Discrete Element Study On Granular Materials Considering Liquid Bridge Force Quantitatively

Studying the interaction between particles and liquid bridges at the micro scale and its impact on the macro micro mechanical properties of unsaturated granular materials is of great theoretical and engineering significance for further revealing the mechanism of soil water interaction in unsaturated soil.After deeply understanding the contact model of liquid bridge effect in granular materials and its research status in discrete element analysis,this article uses PFC6.0 software to conduct particle column collapse tests,unconfined compression tests,and triaxial shear tests considering liquid bridge effect,to study the macroscopic and microscopic mechanical responses of granular materials under different test conditions.In addition,the effects of preparation methods of numerical triaxial specimens and test boundary conditions on the mechanical properties of granular strength of materials were also studied.The results show that:(1)The finite element software Surface Evolver was used to conduct numerical experiments on liquid bridge tension,and the experimental results provided a theoretical basis for the value of liquid bridge force parameters in the contact model,achieving the application of liquid bridge force between particles in particle assemblies;Indoor triaxial shear tests and numerical simulations were conducted on unsaturated granular materials,establishing the corresponding relationship between macro and micro parameters of unsaturated granular materials,and providing methods and experiences for parameter calibration.(2)Under the action of gravity,the smaller the particle size,the greater the role of liquid bridging force in the particle aggregation,and the more obvious the phenomenon of cohesion;Under the same particle size,the larger filling angle corresponds to a larger number of liquid bridges in the particle aggregate,resulting in better cohesion of the sample.Therefore,the unconfined compressive strength of the sample depends more on the number of liquid bridges rather than the maximum liquid bridge force between particles;When the liquid bridge structure between particles is pendulum shaped,the unconfined compressive strength of the sample increases with the increase of water content.(3)In triaxial shear tests,different sample preparation methods can cause the samples to exhibit different mechanical properties during the shear process.Under the same confining pressure,the specimens prepared by the layered compression method have the highest secant modulus and peak strength;As the confining pressure increases,the difference in peak strength between the samples prepared by the layered compression method and the other two methods gradually increases;When the confining pressure is 100 k Pa,compared with the flexible membrane boundary,the softening characteristics of the Stress – strain curve of the sample under the rigid wall boundary are more obvious,the secant modulus is larger,and the peak strength is about 25% higher;Using a flexible boundary coupled with PFC and FLAC,the specimen exhibits a drum shape after shearing,which is more consistent with the indoor test results.(4)In the indoor triaxial shear test,the outer contour of the dry sand sample after shearing presents a drum shape without obvious shear bands,while the shear bands of the low moisture content sand sample are obvious;Compared to the dry state,the peak strength of granular materials with lower moisture content is higher,and the increase in peak strength caused by low moisture content decreases with the increase of confining pressure.(5)The results of the triaxial numerical test are consistent with the indoor test results,which can provide effective reference for the values of micro parameters of similar experimental materials;The change of particle displacement field reveals the development law of the shear surface of the sample,and the change of Contact force chain explains the reason why the peak strength of the sample increases with the increase of confining pressure;With the increase of the axial strain,the high stress sphere in the middle of the specimens at all levels of confining pressure increases,the porosity of the specimens decreases first and then increases,and the Coordination number increases first and then decreases.