Study On The Influence Of The Cantilever Diameter On The Failure Mechanismof NT-CEP Pile Group Piles

With the continuous development of pile foundation technology,cast-in-place pile technology has been widely used in engineering.New Type Concrete Expanded-Plate Pile(hereinafter referred to as NT-CEP pile)is a new type of pile developed on the basis of equal section bored pile.Its principle is to set up bearing expansion plate at the appropriate position of pile body,increase the end bearing effect of pile body,and make full use of the bearing capacity of soil around pile body.Compared with ordinary equal-section straight-hole piles,NT-CEP piles can increase the bearing capacity by 56 %,better adapt to complex geological conditions,and have the advantages of saving materials,small settlement,short pile-forming period,and less waste mud discharge.At present,most of the research on NT-CEP piles focuses on the bearing capacity of single piles,and there is still a lack of in-depth and systematic theoretical research and effective results in pile groups.Due to the particularity of NT-CEP pile body structure,under the action of external load,the interaction between adjacent piles will be far greater than that of straight hole piles,especially the influence of uplift piles is more obvious.The bearing capacity of pile groups cannot be simply calculated by multiplying the number of pile groups by the bearing capacity of single pile.At the same time,the diameter of the cantilever is the main factor affecting the influence.Therefore,this paper will deeply study the failure mechanism of the soil around the pile and the bearing capacity of the pile under the action of the vertical tension of the NT-CEP pile group.In view of the importance and research gap of NT-CEP pile group,this paper studies the bearing capacity and pile group effect of NT-CEP pile group under vertical tension by using ANSYS finite element simulation analysis method from the perspective of pile group and taking the cantilever diameter as the main influencing factor.The stress and displacement data of pile body and soil in the simulation analysis are extracted,and the displacement,stress curve and displacement cloud diagram of pile body and soil on both sides of pile body are compared and analyzed.The bearing mechanism of NT-CEP pile group and the change of soil around pile under different cantilever diameters are revealed,and the reasonable ratio of cantilever diameter to pile diameter is determined.Combined with the previous research results of single pile and double pile,the reduction coefficient of uplift bearing capacity of NT-CEP pile group under the influence of cantilever diameter is determined,and the calculation model of uplift bearing capacity of NT-CEP pile group is proposed.The results show that the failure state of the soil on the disk of the NT-CEP pile group is still in line with the slip line theory proposed by the research group.Because the degree of soil damage between piles will be affected by the cantilever diameter of the disk,under the condition that the pile spacing is constant,with the increase of the cantilever diameter of the disk,the net spacing between the pile disks will decrease,the soil interaction between the pile groups will increase,and the slip range of the soil on the disk will also affect each other,resulting in the reduction of bearing capacity.Considering the uplift bearing capacity of pile body and economic benefits,under the condition of reasonable pile spacing,it is determined that the ratio of the best cantilever diameter to the pile diameter should be between 1.5 times and 1.75 times,and the reduction effect caused by the influence of the cantilever diameter should be considered in the calculation model of the uplift bearing capacity of NT-CEP pile group.Through the research on the uplift failure mechanism of NT-CEP group piles in this paper,the influence of the cantilever diameter on the uplift bearing capacity of NT-CEP group piles is studied for the first time,and the ratio of the cantilever diameter to the pile diameter is used as the reference value.The design concept of uplift bearing capacity of NT-CEP group piles is improved,which lays a reliable theoretical foundation for the design and application of NT-CEP group piles in practical engineering.