Theoretical And Experimental Study On Scale Effects Of Soil

Soil consists of solidoid, liquidoid and gas under changeable and complex conditions.When subjected to natural forces such as weathering, transport, deposition and erosion, thesolidoid of soil develops into soil particles spanning6~7orders of size magnitude, from clayparticles with diameters smaller than5microns to pebble and rubble with diameters largerthan10centimeters. The disordered spatial arrangement of soil particles at different scalesforms the soil skeleton, which bears the gravity and external loads, and has an importantinfluence on the complex physical and mechanical properties of soil. Different combinationsof soil particles at different scales form various microscopic soil structures, which greatlyaffect the macroscopic mechanical behaviours of soil. Therefore, the size, shape and gradationof soil particles have great influence on the physical and mechanical properties of soil,presenting remarkable particle size effect. In order to solve the problem that conventionalcontinuum mechanics cannot interpret the particle size effect of soil, a soil cell element modelof multi-scale characteristics is constructed to analyze the mechanical mechanism of particleinteractions and to establish a mechanical model that can describe the relationship betweenthe macroscopic mechanical properties and microscopic soil structures by introducing anintrinsic length parameter that can characterize the microscopic structures of soil.The theoretical and experimental research done in this dissertation and the relativeconclusions are as followed：(1)According to the geometric characteristics of reinforcement particles measured by thedigital image processing technique, the particle size distribution characteristics are obtained.The test results indicate that: the equatorial projection shape parameter of various particleswith the same lithology and production processes has similar distribution; the minor axis ofparticles is highly symmetric, presenting typical standard normal distribution; the square meshhas a good sieving on the minor axis of particles.(2)On the basis of multi-scale model, the mechanical model of particle-reinforced soil isanalyzed. According to the type of acting forces of particles at different scales, soil isdecomposed into matrix and reinforcement particles. The density of coordinate micro-cracksand the effective strain gradient of soil are calculated to determine the flow stress of particle-reinforced soil. The results indicate that: the strength of particle reinforced soildepends on the properties of the matrix and the reinforcement particle size characteristics; thegeneration and development of coordinated micro-cracks induce dissipation of strain energy,showing as the enhancement of soil mechanical properties; the particle size effect ofparticle-reinforced soil is caused by the strengthening of the soil cell element.(3)The influence of the matrix properties and reinforcement particle size characteristicson the particle size effect of soil is studied. According to the test results, the decreasing matrixliquidity index strengthens the interaction of particles and weakens the lubricating capacity ofinteracting particles caused by water molecules; the decreasing reinforcement particle sizeintensifies the dissipation of strain energy in a unit volume of soil; the flow stress ofparticle-reinforced soil has an approximately linear dependence on the body surface area ofthe reinforcement particles.(4)On the basis of the flow stress calculation methods of particle-reinforced soil, theinfluence factors of the variable intrinsic length are analyzed. According to the analysis results,the intrinsic length is a reflection of the physical and mechanical properties of the matrix andreinforcement particles, and is capable of revealing the influence of the coupling of these twomaterials on the mechanical properties of soil and can describe the size of the inhomogeneoussoil cell element. The expression of the intrinsic length includes the parameter of the matrixshear strength and the reinforcement particle size characteristics and is capable of reflectingthe ability to store and release energy, shear properties, nonlinearity and plasticitycharacteristics.(5)On the basis of the test results of the variable intrinsic length, the influence factorsand sensibility of the variable intrinsic length are analyzed. The analysis results indicate that:as the axial strain increases, the variable intrinsic length gradually achieves a stable value; theintrinsic length firstly increases as the matrix liquidity index increases, after a critical matrixliquidity index, decreases with increasing that parameter; the intrinsic length and itsamplification increase with an increase in the content of the reinforcement particles; theintrinsic length decreases with decreasing reinforcement particle size, and a largerreinforcement particle content relates to a more intense nonlinearity.