Research On Consolidation Settlement And Negative Skin Friction Of Piles In Reclamation Project

With the need of scientific layout of national petrochemical industry and intensive development of safety and environmental protection,many of the refinery bases are built on land reclamation sites,and the pile foundation is the main basic form.Due to the deep soft foundation in the reclamation areas and the consolidation after site pretreatment,negative friction resistance is likely to occur after construction of the pile foundation,which will significantly increase the amount and investment of the pile foundation.However,the current pile foundation theory cannot reasonably solve this problem.Taking the petrochemical base project in Zhoushan as an example,the pre-treatment of rapid land reclamation for land reclamation employs a plastic drainage board and surcharge preloading scheme,and the on-site monitoring of ground settlement after pretreatment is significant.If the consolidation has not been completed,follow-up foundation treatment would be carried out due to the need of construction period.Prestressed pipe piles and bored piles will be used.The consolidation settlement of the foundation will inevitably produce negative friction resistance at the pile circumference.The accurate analysis of the post-construction settlement and the negative friction resistance of the pile foundation in the rapid-reclamated site is a prerequisite for engineering design and construction.In this dissertation,both the centrifugal foundation model tests and the numerical analysis were employed to study the post-construction settlement and differential settlement of soft foundation and negative friction resistance of pile foundation.Through the comparative analysis of the main physical and mechanical parameters of the foundation soil before and after treatment.the consolidation settlement of land reclamation is studied,the main influencing factors of consolidation settlement are analyzed,and the long-term ground settlement deformation laws of the site are preliminarily predicted by using centrifugal model test,plaxis finite element method,and hyperbolic curve prediction method.The research on the negative friction resistance of pile foundation adopts the combination of centrifugal model test and midas finite element numerical simulation.The distribution law of axial force and negative friction resistance of the pile body and its variation with time were analyze,and technical support for the follow-up upper structure design and construction were provided.The main research work and achievements of the paper are as follows:1)It is obvious that the pretreatment with plastic drainage plate and stacking preloading,not only accelerate the drainage of the drainage plate area,but also promote the consolidation of the silty clay layer under the reinforcement zone.Through two centrifugal model tests,the conditions of thefoundation treatment were compared and analyzed.The results show that in the same formation conditions,the total settlement amount of 1 year after working without drainage plate is 550 mm,while that of the drainage plate is 1141 mm,and the consolidation rate of the drainage plate is about2 times that of the drainless plate.The total settlement amount of the five years after their work is803 and 1338 mm respectively,the latter has been basically consolidated,and the consolidation degree of the drainless plate is only about 50%.2)There are significant influences on the total settlement of soft soil layer in the subterranean formation of soft soil foundation.Through centrifugal model test of three groups of different strata,the underlying soil layers were studied as bedrock,20 m thick silty clay layer and 5m thick gravel layer respectively,The result shows that the soft soil layer of the bedrock is about 1338 mm,the soft soil layer of the lower layer is silty clay and is about 2020 mm,and the gray clay layer of the lower layer is about 1596 mm.Gray silty clay accounts for 66% of the total settlement.3)The comparison with the field measured results shows the rationality of the centrifugal model test and the numerical analysis.Centrifugal model tests showed that the total settling settlement after1 year was about 1587 mm,the value obtained by numerical analysis was 1528 mm,and the average value obtained from field measurement was 1413 mm.The foundation settlement amount obtained by centrifugal model test and numerical analysis is consistent with the field monitoring results.4)The position of neutral point in the negative skin friction of piles is significantly affected by the holding force.Through centrifugal model test of the negative skin friction of piles in a study of the factors,such as pile bearing strength layer,pile diameter.the results show that when the pile bearing strength layer is a deep soft foundation,the depth ratio ln/l0 of the neutral friction point of pile foundation is 0.5~0.62(ln is the depth of the neutral point and l0 is the minimum deformation depth of the soil deformation around the pile).When the pile bearing layer is silty clay,the depth ratio of neutral point is ln/l0 is about 0.62~0.7.when the bottom of the pile end is placed on the bedrock,the depth of the neutral point exceeds ln/l0 by more than 0.9 and approaches 1.5)There are obvious differences in friction resistance distribution between friction piles and end-bearing friction piles.For friction piles,the axial force of the pile first increases and then decreases in the depth direction,and the position of the neutral point is at the place where the axial force of the pile is the largest.And with the increase of consolidation time,the neutral position gradually rises,and eventually becomes stable.For end-bearing piles,the axial force of the pile gradually increases along the depth direction.The maximum value of the axial force is basically nearthe bottom of the pile.Basically,the entire length of the pile bears negative friction resistance,and the neutral point is located near the bottom of the pile.