Pile-soil Interaction And Database Assessment Of Design Methods For Axial Loaded Piles Driven In Sands

Driven piles are commonly used to support offshore structures such as major bridges,platforms and wind turbines,the axial capacity has been a major factor that is concered in routine design.Unlike on shore piles,static load tests are prohibitively expensive in the offshore environment,empirical design methods tends to be much more efficient.The newly developed CPT-based design methods take the new findings of stress regime developed during pile penetration into consideration,their reliability are eventually verified to be better through sufficient database assessment.However,the lab experiments 'meet a severe lack of data point close to pile shaft and the field tests are almost unfeasible to measure stresses in sand mass,the basic mechanism of stress regime is still not clear especially for circumferential and vertical stresses.Systematecal experimental and theoretical studies on axial loaded piles should be conducted to improve the reliablility of existed design methods.The following three aspects are the key tasks to study the basic mechanism of pile-soil interaction and to improve the reliability of design methods for axial loaded driven piles at present:(i)to make improvements on laboratory model pile tests and field load tests.Advanced testing techniques should be adopted to solve the difficulties met when measuring the stress regime,pariticle breakage,creep and time effect to make clear of the demonstrating factors governing the axial caparity of piles driven in sand,improvements will be made on the pile design methods then;(ii)to carry out systematical numerical simulation researches on pile driving process.Advanced numerical methods including the Aribitray Lagrangian-Eurian(ALE)method,the Coupled Eurian-Lagrangian(CEL)merhod,the Material Point Method(MPM)and also the Discrete Element Method(DEM)are considered to solve the large deformation problems met during the simulation of pile driving,newly developed soil constitutive models which could better reflect the properities of soil will be introduced to make precise precdiction on the stress regime,particle breakage and etc.during pile driving.The difficulties of measuring during driving will be completed solved,more complete information will be provided to deepen the understanding of axial loaded driven piles;(iii)to build a more comprehensive pile load test database,further detailed database assessment will be conducted against the existed pile design methods to make clear of the functions of the new findings form experiments and systematically evaluate their advantages.From the above,the main contributions of this work include:1.The numerical simulation researches were conducted based on the calibration chamber experiments.ALE technique was the appropriate method that could solve the large deformation problem and also the complex boundary and loading conditions in the meantime.Correspond results were obtained from numerical simulation which remedy the crucial lack of data point in sand mass and difficulties with measuring were absolutely solved.New changing trends for circumferential and vertical stresses were observed,the h/R effect was verified as well.The CEL technique was also introduced to simulate the driving process of large diameter closed-ended piles to study the difference between full scale field tests and lab model tests.2.On the basis of the numerical simulation of calibration chamber experiments,researches on real size pipe piles were conducted in the meantime.CEL technique was a better choice to simulate the driving process of pipe piles.Difference were observed between model piles and real size piles,stress changing trends during penetration were obtained and also the plugging behavior,comparison were also made between empirical results and field test results to make clear make clear of the demonstrating factors governing the axial caparity of piles driven in sand.3.The ZJU-ICL sand database was build through literature searches,the Authors'industrial and academic networks which was said to be the most comprehensive database at present with 117 high quality pile load tests included.A systematical database assessment was then conducted on eight commonly used pile design methods to verify their reliability.The front-burner issue such as effects of L/D,D/t,time effects and etc.on pile axial capacity were also carefully discussed.4.Field test experiments were conducted through test pile program for the Second Wuhu Bridge crossing of the Yangtze River in Anhui Province.Frist hand field test results were obtained and four newly developed CPT based design methods were adopted for prediction beforehand,corresponding results were got compared with the field tests results.The test pile program also enable the researches with time effect and weak sublayers and remedy the lack of load tests with PHC piles and offer data from early pile test results.