Study On Force Behavior Testing And Engineering Application Of Cut-and-Expanded Branch-disc Pull-out Pile

With the continuous emergence of super high-rise buildings,the development and utilization of multi-storey underground space is also increasing.The anti-floating problem of underground structure is also in front of engineers.The anti-floating problem of underground structure is also placed in front of engineers.Rotary cast-in-place piles are widely used in the construction of foundation piles of railway bridges and large buildings because of their high working efficiency,good construction quality and less pollution of dust and mud.When the rotary excavation grouting pile encounters deep residual soil and weak stratum with abundant groundwater and the condition of pile formation is not good,the uplift bearing capacity of the rotary excavation grouting pile with equal diameter may not meet the design requirements.In this paper,two solutions are proposed to solve the problem of insufficient uplift bearing capacity of equal-diameter rotary excavation cast-in-place piles,one is post-grouting at pile end,the other is cutting and expanding branch-disk piles.Based on the analysis of the bearing mechanism of post-grouting cast-in-place pile and branch-disc pile,the vertical uplift bearing capacity of two types of single pile is calculated theoretically.After the same pile diameter and length of rotary excavation pile is completed,the post-grouting treatment of pile end is carried out,and the static uplift test of single pile is carried out after the strength of pile body reaches the age requirement.During the drilling process of rotary excavation pile with the same pile diameter and length,the soil is cut and expanded laterally at the bottom of the pile and about 6.0m above the pile with the cutting and expanding drill bit.After the cutting and expanding,the disc cavity is formed,and then the pile is formed.After the strength of the pile reaches the age requirement,the vertical pull-out static load test of the single pile is carried out.Through the static uplift load test of single pile of the two schemes,the mechanical and deformation properties of uplift pile are tested.The feasibility of the two improved pile types in improving the vertical uplift bearing capacity of single pile is verified by quantitative analysis of specific data.The control of uplift displacement,quality and cost of the two schemes are also discussed.The actual effect of the improved pile type is evaluated by comparing and analyzing with the project.The results show that the standard value of lateral friction resistance used for theoretical calculation of uplift bearing capacity of ordinary piles with equal diameter and post-grouting piles is 0.61 times of the lower limit value provided by geological prospecting report,and the bearing capacity obtained from the actual test is consistent with that obtained from the theoretical calculation of the vertical resistance of single pile by the two improved schemes mentioned above.The uplift bearing capacity has been greatly improved,and the theoretical calculation value of the vertical uplift bearing capacity of the single pile with cut-enlarged branches and discs can reach 1.4 times of that of the post-grouting pile.The actual uplift test shows that the bearing capacity of the two schemes can meet the design requirements,which is consistent with the theoretical calculation.The uplift bearing capacity of the post-grouting cast-in-place pile with rotary excavation can be increased by at least 66%.Above all,the maximum displacement of the pile top can be controlled within 15 mm,and the U-delta curve basically shows a slow change;Compared with the post-grouting scheme,the cut-and-enlarge branch-disk pile scheme has shorter construction period,better quality assurance and at least 50%cost savings compared with the post-grouting scheme.;on the whole,the scheme of cutting the expanded branch-disk pile saves at least 50%compared with the post-grouting scheme.Compared with post-grouting scheme,the scheme has advantages in bearing capacity,displacement control,quality,cost and duration.This project has solved the technical problems in engineering design and construction.