Study On Earth Pressure Of Ultra-deep Pile-anchor Supporting Structure And Analysis Of Collapse Cases

With the development of ultra-deep excavations,designers find that the measured earth pressure data in excavation engineering is non-linear distribution along the depth direction.It is not consistent with the classical earth pressure calculation model which currently used in the calculation of earth pressure.Therefore,the development and optimization of earth pressure envelope is an important problem to be solved urgently in excavation engineering.In addition,the collapse accident in excavation engineering has also caused a great social impact on the construction engineering industry.In order to prevent and avoid the occurrence of the safety accident,the analysis of the collapse resistance of its supporting structure is also the key problem to be solved in the excavation engineering.In this paper,the Plaxis3 D finite element analyses were carried out to study the changing rule of pile displacement and anchor cable axial force in ultra-deep pile anchor structure.Based on this numerical study,the earth pressure distribution of the supporting structure is analyzed by the stress of anchor cable after the completion of excavation construction,so as to obtain the earth pressure AEP envelope distribution mode which can apply to the ultra-deep excavation.Subsequently,the numerical model under a collapse excavation analyses were carried out to study the stress of prestressed anchor regularity in Nanning,Guangxi.Meanwhile,the determination factor of collapse was put forward to study the collapse resistance of excavation by analyzed the change of internal force of supporting pile,under the condition of the destruction of the anchor rope supporting function failure.The following conclusions are obtained:1.By comparing with the measured vertical settlement value of soil outside the excavation,HS constitutive model can well simulate the whole excavation process of the construction in the pile-anchor supporting structure.Based on the study of the change of pile displacement and anchor cable axial force,the conclusion have been found that when pile displacement increases,the anchor cable axial force also increases,and when pile displacement is stable,the anchor cable axial force value decreases and tends to be stable.Especially,the displacement growth rate is large in the range of 1/4 to 3/4 of the depth of the excavation during the process of excavation.2.Based on the actual simulated earth pressure distribution,as well as the existing earth pressure data and envelope line,the empirical charts have been proposed for the ultra-deep excavation by modifying the existing earth pressure envelope.3.A detailed study between the displacement of pile top and the anchor cables were carried out through the case study and analysis of collapse.The displacement of pile top decreases with the increase of the prestress of central anchor cable.When the amount of prestress is determined,the displacement of pile top increases with the increase of water level,and the rate of increase is basically the same under different prestress conditions.Under the condition of constant water level,the displacement of pile top can be reduced by8.4% ～ 10.7% with the increase of prestress.4.The determination factor of collapse RBM is put forward to evaluate the anti-collapse performance of the retaining structure of the excavation.The RBM value at the bottom of the excavation is higher than that in the middle of the supporting pile by studying the failure conditions of different anchor cables.Specially,with the expansion of failure range of anchor cable,the growth rate of RBM value at the bottom of excavation also increases gradually.5.The failure of anchor cable has a great influence on the change of internal forces at the bottom of the excavation,which makes the supporting structure at the bottom of the excavation reach the limit of bending moment before the central part of the supporting pile,so that it collapses.Based on the study,the conclusion has been found that it can effectively improve the safety of the excavation by improved the collapse resistance of its bottom.