined with the bending element and the Hall effect sensor,revealing the small strain characteristics and path-related characteristics of the soil during the excavation of the subway foundation pit.The research shows that the shear modulus of soil is highly nonlinear in the small strain range.With the increase of shear strain,the shear modulus of soil is significantly attenuated.The shear modulus of the soil is closely related to the stress path.The shear modulus of the soil sample under the TE stress path is significantly larger than that under the RTC stress path.The maximum shear modulus of the soil obtained by the bending element and the Hall effect sensor is significantly different.（4）Based on the foundation pit of a Nanjing subway station,PLAXIS 2D numerical simulation of the foundation pit project considering the small strain characteristics of the soil was carried out.Combined with the on-site monitoring data,the deformation of the foundation pit and influencing factors during the excavation of the subway pit were discussed.The results show that the numerical simulation results of the excavation of the foundation pit considering the small strain characteristics of the soil are in good agreement with the field measurements.The ground surface settlement of the soil behind the wall is funnel-shaped,and the horizontal displacement of the wall increases first and then decreases with the depth of the wall.Both surface settlement and horizontal displacement increase with the increase of the excavation depth.Applying the support prestress,increasing the thickness of retaining wall and reducing the support spacing can all reduce the ground surface settlement and horizontal displacement of retaining wall.However,none of these changes the deformation morphology,and does not change the position of maximum surface settlement and maximum horizontal displacement.