Study On The Adaptability Of Pile Planting Process Based On The Special Stratum Of Nanning Basin Static Pressure Followed By Drilling Pipe

Pile foundation has a long history of development and is widely used in high-rise and super high-rise buildings because of its high bearing capacity,along with the continuous development of societ.Nanning Basin has undergone complex geological structure movement,with well-developed strata,in which pebble stratum and clastic rock stratum are widely distributed,and serve as the main bearing stratum of foundation.Due to the particularity of the stratum itself,the stratum stress can be released after the drilling(punching)pile is used in the round gravel stratum,and it is easy to make the stratum loose and even have the risk of hole collapse.Besides,water has a great influence on the mechanical properties of the clastic rock,and the drilling(punching)cast-in-place pile is wet operation,so the strong disturbance of construction machinery will lead to a sharp decline in the bearing capacity of the stratum.Moreover,when the traditional hammering pipe pile and static pressure pipe pile encounter hard rock stratum,the end of the pile can not enter into the rock,and the high bearing capacity of the prefabricated pipe pile cannot be exerted.The technology of static pressure pile while drilling combines the advantages of static pressure pipe pile and rotary pile construction.During the process of rotary drilling,the pile body plays a role of wall protection to prevent hole collapse,and rotary drilling assists pile sinking solves the problem that pile end cannot enter the rock when the stratum is hard.In addiction,dry operation construction method avoids the adverse effect of water on the mechanical properties of the stratum.Therefore,it is of great significance to study the adaptability of this technology in the special stratum of Nanning Basin to solve the engineering problems caused by the stratum peculiarity in this area.In order to solve the engineering problems existing in the application of traditional pile foundation construction technology in gravel and clastic rock stratum,this paper studies the adaptability of static pressure drilling with pipe planting pile technology by combining theoretical analysis,field test and ABAQUS finite element simulation,based on the geological situation of Nanning basin.The main work and conclusions of this paper are as follows:(1)This paper analyzes the stress state of foundation soil in the process of pipe following while drilling,compares the field test value of single pile bearing capacity with the calculation value of traditional empirical formula,and concludes that the bearing capacity of single pile of this pile type should be multiplied by the corresponding enhancement or reduction coefficient when the traditional empirical formula is used to determine the single pile bearing capacity.Based on this,two calculation formulas for the vertical ultimate bearing capacity of the pile type single pile are proposed,and the main influencing factors of bearing capacity,side friction and tip resistance are analyzed according to the characteristic of pile-forming.(2)Two sites with typical representative strata are selected for the test design,and the round gravel stratum and the clastic rock stratum are used as the bearing stratum of the foundation.Then,the test pile design is carried out combined with the proposed building of the site,and the static pressure drilling with pipe planting technology is used for construction.The pile forming characteristics and bearing characteristics of single pile are analyzed by low strain method and field static load test.The construction process of each test pile is smooth and barrier free,the pile forming speed is fast,and there is no adverse phenomenon.In the detection results of low strain method,except one test pile’s result in moderately weathered siltstone stratum is II class,the detection results of other test piles are class I pile.There are slight defects in the pile body of type II pile,which does not affect the bearing capacity of pile body.What’s more,it shows that the quality of pile body is not affected by the use of static pressure drilling with pipe planting technology in the round gravel and clastic rock in Nanning Basin.According to the analysis of the test results,the bearing capacity of the test piles can meet the design requirements,and the rebound rate of the pile body is high,which indicates that the test piles still have great bearing potential.So,it can be concluded that the technology has good adaptability in the round gravel stratum and clastic rock stratum in Nanning Basin.Based on the above research,the static pressure while drilling with pipe pile technology is applied to the actual project,The acceptance test shows that the pile body quality of the inspected foundation pile is in good condition,the bearing capacity meets the design requirements,and still has a large bearing capacity margin.The good practical application results show that the technology has high application value.(3)ABAQUS three-dimensional finite element model is established according to the actual parameters of pile-soil test,and the correctness of the finite element model and the rationality of parameter selection are verified by comparing the Q-S curve of the results of field test and finite element simulation.The finite element simulation can be used to guide the design and construction of subsequent practical projects instead of test piles,so as to shorten the construction period.