Study On Design Method And Water-Soil-Retaining Structure Interaction Of Ultra-deep Foundation Pit

The research work of deep foundation pit has been made widely,and many important results were achieved.Many countries have promulgated Industry Standards of foundation pit.Precious design and construction experience of ultra-deep foundation pit has also been accumulated.Due to excavation depth increase,new supporting methods and construction technologies were used and higher security requirements were proposed.New questions of ultra-deep foundation pit appeared,which requires design methods and analysis theories to be improved constantly.The research works in this paper are based on two major ultra-deep foundation pit excavation projects of Hangzhou Metro Line #1 and Shanghai World Expo substation.Based on the study on water-soil-retaining structure interaction of ultra-deep foundation pit,one new retaining structure design method was proposed. The major achievements are as follows:(1)Through the stress path tests of ultra-deep foundation pit,the influence of stress path on soil parameters was studied.The test results of sandy silt in Hangzhou are as follows:for lateral unloading test,the influence coefficient X of initial tangent modulus is E_i 80;for vertical unloading test,that is changed from 470 to 510, for vertical unloading test,the strength parameter of stress-strain curves can be expressed as b=1/(σ_x^′-σ_z^′)_ult= 4.6 - 0.0043σ_z^′.(2)Through the analysis of foundation pit loading mode,one force analysis model of retaining structure was proposed,which can take into account the influence of stress path and space dimensions.Based on the Melan solutions to elastic half-space problems,stress solution and displacement solution to the problem of strip load in elastic half-space were gained.Further,the horizontal foundation coefficient applied to the above analysis model was gained,which can be expressed as K =α·λ·γ_s·z.The coefficientαreflects the influence of space dimensions, and the coefficientλreflects the influence of stress path.Combined with Duncan-Chang model,this horizontal foundation coefficient can take into account the influence of load level.The calculated results by this method were consistent with the results of continuum finite element method,which shows that the analysis method in this paper can reflect the influence of space dimensions accurately,path stress and load level.At the same time,this method can simulate the change of earth pressure accurately.(3)Combined with the constitutive relations of transversely isotropic material, one composite foundation model was gained through theoretical derivation,which can take into account the influence of engineering piles to soil in different directions. The influence of engineering piles to horizontal foundation coefficient was also gained.Due to the influence of engineering piles,the rebound of pit base decreased obviously,and the deformation of retaining structure decreased to some extent.(4)Using one homemade model test device,the model tests of foundation pit seepage flow and seepage failure were made.The influence of soil density,soil characters,Friction characteristics of pile-soil interface,engineering piles and leakage to seepage flow and seepage failure were analyzed.Through the model tests, the seepage failure mode and the change rules of seepage field and displacement field were revealed.The seepage failure mode is Wedge heave when the surface of -retaining structure is slick,and the seepage failure mode is boiling when the surface is rough.(5)Through the seepage analysis of foundation pit in homogeneous foundation,the influence of seepage to water pressure,earth pressure and horizontal foundation coefficient was gained.Based on the above study results,one retaining structure design method of ultra-deep foundation pit was proposed,which can take into account the influence of stress path,space dimensions and seepage.In this model,the lateral loading is static earth pressure and water pressure considering the influence of seepage,and the horizontal foundation coefficient should take into account the influence of stress path,space dimensions and seepage.(6)Combined the model tests with finite element simulation,the seepage failure mechanism of foundation pit was analyzed.At the same time,the standard of criterion and the decision method were proposed.Vertical efficient stress is zero, which is not the unique standard of criterion.According to plastic failure criteria k =(σ₁^′-σ₃^′)/(σ₁^′+σ₃^′),the failure process can be divided into three stages.First is critical edge stage,second is plastic failure stage and last is quicksand.(7)Through theoretical analysis and numerical simulation,the safety factors of foundation pit seepage failure were analyzed,which depend on foundation pit depth,width,retaining structure depth and substratum depth.Taking into account the safety factors and the influence of seepage failure to retaining structure,one control analysis method of ultra-deep foundation pit seepage failure was proposed.(8)Through the case study of Hangzhou Metro Line #1,the calculated results of structure deformation and earth pressure are close to the measured results.In the end,parts of study results were applied to the ultra-deep foundation pit excavation project of Shanghai World Expo substation.