Field Test And Numerical Study On Mechanism Of Long-short Composite Retaining Piles

In recent years,with the economical development in China,the number and scale of excavation engineering increase largely and the supporting methods of foundation pits are also presenting new changes.In the existing design methods of retaining structure for excavation,the method of equal-length pile is generally adopted.As the bending moment of the piles below the pit bottom decreases with depth,the bending moment at the low part of the piles cannot give full play when the pile is arranged in the form of equal length.However,the bending potential at the lower part of the piles can be fully developed by applying combination of long and short piles,which is gradually applied in engineering.Firstly,this paper summarized the research progress of numerical analysis of long-short composite retaining piles and excavation engineering at home and abroad,and concluded the deficiencies of existing researches.Based on this,some researches were made through field tests and numerical simulation on the working characteristics of long-short composite retaining piles.（1）Based on the deep excavation of niandaixingceng basement in Nan Chang,the scheme design of field test for retaining piles in a certain area of the project was carried out.The spacing arrangement of the long pile and the short pile was conducted.The lengths of the long pile and the short pile were 21.85m and 19.85m respectively.During the excavation process and the removal of internal support,the internal force and deep horizontal displacement of the piles were measured using the inclinometer and vibration string frequency receiver,under the condition that the ratio of long and short piles was one long and one short,one long and two short,and one long and three short.（2）Based on the small strain theory,the SS-HCA GDS system and oedometer apparatus were used to complete a large number of laboratory tests,which are obtained as the prime parameters of HSS model,such as c,,?,,R_f,E₅₀^{re f},E_ur^{re f},E_oed^ref for site of clay layer No.2,muddy silty clay layer No.3,silty clay layer No.4 and sandy powder soil layer No.5 in Nan Chang,The relationship among the modulus was eatabilshed and the experimental results were compared with those of existing literatures.（3）The PLAXIS 3D finite element software was used to carry out numerical simulation of the field test project.The HSS model was selected as the soil constitutive model,using the parameters taken from laboratory test.The numerical simulation results were compared with the measured results.Subsequently,under the condition of different lengths of short piles and spacings,the deep horizontal displacement and bending moment of the long and short piles,as well as the ground settlement,bottom heave and the axial force of supporting were studied through PLAXIS 3D.The three dimensional working mechanism of long-short composite reraining piles is discussed in depth.