Research On Drainage Effect And Bearing Characteristics Of Pervious Concrete Pile

The construction of embankment on deep soft soils is a widely encountered challenge,and in the eastern coastal region of China,many soft grounds with the thickness of 30～50 m need to be treated.For the conventional methods,the drainage consolidation method is time-consuming,and being low efficiency.For PHC piles,it is difficult to determine the quality of pile joints,and the economic efficiency is low.For the cement mixing piles,it is difficult to control the quality of piles after the depth of mixing piles exceeds 15 m.Therefore,a pervious pile foundation treatment that combines the advantages of rigid pile and drainage consolidation method is proposed.It has been shown that the pervious piles can significantly improve the seismic and liquefaction resistance of foundations.However,for the pervious concrete piles,the pressure-reduction effect in the pile installation process,the bearing characteristics in the loading process and the settlement calculation method have not been fully revealed.and the current results are not enough for the guidance of practice.By means of indoor tests,model tests,numerical simulations and theoretical analyses,this paper systematically studied the drainage effect,the bearing characteristics and the settlement calculation method of pervious concrete piles in deep soft soils.The main research contents and results are as follows:（1）The pervious concrete specimens with different particle sizes（3-5 mm and 4-6 mm）and porosities（20%,25%,30%and 35%）were prepared,by comparing strength,water permeability and effective porosity,the aggregate size of 3～5 mm and the porosity of 30%were selected as the preferred mix proportion.Considering both the similarity constants and model dimensions,the model pervious concrete piles were designed and prepared,with the modulus of elasticity being 20.1 GPa.（2）In the range of 1D（D=70 mm）from pile center,the excess pore water pressure of soil is significantly influenced by pile installation.And the acceleration effect of excess pore water pressure dissipation by pervious concrete piles increases with the increase of foundation depth.Compared with the impervious concrete piles,the pervious concrete piles can increase the dissipation rate of excess pore water pressure by about one time integrally.,and it can reduce the peak value of excess pore water pressure by 46%at 200 s after installation.Compared with the soil range of 4D,when the consolidation degree exceeds 60%,the difference of consolidation rate of soil between the 2D and 4D range by pervious concrete pile gradually becomes obvious.The influence ranges of the final lateral deformation and lateral stress are4.9D and 6D respectively,and they all decay exponentially along the radial direction with the increase of distance.During the installation process of pervious concrete pile,the peak value of excess pore water pressure increases linearly with installation rate and pile diameter ratio.With the increase of pile diameter ratio,the horizontal lateral deformation,the range of pile tip extrusion zone and pile side plastic zone all increase,and the growth increases first and then decreases.The maximum effective stress in installation and consolidation process increases linearly with pile diameter ratio,and the maximum effective stress in consolidation process is smaller but the growing rate is larger.（3）When the pervious concrete pile is used as the vertical reinforcement in composite foundation,compared with impervious concrete pile composite foundation,the bearing capacity characteristic value of pervious concrete pile composite foundation is 21%higher,and the average skin friction of pile is 64%higher,the pile-soil stress ratio is smaller before pile doward penetration occurs,and the load sharing mode is more reasonable.The results of the additional stress in the foundation show that the stress dispersion effect of the reinforcement layer formed around the pervious concrete pile is more obvious.Compared with clay foundation,the bearing capacity of the silt soil foundation reinforced by the pervious concrete pile is 16%higher,and the average skin friction of pile is 38%higher.In the loading process,the bearing capacity of the foundation soil is improved obviously and more loads is shared by the soil,so the pile-soil stress ratio and axial force in the early stage are smaller.Although the initial settlement of composite foundation is relatively large when the underlying layer of pervious concrete pile is silt soil,the skin friction of the pile is 3.5 times that of pervious concrete pile in the underlying sand soil layer.Therefore,the characteristic value of the bearing capacity of pervious concrete pile composite foundation in the underlying silt soil layer is only reduced by33%.At the initial stage of loading,although the maximum difference of pile-soil stress ratio between the two foundations is 3.8 times,with the increase of pile bearing capacity in the underlying silt soil foundation,the pile-soil stress ratio eventually tends to be the same,showing similarities in bearing characteristics.（4）Based on the skin friction of pile and DPT results of the foundation soil,the characteristic value of bearing capacity was estimated.The results show that,when the replacement ratio of composite foundation was reduced from 9.26%to 2.32%,the characteristic value of bearing capacity was reduced by 14%,and appropriate loading can significantly improve the bearing capacity of pervious concrete pile composite foundation with low replacement ratio.Since the compaction of soil around the pile and the consolidation effect of drainage decreases radially,the pile-soil stress ratio in the foundation of m=2.32%is larger at the initial stage of loading.As the total load shared by the foundation soil is more,and less load shared by the pile,the axial force and skin friction of the pile did not show a significant increase with the load.When the load is 2800 N,the skin friction of pile when m=2.32%is only 51%of the pile when m=9.26%（5）The reason for the high bearing capacity of pervious concrete pile composite foundation is that,an artificial reinforcement layer with higher bearing capacity in the foundation was formed by the pile drainage consolidation,which can share more loads,and the additional stress in the foundation is more equally distributed.Besides the consolidation can accelerate the skin friction increase rate of the pile,both of these are beneficial to reduce the settlement of the underlying soft soil.As the thickness to width ratio（H/B）and elastic modulus ratio(E_hard/E_soft)of the reinforcement layer increases,the stress dispersion effect and board-like properties will also gradually increase,but the growth gradually decreases.The skin friction calculated byβmethod suggested by Chandler is in good agreement with the measured results,and the ultimate skin friction is larger at top and bottom,and being smaller in the middle.The calculated shear strength increment results show that,the increment of shear strength around the pile decreases first and then increases with depth,and the average shear strength increment of soil in the 1D range around the pile is 18%higher than that of the soil in 2D range.If the strength of pile-soil contact surface is sufficient,for the adjacent foundation soil of the pile,shear failure will occur at the position of 0.7～0.8 times the pile length and at the bottom of the reinforced layer first.（6）Based on the distribution of skin friction of pervious concrete pile obtained from the test,the settlement calculation method of pervious concrete pile composite foundation under embankment load is proposed.According to the reinforcement mechanism of pervious concrete pile composite foundation,the consolidation of soil around piles in the reinforcement layer of composite foundation is simplified as a spatial axisymmetric problem.Assuming that the permeability coefficient of pervious concrete pile decreases with time as an inverse proportional function,an analytical formula of the consolidation degree for pervious concrete pile composite foundation is proposed.