Compacting Effect Of Precast Square Piles In Lianyungang Soft Clay Foundation

Precast pile belongs to displacement pile which would replace foundation soils of the same volume.Hence it would induce predominant compacting effect in deep soft soils when group precast piles were driven,inducing excess pore water pressure and lateral deformation of soils,which brought threat to the security and stability of engineering.The research of compacting effect had a long history but for the complexity of this issue,there existed few conclusions that could be applied to engineering project,which called for more research of the calculation methods and controll method.In addition,the compacting effect had close connections with soil properties and it tended to be different in different regions and different soils.This paper was based on the wind power project of pile testing in Lianyungang,the soil foundation of which was special for deep marine muddy soils above and silty soils and silty sandy below.The compacting effect of precast piles were studied,aiming to provide experience for later large areas of pile driving construction and provide reference for the following study of theory of compacting effect.The main research work and conclusions are following.(1)Soil tests of marine muddy soils were conducted.Different depths of undisturbed soils were sampled and basic physical property tests,one-dimensional consolidation tests,GDS triaxial tests were conducted to analyze physical and mechanical properties of Lianyungang deep marine muddy soils systematically.The natural soil structure of marine muddy soils was clarified emphasically.Combined with the structural characteristic of soils,the disturbance of soil structure and the feasible engineering problems of pile driving were analyzed,providing reference for the following field tests.(2)Field tests of driving of group square precast piles were conducted,the dynamic variation of excess pore water pressure and lateral soil deformation was analyzed emphasically in terms of spatial distribution and time effects.Connections between bearing capacity and excess pore water pressure were pointed out according to measured pile skin friction.Combined with measured results of other pile test sites,some questions were discussed such as the vertical distribution of excess pore water pressure,the dissipation of excess pore water pressure and water split action,the shape of lateral soil deformation and so on,more general conclusions have been achieved.Finally,considering the low speed of dissipation of excess pore water pressure and predominant compacting effect,construction measurements which could be adopted were analyzed and summed up,providing suggestions for the construction of the project.(3)The analytical methods of excess pore water pressure induced by pile driving were clarified.The Lo&Stermac method,cavity expansion theory method and ultimate balance theory method were introduced and applied to this project.The comparison of different methods was done with reference to the measured results of excess pore water pressure.The problems in the application of analytical methods were pointed out.The lateral soil deformation induced by pile driving was calculated by Segerata sources and sinks method and the influence of different parameters on the calculated results was explored.(4)Some complex problems such as large deformation,elastoplastic constitutive law,sliding friction between pile and soil and stress field of soil's dead weight were taken into account by using the displacement penetration method and the finite element model of pile driving was established.The complete process of successive penetration of precast pile was simulated and the distribution of stress,lateral and vertical deformation were analyzed with different penetration depths.The influence of mechanical parameters and friction coefficient on the simulation results were analyzed to provide reference for engineering project.Finally,the simulation results were compared with analytical method results.