Experimental Study On The Jacked Loading Characteristics Of Single Pile In Clayey Soil

In the field of traditional industrial and civil buildings as well as the rapid development of offshore engineering and high-speed traffic engineering in the 21 st century,huge amounts of pile foundations are used.The method of building prestressed high strength concrete pipe piles cooperated with jacked method has advantages of high bearing capacity,strong penetration,cost low,environmental protection and so on.Therefore,the method has been applied more and more widely.In the process of jacked pipe piles in clayey soil,there will appear some gaps between pile and soil.What's more,there will be excess pore water pressure in the soil around the pile.Hence,and the true effective pile pressure needs to exclude the pore water pressure.The loading characteristics of the pile foundation considering the effective lateral pressure of jacked piles in the clayey soil are increasingly concerned by the designers,especially the soil pressure and pore water pressure of the pile-soil interface act directly on the pile body,which is more important than the soil of the pile-soil interface.The pressure affects the change of the frictional resistance of the pile.However,the loading mechanism of the pile-soil interface during the jacked process has not known yet.Therefore,it has great practical and theoretical significance to study the loading characteristics of the pile and the pile-soil interface during the jacked driving.In this paper,the mechanical properties of jacked piles in the clayey soil were studied systematically by means of pile-soil interface direct tests with constant normal stiffness,large-scale model tests and full-scale tests in the field.In addition,innovative installation methods for test components about indoor and field test methods were proposed.The research results provided basis and references for the lack in the study of pile-soil interface during the jacked process in clayey soil.The main work and research results of this paper are as follows:1.The mechanical properties of pile-soil interface was studied using the large-scale pile-soil interface direct shear apparatus with constant normal stiffness,and the shear stress peaks and shear failure displacement of the pile-soil interface under experimental conditions with different roughness,content and shear rate were summarized,which provided references for practical engineering applications.What's more,the method of installing micro earth pressure and pore water pressure at the pile-soil interface was successfully used in the direct shear test,and the shear mechanical properties of pile-soil interface considering the excess pore water pressure in the clayey soil was studied.2.The double-wall model pipe pile was designed and manufactured,and we proposed the innovative installation,testing and calibration techniques for the axial force sensor of the inner and outer tubes as well as the earth pressure and the pore water pressure sensor of the pile-soil interface was proposed.Three sets of jacked pile tests were carried out based on the large-scale model test system of the Qingdao University of Technology.The systematic test study on the mechanical properties of the soil plug production,opening and closed model pile penetration process of open model piles in clayey soil was carried out.3.Based on the model test,the variation of soil plug height of open model pile,the law of load transfer and the change law of pile-soil interface stress during the jacked process of closed model piles in clayey soil were obtained.The influence of excess pore water pressure,the total radial stress and the effective stress of the pile-soil interface on frictional resistance during the jacked process were analyzed.Then we found that there was a "side pressure degradation" phenomenon at the pile-soil interface.The variation of effective radial stress was similar as the total radial stress of the pile-soil interface,and the effective radial stress and the lateral frictional resistance of the pile-soil interface reached the peak at the same time,then the effective radial stress decreased,while the lateral friction resistance remained stable.It shows that the fact of frictional resistance degradation of the pile side is also the reduction of the effective radial stress of the pile-soil interface.4.The PHC test piles with implantation of fiber Bragg grating sensors in the pile during the PHC pipe pile production process in complex environment were developed.At the same time,micro earth pressure and pore water pressure sensors were installed on the surface of the pile body to form an on-site tests method corresponding to indoor testing.Comparing with the field test results of embedded fiber in the pile body,the accuracy of the implanted fiber test technology was verified.The change law of the excess pore water pressure,total radial stress and effective radial stress of the pile-soil interface during loading,sinking and unloading were compared with the laboratory test,it has verified the feasibility of testing technology of the pile-soil interface.5.Based on field tests,the law of pile load transfer,excess pore water pressure,total radial stress and effective radial stress of the pile-soil interface were analyzed.It was found that the excess pore water pressure of the pile-soil interface increased sharply at the initial stage of the sinking.The excess pore water pressure of the pile-soil interface increased to its peak value and then declined during sinking pile.And it would quickly disipated at the end of the pile sinking.The total radial stress of the pile-soil interface was the linear relationship with the depth of the soil.At the same penetration depth,the total radial stress of the pile-soil interface decreases with the h/B increasing,which it revealed that the frictional resistance of the pile side was essentially a reduction in effective radial stress of the pile-soil interface.The "side pressure degradation" phenomenon based on the reduction of effective radial stress was proposed.