Study On The Mechanical Properties And Crushing Mechanisms In Volcanic Soil From Macroscopic And Microscopic Perspective

Volcanic soil is a special type of soil that originates from the eruption of volcanic magma and condensed and deposited on the earth’s surface.Its distinctive characteristic lies in the presence of distributed internal voids within the soil particles,which makes them susceptible to particle breakage even under low external forces.This characteristic changes the gradation and mechanical properties of volcanic soil,resulting in significant settlement and volumetric deformation of embankment and foundation works that utilize volcanic soil as fill material.Consequently,this can severely impact the safety and functionality of structures and buildings constructed on such foundations.With the continuous development of geotechnical engineering and new technologies,researchers have realized that the micro-mechanical characteristics of rock-soil body determine its macro-mechanical performance.A few scholars have been migrating their research spotlight from the macro-mechanical scale to the micromechanical level,trying to acquire the mechanistic explanations.This research has yielded more achievements in conventional materials such as sand and gravel.However,for volcanic soil,such a fragile particle material,there have been few studies on its own crushing mechanical characteristics,most current studies usually apply it as modified material to reinforce other soils,and the research mainly focuses on the macromechanical level.Carrying out research on macro-mechanical level crushing characteristic test of volcanic soil and its micro-mechanical level can not only essentially clarify the mechanical properties of the macroscopic performance and the intrinsic mechanism,but also help to provide rational application of land resources in the volcanic soil area and effective theoretical support for foundation design in engineering construction.This paper is based on a research program funded by the National Natural Science Foundation of China(NSFC)to study volcanic soil in Changbai Mountain region of Jilin Province.Indoor laboratory mechanical tests,theoretical analysis,and numerical simulation of particle flow based on code programming were adopted to carry out an in-depth research on its macro-mechanical properties and micro-mechanical crushing mechanism.The main contents and findings of this study are as follows:(1)The basic conditions and mechanical properties of volcanic soil in the sampling area were investigated and analyzed,including the mineral and chemical composition,particle morphology characteristics and microscopic features.Taking into account the scattered size of volcanic soil particles,and in conjunction with actual engineering requirements,two sets of gradation conditions for mechanical tests were established to analyze relevant shear strength indexes.(2)A study on the mechanical behavior and breakage characteristics of volcanic soil particles under a triaxial large shear strain condition(30% axial strain)was carefully carried out based on the sand rain sampling technique.The macro-mechanical characteristics of volcanic soil were analyzed,the influencing factors on volcanic soil particles crushing were evaluated,and changes in the particle crushing index and fractal dimension were explored.Additionally,the particle breakage patterns were assessed,and the relationship between macro-stresses and the corresponding breakage indexes was established based on energy theory.(3)Single-particle strength tests of volcanic soil particles with specific grain sizes were conducted using a modified single-particle pressure testing apparatus.The modes of failure for single-particle crushing,the relationship between characteristic strength and survival probability,and the geometric self-similarity parameter of the volcanic soil’s crushing strength size effect was corrected in conjunction with the experimental results.(4)A set of granular film conventional triaxial flexible boundary consolidation shear system was systematically compiled based on FISH language.The reliability test showed that the flexible boundary model constructed by the code presented in this paper could accurately reflect the local large deformation characteristics of the real shear specimen.It provides the basis for the subsequent particle flow numerical simulation to better approximate the real physical triaxial boundary in crushable volcanic soil.(5)A three-dimensional numerical model of volcanic soil,which incorporates particle breakage,was established using the fragment replacement model(FRM).The model featured a flexible membrane boundary and utilized a microscopic octahedral breakage criterion that considered the size effect of breakage strength.This criterion was corrected based on experimental results presented in this paper.Additionally,the model employed a 14-sphere Apollonian packing algorithm,which exhibited a fractal dimension similar to that observed in the experimental results,as the sub-particle replacement mode.Based on the above model,the dynamic evolutionary process of volcanic soil particle crushing,contact force chains,displacement and velocity fields,coordination numbers,contact force probability distribution evolution,and fabric anisotropy level of volcanic soil during the loading process were analyzed at the macroand micromechanical levels.