Experimental Research On Vertical Bearing Capacity Of Cement-soil Composite Pipe Piles Based On BOTDR Distributed Optical Fiber Sensing Technology

Cement-soil composite pipe pile,composite pipe pile,also known as vigorous mixing pile,is a new type of composite pile foundation made of prestressed high-strength concrete pipe pile,which is concentric implanting soil-cement mixing pile as core pile,and filled with concrete of a certain length in the inner cavity of core pile according to structural requirements.The technical characteristic of this type of pile is that it can give full play to the advantages of the large rigidity of the prestressed high-strength concrete pipe pile and the large lateral resistance of the cement-soil pile by optimizing the size and material strength matching of the peripheral stirred cement-soil pile and the prestressed high-strength concrete pipe pile.In addition,the cement-soil pile can form an expansion head with a certain diameter and height at the bottom of the hole according to the design requirements.The bottom of the pipe pile can be threaded or bamboo pile to increase the lateral friction resistance between the pipe pile and the cement-soil pile.The pile type is mainly suitable for plain fill,silt,clay,sand and other soil layers,especially the soft soil layer,which provides new methods and new ideas for the construction of large-scale buildings(structures)in the soft soil area.Cement-soil composite pipe pile is a new type of composite pipe pile developed on the basis of the cross-integration of existing technologies.It has drawn on the technical advantages of cement-soil mixing pile and pipe pile,and has the characteristics of large diameter,long pile,high bearing capacity,high economy,high construction efficiency and low pollution.Compared with single cast-in-place piles,pipe piles,cement-soil piles and other technologies,cement-soil composite pipe piles not only save a lot of steel,sand and other raw materials,but also have no mud pollution,noise pollution and low squeezing soil,which is in line with the national "Four Sections and One Environmental Protection" policyand the development of green buildings.In this paper,based on the Taiyuan Evergrande Jinbi world project,conduct site full scale of cement-soil composite pile static load test,using the method of low strain,core test and BOTDR distributed optical fiber sensing technology of pile body completeness,cement-soil unconfined compressive strength,strain,settlement and side resistance test,based on the experimental data obtained in the process of analysis,studies the cement-soil composite pile under the vertical load of the settlement,the change law of axial force and pile side friction and the analysis of the load transfer mechanism of composite pile and load-bearing characteristics.At the same time,the effects of core length ratio and core content ratio on cement-soil composite piles were studied.Combining design,experiment and engineering experience,the calculation method and failure mode of the bearing capacity of cement-soil composite pipe pile were discussed,some suggestions for improving bearing capacity were put forward,and compared with traditional cast-in-place pile in economy.Finally,the research work that needs further development is discussed.The main results are as follows:(1)The cement-soil mixing pile machine has the advantages of high pile forming quality and strong pile forming ability,and can fully meet the construction design requirements of Taiyuan area.The cement-soil composite pipe pile has no obvious wave reflection at the pipe pile connection position,non-composite interface and pile bottom,and the pile body quality is high.The pile forming ability of the construction machine is high,and the pile forming depth can reach 55 m.(2)The cement-soil composite pile meets the design requirements of bearing capacity and settlement,and can replace the cast-in-place pile equivalently.The distribution of the side friction resistance of each test pile is basically similar,and they are all distributed in a single peak shape.The effect of the load applied to the core pile on the friction resistance is more obvious in shallow soil than the composite pile.The use of slow-speed and low-level loading of composite piles helps to exert the friction resistance of each soil layer.The empirical values and survey and design recommendations of the friction resistance areas at various levels areconservative.The representative value of compressive strength of cement soil is 6MPa,which is five times the representative value of regional experience.Cement soil has a high gripping force on core piles.(3)The highest end resistance ratio of cement-soil composite pipe pile is 1.26%,2.33%and 5.31% respectively,which is pure friction pile.The ratio of end resistance showed a significant "U" shape change with loading level,and the end load ratio was basically the same as the initial load.In addition,the error of the end resistance estimation value in the distributed optical fiber strain test is relatively large,and it is recommended to use the earth pressure sensor to collect.(4)The core length ratio and core content ratio have a greater influence on the bearing capacity of composite piles,especially in the later loading stage.Within a certain range,the greater the core length ratio and the core content ratio,the higher the ultimate bearing capacity.The higher the core ratio,the higher the load concentration of the core pile.The optimal core length ratio in this test is 0.92.The failure mode of cement-soil composite piles is that the concrete at the top of the core pile is crushed after shear failure,the shear failure surface is located about 3.85 m below the pile top,and the cross-section difference is about 0.10 m.(5)Compared with traditional cast-in-place piles,cement-soil composite pipe piles have the advantages of “three highs” with high economy,high controllability and high environmental protection.The comprehensive cost of cement-soil composite pipe piles in this project is 1/5 less than traditional cast-in-place piles.(6)Replacing traditional cast-in-place piles with cement-soil composite pipe piles is effective in Taiyuan area,and provides new ideas and methods for foundation treatment in this area.However,the pile type needs to be further optimized.Among them,the optimization of core-length ratio and core-containing ratio requires comprehensive consideration.At the same time,it is recommended that the core pile be grouted within a certain length and anchored with the pile foundation cap.If necessary,the cement-soil pile should be re-sprayed or mixed within a certain length of the pile top or increase the water-cement ratio to achieve optimal damage mode.