Experimental Study On Bearing Characteristic And Failure Mechanism Of Rock-socketed DX Pile

With the rapid development of economy,the scale of modern engineering constructions is ever increasing,which has stringent requirements of the foundation.Great progress has been made in pile foundation,the pile types are mainly large-diameter,overlength and variable cross section.However,the quality of piles may not be guaranteed when the diameter and length are too large.The new type cast-in-place pile with expanded branches and bulbs(commonly known as DX pile)has advantages of advanced technology,facility and high quality and used widely in high-rise building foundation,large industrial buildings,LNG storage tanks,railway and highway bridge foundation engineering.On this basis,attempts have been made to replace the conventional rock-socketed piles with DX pile by setting a bulb in the highly weathered stratum and resting the pile tip on the upper section of moderately weathered layer,which adequately meet the demands of bearing capacity and pile head displacement,reduce pile length and diameter and be successfully applied in the foundation engineerings in Guangdong and Guangxi with significant economic profits.The native researchers focus mainly on the bearing characteristic of DX pile in soil strata,while relevant study on the rock-socketed DX pile is inadequate.Figuring out the bearing performance of rock-socketed DX pile and establishing scientific design methods are of great significance at the stage when large amount of overlength piles are need in foundation engineerings of modern constructions.Based on the stock yard of steel project in Fangchenggang,series of field load tests,model tests and numerical simulation are conducted in order to explore the bearing characteristics and failure mechanism of rock-socketed DX pile,the conclusions are mainly as follows:(1)The Q-s curves of rock-socketed DX piles increased progressively,no sign of failure occurred at the later stage of loading.When unloaded,the residual displacement and rebounce of pile head were small,which indicated that the method of setting bulb in highly weathered argillaceous siltstone and resting the pile tip on the upper section of moderately weathered argillaceous siltstone layer would enhance the capacity of single pile to significant extent,and the pile was uncompressible,with small differential settlement and outperforming ability of displacement control.(2)The Q-s curves of rock socketed model DX piles were gradually increasing,while the settlement of straight piles increased sharply in the later stage of loading.The bearing capacity of rock-socketed model DX piles were 118?131%higher than straight piles under the same condition,also,when straight piles reached the limit,the pile head displacement of DX piles under the corresponding load were 80?86%smaller than straight piles zka.When socketed depth of straight piles zkb increased by 2d-3d,the bearing capacity of DX piles were still 56?64%higher than the former,and when straight piles reached the limit,the pile head displacement of DX piles under the corresponding load were 63?66%smaller than straight piles,which indicated that the bearing and settlement resisting ability of rock-socketed DX piles were excellent.(3)The bearing capacity of pile side,bulb and pile tip was mobilized simultaneously when the pile head settlement was small.When the applied load was comparsively small,the side resistance was the highest;bulb resistance was larger than the tip resistance.When the applied load increased,the load sharing ratio of side resistance,bulb resistance and end resistance remained stable;when the applied load further increased,the load sharing ratio of bulb resistance began to decrease while the end resistance increases gradually and bulb resistance was exceeded by tip resistance.(4)The rock socketed DX pile was classified as frictional end-bearing pile in that the bulb could effectively bear the load in the early stage of loading and control the displacement of pile head.With the increasing of the applied load,regional shear failure occurred in the rockmass beneath the bulb,and the excessive load was transferred to the pile tip.The bearing capacity of pile tip was mobilized to a significant extent,which coordinated with the bulb and pile side to ensure stability of bearing system.(5)The additional stress in rock mass distributed nonuniformly beneth the bulb,of which the outer portion was higher and shearing fracture would occur in this place and evolve towards the pile due to stress concentration.The additional stress value of the rockmass beneath the pile tip remained the highest and decayed with depth.When the shear fracture bands developed,the stress value beneath the pile tip increased rapidly.(6)Four stages of the failure process of rock-socketed DX pile were put forward according to the Q-s curve obtained from the model test,stress increasing characteristicsin the rockmass around the pile and results from the numerical analysis,namely:elastic stage,plastic stage before yielding,plastic stage after yielding and failure stage.