Research On Bearing Characteristics Of Improved Deep Cement Mixing Pile Composite Foundation In Yellow River Flooding Area

Deep cement mixing pile(DCM pile)has advantages such as small disturbance,rapid construction,low cost,and large depth of foundation improvement.Since its import into China since the 1970s,it has been widely used in foundation reinforcement projects in the fields of transportation infrastructure,industrial and civil buildings,etc.In the Yellow River flooding area of Shandong Province,quality defects such as uneven mixing and incomplete pile in deep areas commonly occur in DCM pile projects,which greatly restricts the engineering application.Many projects have changed to foundation reinforcement methods such as CFG piles,concrete pipe piles,and jet grouting piles,resulting in a significant increase in engineering construction costs.Aiming at the quality problems of traditional DCM piles in the Yellow River flooding area,this paper compares the differences between the soft soil in the Yellow River flooding area and those in the sea and lake facies,selects typical strata in the Yellow River flooding area to conduct slurry diffusion field tests,reveals the reasons for the failure of traditional DCM piles,proposes a new engineering technology of improved DCM piles,conducts in-situ improved DCM pile tests,and explores the bearing characteristics of it through ABAQUS numerical simulation.A design and calculation method for composite foundation of improved DCM pile based on existing specifications is presented.The main work content and research results are summarized as follows:(1)Based on the engineering construction data in the Yellow River flooding area of Shandong Province and engineering geological data of marine and lacustrine soils,the differences in their engineering properties are compared and analyzed.The foundation soil in the Yellow River flooding area is formed by the erosion and deposition of the Yellow River.The soil layer is distributed in a staggered layer of silty(sand)soil,silty clay,and clay layers,with the silty(sand)soil layer being the most widely distributed.Compared with the characteristics of high fluidity,high water content,and low strength of marine and lacustrine soft soil,due to the high permeability of silty(sand)soil layer,after long-term consolidation,the silty clay and clay layer in the Yellow River flooding area have the characteristics of high degree of consolidation and strong constitutive property.(2)Based on the quality problems of DCM piles in the Yellow River flooding area,field tests of slurry diffusion were conducted in silt,silty clay,and clay layers in the Yellow River flooding area.Combining the test results with the theoretical formula of slurry diffusion,it was proved that insufficient slurry pressure was the main reason for the quality problems of traditional DCM piles.The slurry pressure parameters in the silt,silty clay,and clay strata in the Yellow River flooding area have been adjusted,and targeted equipment improvements have been made to form the construction equipment and method for improved DCM piles.(3)Field test and process parameter optimization test were conducted on typical silty clay,silt,and mucky silty clay layers in the Yellow River flooding area.The test results show that under the condition of ensuring the effective construction pressure of each soil layer in the Yellow River flooding area,a lifting speed of 20cm/min can construct a solid pile.Reducing the pressure or increasing the lifting speed can form a cylindrical pile,which is caused by the decrease of the reflux slurry leading to the lower strength of the pile center.The strength of the improved DCM pile conforms to a normal distribution,and the strength distribution at the pile center is less volatile than that at the pile side.(4)Based on the field test results,the numerical simulation of static load test of single pile,static load test of single pile composite foundation,and pile group composite foundation covered with embankment were conducted using ABAQUS.The improved DCM solid pile has excellent bearing performance.The improved DCM cylindrical pile has a maximum stress concentration at a distance of 0.7m from the pile top.During construction,capping for 1m can alleviate the stress concentration.The effect of improved DCM pile on controlling settlement is better than that of traditional DCM pile under the conditions of equal pile spacing and equal area replacement ratio.Based on the results of in-situ tests and simulations,the existing design and calculation methods have been modified.