Three-dimensional Discrete Element Method Investigation Of Stress And Deformation Fields In Sand During Pile Penetration

Over the past few decades,the ever-growing exploitation of marine resources in China has led to the construction of numerous offshore structures,such as ocean platforms,wind turbines,and long-span bridges.Driven piles are a common foundation type frequently used to support these structures in offshore engineering.As the pile axial capacity is critical to the safety and reliability of the structures,design methods that take into account the large vertical surcharge and complex marine environment are necessary.Conventional design methods such as the static load tests may not be feasible for offshore piles.Comparatively,the cone penetration test(CPT)-based design methods offer pile axial capacity predictions with increased precision and reliability by considering factors including the stress field around the pile shaft.To aid the CPT-based design,calibration chamber tests are usually performed to obtain the stress,displacement,and strain fields in the soil mass.Compared to in-situ tests,they offer advantages such as controlled boundary conditions and cost-effectiveness,and are widely used to study the soil behaviors and pile-soil interaction mechanisms during pile driving.However,calibration chamber tests have their own drawbacks,such as difficulties in arranging and protecting stress sensors,and limited measurement points with uncertain accuracy.On the contrary,the numerical simulation using discrete element method(DEM)allows for comprehensive acquisition of data during the whole pile driving process,revealing the pile-soil interaction mechanism from micromechanics point of view,and providing reference for improving the design methods for driven piles.In this thesis,a high-performance three-dimensional(3D)DEM code MUSEN,empowered by GPU parallelization,is used to simulate the closed-ended pile penetration in sand.The major contributions are listed below:1.A series of conventional drained triaxial compression tests is conducted and compared with the experimental results on the Fontainebleau sand used in the calibration chamber test in terms of the peak friction angle and dilatancy characteristics.The agreement between the DEM and experimental results verifies the validity of DEM parameters.2.Axisymmetric assumption and particle refinement method(PRM)are adopted in the 3D DEM in modeling the monotonic pile driving process of a closed-ended pile in sand.The reliability of the numerical results is verified by analyzing the pile tip resistance curve.The use of axisymmetric model and PRM effectively reduces the size and boundary effects in DEM simulations and ensures result accuracy with a reduced particle number and improved computational efficiency.3.The in-depth investigation of the evolution of soil stress around the pile during pile driving confirms the stress hysteresis loop experienced by soil elements near the pile during the penetration process.The simulation results supplement the calibration chamber test data near the pile and indicate significant differences in the stress levels and principal stress directions of soil elements at different locations.Strong stress concentration occurs near the pile tip,and the stress distribution exhibits notable h/R and r/R effects.4.The soil around the pile can be divided into three zones,namely,vertical compression zone,transition zone,and radial compression zone and the largest displacement occurs in the transition zone.The sand beneath the pile tip is mainly under vertical compression,with strain concentration at the tip,while the soil close to the pile shaft is mainly under radial compression,with strain concentration at the shoulder.