Research On The Application Of Material Point Method In Geotechnical Dynamic Contact Problems And The Simulation Of Dynamic Compaction Of Rockfill

There are many geotechnical large deformation problems accompanied with the contact between soil and structure,and the interaction between soil and structure will significantly affect the deformation of soil around the structure and the safety of the structure.Numerical simulation of geotechnical contact problems has become an active research area and a persistent chal enge in various numerical methods.For geotechnica l dynamic contact problems,since the dynamic load is often much larger than the static load,it will cause more obvious damage and deformation of the structure or soil.Dynamic compaction is a typical geotechnical dynamic contact problem.As an increasingly important foundation treatment technology in China,it is of great value to systematica l ly study its mechanism,reveal its laws,and guide engineering practice.In this paper,the material point method(MPM)especially suitable for simulating large deformation is used as a numerical tool to study the geotechnical dynamic contact problems,and the numerical simulation and systematic research on dynamic compaction of rockfill are emphasized.The main research achievements of present study are as follows:1)The MPM program is improved.The dual domain material point method(DDMP)has higher accuracy of the overall stress calculation results except at the material boundary.The distribution coefficient algorithm is introduced to deal with the small-mass background grid nodes of the MPM.The empty element removal method is developed to deal with the zero-mass background grid nodes accompanied with non-zero internal forces of the DDMP.A consistent method of calculating internal and external forces is also proposed for the DDMP.The calculation accuracy and efficiency of the program are improved.2)The contact algorithm is improved.The continuous displacement condition is modified by the energy conservation condition,and a technique to overcome the false contact is introduced to effectively prevent contact fro m occurring in advance.The accuracy of displacement calculation and the conservation performance of energy are improved.3)The viscous boundary and the viscoelastic cone boundary are introduced to the MPM,and corrected formulas of these two silent boundaries are proposed for a density-dependent constitutive model.The applicability of these two silent boundaries in the MC model and the density-dependent constitutive model is investigated.It is found that under different impact load levels,the viscous boundary is more app licable in the aforementioned two elastoplastic models.4)The improvement mechanism of dynamic compaction of rockfill is investigated.Several concepts of evaluating the energy conversion efficie nc y during dynamic compaction are proposed,including the effective energy ratio,the total plastic energy ratio,and the shear plastic energy ratio.The deformatio n laws and energy conversion laws,the specific differences between low drop of a heavy hammer and high drop of a light hammer,the effects of the spa cing and the layout of tamping points,and the effects of the soft substratum,of dynamic compaction,are systematically investigated.The calculation formula of the improvement range and the construction suggestions of dynamic compaction are also proposed.