Slurry Preparation And Engineering Application Of Super-Long Bored Pile In Soft Soil

Super-long bored pile is widely used in deep soft soil foundation engineering because of its high bearing capacity and little settlement.With the increase of pile length,there are some problems in the construction of super-long pile in complex soft soil layer,such as hole collapse,necking and sediment thickness.As the“blood”of the bored pile,slurry characteristics is of vital importance for pile foundation successfully forming holes,keeping the pore wall stable and carrying sediment.Slurry with special properties is needed in order to meet the construction requirements of super-long pile foundation in special areas.From this,the super-long bored pile of a great project Yellow River Bridge under construction in Jinan,Shandong Province is taken as the research object in this paper.Theoretical analysis and numerical simulation are carried out on the mechanism of slurry wall protection and hanging slag in super-long piles,and the optimal ratio of slurry is obtained by indoor slurry modification test.The new modified pore-forming slurry is successfully applied to the construction of super-long pile in this project,which provides support for the dynamic adjustment of slurry ratio in field construction,solves the problem of pile foundation pore formation,and offers reference for the construction of super-long pile in this area.The main research contents and conclusions are as follows:(1)Studied the mechanism of slag suspension and wall protection of super-long pile drilling slurry.In this paper,the drilling slag is assumed to be a spherical particle with uniform diameter,analyzed the migration law of drilling slag particles in the drilling,established the calculation formulas of drilling slag settling velocity and slurry return velocity,proposed the discriminant method of drilling slag return under positive and reverse circulation,and the feasibility of this method is verified by engineering examples;The three-dimensional instability model of drilling is established in order to research the three-dimensional stability of drilling hole with slurry wall protection.Considering the annular force of hole wall and energy dissipation of slurry circulation work in hole,the upper limit solution of three-dimensional stability coefficient of pore is derived by the limit analysis method and combining with the virtual power equation and strength reduction method.This solution can provide a theoretical reference for the analysis of the influence of slurry on the stability of the hole wall during the construction of bored piles.(2)Based on the Fluent and FLAC 3D,numerical simulation of slurry carrying effect and wall protection effect is carried out under different working conditions.In the numerical simulation,on the one hand,many factors are researched to affect the slurry carrying performance,such as different drill pipe rotation speed,inlet velocity,particle diameter,relative density and slurry viscosity.The results show that,the increase of the diameter of the drilling slag particles is not conducive to the slag carrying of the slurry.Increasing the rotation speed of the drill pipe,the inlet velocity of the slurry,the relative density and the viscosity is beneficial to improve the slag carrying effect of the slurry.It should be noted that the slurry indicator should not be increased blindly,and dynamic adjustment should be made according to the actual working conditions.On the other hand,many factors are also researched to affect the slurry wall protection,such as borehole diameter,borehole depth and slurry specific gravity.The results show that,the larger the borehole diameter and the deeper the depth,the smaller the mud proportion,the greater the transverse displacement of the drilling hole wall,the less conducive to the hole wall stability.It can provide reference for the field construction according to the simulation results.(3)Carried out the wall protection slurry modification test for the Yellow River alluvial plain area with silt,silty clay and fine sand geology.The effects of soda ash,carboxymethylcellulose,polyacrylamide coagulant and barite powder on the main indexes of bentonite slurry were analyzed through laboratory tests.The optimal ratio of slurry for general strata in this area was obtained by orthogonal test and significance analysis method,and the microstructure of modified slurry cake was analyzed by scanning electron microscope test.The results show that,the increase of bentonite content from 6%to 8%can increase the specific gravity of slurry from 1.036 g/cm~3 to 1.052 g/cm~3,and this demonstrates that increase of bentonite content will cause the increase of slurry thickness.The viscosity of slurry increases from 18 Pa·s to 23 Pa·s,and the water loss rate of slurry decreases from 18.2 ml/min to 15.2ml/min when the content of CMC increases from 0.1%to 0.5%,and this demonstrates that CMC can adjust the colloid rate and static shear force.Increasing the soda ash incorporation from 0.3%to 0.5%of the bentonite mass can increase the slurry pH from 6.7 to 10.SEM revealed that the slurry cake still maintained the irregular flake structure of bentonite,and the modified polymer molecules were adsorbed on the surface of bentonite particles,which made the overall structure of the slurry cake more compact and enhanced the wall protection effect and filtration loss reduction effect of the modified slurry.(4)Combined with the field application of the project,the drilling slag during the drilling process is sampled and tested.According to the actual geological conditions on the site,a new type of slurry is prepared and tested the slurry index during the construction process.The test is qualified and successfully applied to the project.This application verifies the the applicability of the modified slurry ratio,which provides support for the dynamic adjustment of the slurry ratio in the field construction and reduces the hole cleaning time and the connection time of each process.It effectively solves the problems of hole collapse,difficult hole clearance and thick sediment in early construction,and provides reference for the construction of super-long piles in this area.