Borehole Wall Stability Analysis And Slurry Ratio Optimization For Construction Of Pile In Complex Marine Strata

With the continuous application and development of cast-in-place bored pile technology,many engineering accidents such as hole wall collapse,hole wall shrinkage and stratum fracturing occur in bored pile engineering.At present,the stability of borehole wall mainly depends on the protective mud.It has significant engineering significance and research value to study the stability of borehole wall and the action law of protective mud of bored pile.Based on the Meisha pile foundation project in Yantian area,this paper carried out theoretical model research on borehole wall stability under mud action,mud ratio and mud characteristics test,PFC discrete element simulation research on borehole wall stability under mud protection,and achieved the following results:(1)A theoretical model of hole wall stability under the action of slurry was established by using circular hole expansion theory,and the elastic-plastic theoretical solution of soil stress and strain around the hole wall was solved.The stability of hole wall was defined as slurry stability and mud film stability,and the safety factor of hole wall stability under the action of slurry was given.The influencing factors of hole wall stability and the characteristics of hole wall instability under stratum condition are analyzed.(2)Mud proportioning research results show that mud solid phase material composition mainly controls the mud weight,the increase in water content results in a significant decrease in mud weight,viscosity and sand content,slurry additive sodium carboxymethyl cellulose and sodium polyacrylate in main effect is to improve the mud viscosity,sodium polyacrylate for mud also has the very strong flocculation.Barium sulfate is mainly used to increase mud weight.In addition,according to the actual engineering geological conditions of the pile foundation project of Meisha Street in Yantian district,the performance requirements of the wall retaining mud of complex Marine strata are analyzed,and three optimization schemes of mud ratio are given.(3)In the discrete element numerical simulation of borehole stability under the action of mud wall protection,the particle flow model of mud viscous fluid is firstly established,and the relationship between mesoscopic parameters and mud performance is analyzed.Subsequent drilling construction simulation results show that the sand layer hole collapse phenomenon embodied in nonuniform radial displacement and mud particle invasion of hole wall,clay layer necking embodies as the uniform deformation of hole wall,with the improvement of slurry viscosity,uneven hole wall can make sand gravel layer radial displacement gradually transformed into a uniform radial displacement,however,the reduction of radial displacement is small.With the increase of mud weight,the radial displacement of hole wall will decrease significantly,but the distribution characteristics of hole wall deformation will not change.If the grouting pressure is too small,the deep soil pressure cannot be balanced.If the grouting pressure is too high,it will promote the diffusion of mud particles in the gravel sand layer,which are both unfavorable conditions.If the spacing of holes is too small,the location of holes will shift to the center,so the proper spacing of holes should be established.Based on the analysis of the simulation results,the construction opinions and suggestions of cast-in-place pile are given.