| Due to the excellent physical and mechanical properties, coarse-grained soil has been widely used in the construction of civil engineering. For the high-speed railway in china, all the parts of embankment, i.e. surface and bottom layer of subgrade and embankment body, is mainly filled with coarse-grained soil and, thus, it is very important to study the engineering properties of such soil.By the means of theoretical analysis and laboratory tests, coarse-grained soil’s several important engineering properties is studied deeply. According to the contact relationship among soil particles, the pore characteristics of coarse-grained soil with oversize particles in "floating" state is analyzed, and a new method to determine the dry density of coarse-grained soil containing oversize particles is proposed, i.e. " solid surface pore " method. Based on the void-filling principle, a new method to test soil structure types of coarse-grained soil, i.e. "cutoff particle-void filling" method, is proposed, by which the variation of graded gravel’s soil structure types as the change of particle size distribution in the grading areas offered by the High-speed Railway Design Specifications is investigated. According to the soil particle’s motion feature, the coarse-grained soil’s shear strength formation mechanism and its compaction performance are discussed. Based on the theory of probability and statistics, the method to analyze the embedded particle’s crossing motion in the process of shearing is presented, and the formation mechanism of coarse-grained soil’structure force (similar to clay soil’s cohesion) is revealed.(1) Analysis of pore characteristics of coarse-grained soil containing oversize particle and study on method to determine dry density of such soilThe oversize material usually refers to particles larger than the limit value allowed by laboratory compaction equipment, and how to determine dry density of coarse-grained soil containing oversize material is an important topic. According to the contact relationship among oversize and non-oversize particles, the pore characteristics of coarse-grained soil with oversize particles in "floating" state is analyzed, the pore ratio of oversize particle, i.e. ratio of oversize particle material’s volumes to voids made by oversize particle, and its geometric calculation model is proposed, by which a new method to determine the dry density of coarse-grained soil containing oversize particles, i.e. " solid surface pore " method, is developed. Test results indicate that the pore ratio of oversize particle is influenced by size ratio of oversize to non-oversize particle i.e. k greatly. There exists a special size ratio i.e. ks for coarse-grained soil containing oversize material in "floating" state, when k is more than ks, the pore ratio of oversize particle is so small that the omission of it cause little effect on the dry density, and if k is less than ks, the error of dry density determined by omission of the pore ratio of oversize particle will increase significantly with the decrease of k. Test results also confirm the applicability of the proposed new method on the condition of the oversize fractions being less than 40% by mass.(2) Soil structure testing method for coarse-grained soil and analysis on the soil structure type of graded gravelly soil filling the surface Layer of subgrade of high-speed railwayBased on the void-filling principle, through removing the soil particles and then testing the soil structure from small to large gradually, a new method to test soil structure types of coarse-grained soil, i.e. "cutoff particle-void filling" method, is proposed, by which the variation of graded gravel’s soil structure types as the change of particle size distribution in the grading areas offered by the High-speed Railway Design Specifications is investigated. Test results showed that when the particle size distributions of gravelly soils vary from the upper limit to the lower limit of grading areas offered, its soil structure type will transform from dense suspension structure to dense skeleton structure, and finally change into pore skeleton structure. Accordingly, the grading areas was divided into three portions, i.e. grading areas of dense suspension structure, grading areas of dense skeleton structure and grading areas of pore skeleton structure. The research results are helpful to improve the high-speed railway grading technical standards for graded gravelly soil.(3) Soil particle motion characteristics-based investigation on shear strength formation mechanism of coarse-grained soil and its compaction performanceBased on the characteristics of particle permutation on the shear plane, soil particle’s motion in the process of shearing is studied. According to the results of large-scale direct shear tests, the relationship between particle’s motion forms and internal friction & structure force (similar to clay soil’s cohesion) is discussed, and the parameter which reflects the strength of embedding effect among soil particles is also presented. Based on the mechanical mechanism in the process of coarse grained soil’compaction, the mechanism determining coarse grained soil’ compaction performance is revealed. Studies show that there are three different motion forms in the process of shearing, i.e. crossing, sliding, and rolling. The crossing motion, which is a main source of structure force, mainly occurs in the initial period of shearing. The motion of sliding & rolling, which is a main source of internal friction, occur in the whole process of shearing. The maximum structure force in the process of shearing, i.e. cmax, can be considered as the parameter characterizing the strength of embedding effect among soil particles. The ability to form strong strength of embedding effect is a key requirement for coarse-grained soil to show good compaction performance. The size relation between cmm, which is cmax of coarse-grained soil in the most dense state, and grinding pressure F exerted by roller is the mechanism determining coarse grained soil’compaction performance. When F is much much more than cmm, that is to say, the soil body in the most dense state can not keep stable in the process of rolling, so in this case, the soil body is hard to reach a good compaction. When cmm is much more than F, it indicates that the soil body in loose state can form large cmax to resist grinding pressure F, the soil body is also hard to reach a good compaction in this case. When cmm≈F, the soil particle can move to closer and closer in the process of rolling, and the soil body is easy to reach a good compaction.(4) Probability statistics method-based studies of the motion of embedded particles in coarse-grained soils and the formation mechanism of their structure forceBased on the characteristics of particle permutation on the shear plane, a single embedded particle crossing motion model in relation to shear displacement is set up. According to the randomness of particle permutation on the shear plane, a probability statistics & analysis method for analyzing embedded particle group’s crossing motion in the process of shearing is worked out, and the formation mechanism of coarse-grained soil’ structure force (similar to clay soil’s cohesion) is revealed. Studies prove that there are two characteristic angles of rotation, i.e. appearance of embedding effect and disappearance of embedding effect, and three phases, i.e. Phase Ⅰ, in which particle rotation is not influenced by an embedding effect, Phase Ⅱ, in which an embedding effect works, and Phase Ⅲ, in which the embedding effect disappears, in the crossing process of single embedded particle. Based on the randomness of distribution of characteristic angles of rotation in the embedded particle group, the statistical law of the quantitative proportion of particles in three phases with the change of shear displacement is worked out. The results of large direct shear tests indicate that the shear resistance generated by particles’crossing motion due to the embedding effect among particles is the source of coarse-grained soil’s structural force, similar to clay soil’s cohesion, and its degree of exertion is consistent with the statistics of proportion of particles in Phase Ⅱ. The findings of this study propose a new approach for probing into the relation between coarse-grained soil’s particle motion and macro-mechanical behaviors. |
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