Study On Mechanical Mechanism And Bearing Capacity Of Multi-section Expanded Bored Piles

As one of the most common types of foundation,the vertical bearing performance and the bearing mechanism of the pile foundation have great influence on the safety design of the building.The construction technology of multi-section drilling and expanding cast-in-place pile is the first variable-diameter cast-in-place pile construction technology,which is the first in China.By using the pump suction and reverse circulation technology of the drilling and expanding all-in-one machine,the effect of the pile collapse,the mud and the sediment of the pile body is effectively reduced,and the pore-forming quality and the bearing capacity are greatly improved.Based on the static load test and the stress test of the pile body of a certain project in Beijing,this paper studies the transmission mechanism and the strength failure characteristic of the multi-section drilling and expanding pile in the vertical compression load through the theoretical analysis,and provides the optimal design theory support for the design and construction of the multi-section drilling and expanding pile in the future.On the basis of sorting out and analyzing the static load test results of multi-section drilling and expanding pile,the axisymmetric two-dimensional finite element model of test pile type is established by using ABAQUS software,and the settlement displacement curve and axial force variation curve of the simulation model are extracted and compared with the field test results,so as to verify the reliability of the model.On this basis,the geometric shape,position,spacing,quantity and other working conditions models of equal diameter pile and different expansion body are established respectively,and the differences of stress mechanism and bearing capacity between the pile and the same diameter pile are compared respectively,and the optimal design of multi-section drilling and expanding pile is found.Aiming at the multi-section drilling and expanding group pile,the models of equal diameter pile group and different pile spacing are established respectively,and the difference between the pile group and the same diameter pile group and the influence of pile spacing on bearing capacity and deformation are studied.The research shows:(1)The load-settling curve of a multi-section drill-expanded pile is a slow deformation curve;during the loading process,the drill-expanded pile exhibits the characteristics of a friction pile and the end bearing friction pile in turn.The bearing capacity of the radial body is brought into full play first,and the bearing capacity of the lower expanded body is exerted later,with time and sequence effects.(2)During the load transfer process of the drill-expanded pile,due to the existence of the enlarged diameter body,the soil around the lower part is compacted,resulting in a hardening effect;the upper part forms a "ring-free zone",and the surrounding soil is plastically deformed to produce a weakening effect,which causes the pile to weaken.The side friction resistance and pile body stress change significantly within this range.Therefore,compared with equal-diameter piles,the bearing capacity of single-expanded multi-section drill-expanded piles is increased by20～40% compared to constant-diameter cast-in-place piles;the ultimate bearing capacity of double-expanded drill-and-expanded piles is improved compared to equal-diameter piles 40～70%.(3)Comparing the numerical simulation results under various working conditions,it is determined that the optimal diameter expansion ratio D/d of the drill-expanded pile should be 2～2.6,(D is the diameter of the expanded body,and d is the diameter of the pile);The number of diameter bodies is1～3.Under the premise of ensuring that the soil body is not damaged,it should be set in the upper hard soil layer.The optimal distance between adjacent expanded bodies is 3D～5D.(4)Under the same conditions,the ultimate load of multi-section drilled and expanded group piles is about 140% of equal-diameter piles;the angular pile pile-soil relative displacement is the largest,and the center pile is the smallest.