Research On Penetration Mechanism Of Piezocone Penetration Test (CPTU) And Its Application On Pile Foundation Engineering

The scale of engineering construction has been expanded with the development of the economy. To avoid safely accidents, the accuracy of soil parameters must be guaranteed. The piezocone penetration test (CPTU) is a new in-situ technology in the world. The pore water pressure sensor is added on probe based on the traditional cone penetration test (CPT). Compared with the traditional CPT, CPTU has many remarkable advantages, such as higher accuracy, continuous test, and convenience. However, the appilication of CPTU has been hindered due to the lack of a systematic study on the penetration mechanism such as sensitivities of soil parameters and lead and lag depth. The CPTU probe can be assumed as a model pile with small size. Therefore, methods to determine bearing capacity of piles directly based on CPT and CPTU data are used more often. However, these methods are not suitable for the open-end pipe pile for its soil plug effect, which brings a lot of difficulties on the pile foundation design.The sensitivities of soil parameters, lead and lag depth and new method to evaluate the axial capacity of open-ended piles were studied by numerical simulation and field test The main contents and results are as follows:(1) The effects of internal friction angle, dilation angle, elastic modulus and the permeation coefficient on the CPTU penetration are systematically summarized by simulating the penetration process. It shows that internal friction angle, dilation angle, elastic modulus can increase the cone tip resistance in different levels. The horizontal permeation coefficient plays a more important role in the development and dissipation of the excess pore water pressure than vertical permeation coefficient.(2) The process of the cone penetration test in layered clay is simulated to analyze the relevance between lead and lag depth and the strength characteristic of the layered clay. The calculation formula of the lead depth is proposed by combining the large amounts of field test data in coastal areas and Nanjing with the simulation results, providing the basis on identifying the layer boundary correctly.(3) Several methods to determine bearing capacity of piles directly based on CPT/CPTU data are presented. The applicability of every method on the open-ended pipe piles is evaluated based on the field test data. The relevance between the incremental filling ratio (IFR), pile depth, and the base capacity have been analyzed through the bearing capacity of open-ended prestress high strength concrete (PHC) piles of Jiangsu Chongqi bridge combining CPTU data and capacities obtained from static loading tests.A new method to evaluate the axial capacity of open-ended piles has been proposed by revising the existing domestic method proposed by Eslami and Fellenius.