Research On The Adaptability Of Spiral Extruded Cast-in-place Pile Based On Nanning Mudstone Stratum

Due to the special engineering characteristics of low strength,poor cementation and strong water-rock action of mudstone in Nanning region,there are certain drawbacks of pile foundation projects in mudstone strata in Nanning basin,such as water-rock interaction between mud and mudstone on the pile side and pile end in traditional bored piles with mud wall protection,resulting in softening or even disintegration of mudstone rock,causing insufficient bearing capacity of pile foundation;hammering method and hydrostatic method of The prefabricated pipe pile is difficult to cross the conglomerate layer and hard rock layer,which makes the pile end unable to enter the rock or the depth of the rock is insufficient,resulting in the characteristic of high bearing capacity of prefabricated pipe pile.In addition,environmental protection and civilized construction in the construction field have become the key factors affecting the quality and progress of the project.The threaded squeeze pile technology,which uses threaded drill rod to squeeze the soil into the hole and the center pressure filling,has been in exploratory application in the mudstone strata of Nanning Basin.The threaded pile structure improves the bearing capacity of the pile foundation.Therefore,it is important to study the adaptability of this process in the mudstone strata of Nanning Basin to solve the engineering problems caused by the special engineering properties of mudstone in this area.In order to solve the engineering problems when the traditional pile construction process is applied to the mudstone strata,the paper investigates the stratigraphic adaptability of the threaded squeeze piling process based on the geological profile of the mudstone strata in the Nanning Basin,using a combination of theoretical analysis,field tests and numerical analysis methods.The main research contents and results of this paper are as follows:(1)By analyzing the pile formation process of threaded squeeze grouting pile,the theoretical solution of soil displacement field,stress field and strain field around the pile during the formation of threaded squeeze grouting pile is deduced by using the elastic-plastic model of the expansion of round hole from zero radius of Hai-sui Yu,and the influence of soil squeeze effect on the environment around the pile hole is analyzed;the characteristics of bearing mechanism between threaded squeeze grouting pile and traditional straight pile are analyzed and compared,based on Based on the ultimate equilibrium theory of Taisha base,a kind of ultimate bearing capacity calculation formula considering the continuity and occlusion of threaded squeeze pile screw teeth is proposed.(2)Two sites with typical representative strata of Nanning mudstone were selected,and the bearing capacity calculation models of Technical Specification of Threaded Pile(JGJT379-2016)and Technical Specification of Screw Infill Pile(T/CECS 780-2020)were integrated for test design,and the bearing capacity calculation formula of threaded section and the corresponding enhancement of lateral friction resistance of Nanning mudstone strata were proposed in combination with the results of static load test on site.The empirical value of the coefficient of lateral friction resistance is proposed.(3)Based on the engineering example of the threaded squeeze infill pile,the drilling and bearing model of the threaded squeeze infill pile was established by numerical method,and the numerical solution was compared with the theoretical analytical solution and the field test results to analyze the influence range of the squeeze effect caused by the threaded drilling rod feed and the damage mode under the ultimate bearing capacity,and to verify the adaptability of the threaded squeeze infill pile in the Nanning mudstone formation:1)The plastic deformation of the soil around the pile is mainly in the range of 0.8 to 1.0 times the pile diameter by drilling rod spinning from zero radius expansion to pile diameter,and the influence range of drilling is 5 to 6times the pile diameter.2)The error of the ultimate bearing capacity of the single pile calculated by the correction method and numerical analysis method based on the specification is 0% ～ 6.4%,which is better adaptable than the specification method.3)The analysis of the interaction between the pile side and the soil body shows that the soil body under each screw tooth under the action of the load occurs as a whole shear damage,and with the increase of the load from top to bottom gradually transfer occurs.Under the action of ultimate bearing capacity,all the soil between the screw teeth and the soil at the end of the pile undergoes shear damage,and the soil around the pile forms a pear-shaped shear damage surface as a whole.