The Test On The Stress Grid Of Granular Material And Characteristic Research On Arching Effect

Soil is formed by the weathering of rock, which is generated by the transportation of gravity, water and wind and the deposition. It’s a complicated variable structural discrete partical material composed of mineral grain, liquid and gas. But the soil has been researched as continuum for 200 years nearly. Based on elastic mechanics, plastic mechanics and fluid mechanics and so on, the classical soil mechanics theory is developed which can be widely used in practical engineering, and can explain and analyze the vast majority of rock and soil mechanics problems. But up to now, there are numerous phenomenons that can’t be explained effective by existing theory, such as landslide, channel filing, dilatancy shear shrinkage phenomenon and soil arching effect etc. The main reason is that the traditional soil mechanics ignores the granularity and structural features of the soil. The soil was treated as homogeneous continuum, which annihilate the discontinuity, nonlinear and multi-scale characteristic, etc. So many scholars believe that the key of the soil mechanics changing and continuing to develop is to research geotechnical material from particle size.The discrete nature of particle medium makes the mechanical response of external load different from continuum material. The stress grid and arching effect are its main differences in material internal. The stress grid is a uneven distribution phenomenon of strong and weak force chain caused by stress localization. The arching effect is a stress shielding effect formed by particle lapping. These two features have huge impact on the overall information and macroscopic intensity of granular material. Thus through the research of occurrence and distribution characteristics of stress grid and arching effect, the mechanical response under quasi static loading in mesoscopic level is explorered in this paper, and to simulate the soil arching effect in geotechnical engineering, which can provide guidance and reference for the innovation and development of the basic theory of geotechnical engineering.With the stress visualization advantage of photoelastic experiment, through small particle size of photoelastic particles and homemade loading device, to explorer the stress shielding effect, the formation of shear band and dilatancy and the shear shrinkage phenomenon under the effect of compression and shear. Combined with digital image processing technology, the geometric and mechanical properties in granular material internal are researched by the function relation of photoelastic granular material chromatic stripe gradient value and contact force. Then the above research method is applied to the soil arching effect existing widely in geotechnical engineering to discuss the formation process, evolution mechanics and shape change of the soil arching from particle size. In this paper, the main research work and conclusions are as follows:(1) Excogitate three molding methods of small particle size of photoelastic particles and different annealing curve for different particle size with considering thermal radiation. A kind of load device applicable to this topic research is designed to be used in plane photoelastic compression and shear test, which can implement the force transferring between granular system internal visual.(2) Using stress grid visualization experiment of photoelastic granular materials, the stress transfer in particle medium has a huge difference with the stress distribution in the material based on the continuum hypothesis. A non-uniform stress grid distribution is formed inside granular material with strong force chain and weak force chain and more obvious localization characteristics. On this basis, a new method of multi-scale division is defined, which includes the particles of monomer in micro scale, the force chain shielding grid in meso-scale and particle system in macro level.(3) Adopt the particle geometric contact angle and the strong force chain contact angle as measured parameters to analysis the geometric and mechanical characteristics of granular material internal. Under the initial condition without load, the particle geometric contact angle is not homogeneous distribution. The frequency in six direction is bigger, present ‘hex’ shape distribution, showing obvious initial anisotropy. This suggests that the anisotropy in mesoscopic is the essence properties of granular material.(4) The photoelastic experiment of granular material is applied in the soil arching effect existing widely in geotechnical engineering and an criteria of soil arching formation is defined. With the increase of filling height, the soil arch turns from triangular arch into a semicircle arch or trapezoidal arch. The size of the load change will not affect the appearance of the soil arch effect, but will greatly influence the structure shape of soil arch. With the increase of pile spacing ratio, the soil arch effect will gradually disappear.(5) Height-span ratio is the important factor that affect soil arch. When the heightspan ratio H/S is less than 1, the soil arch can’t be formed. The interface program StressDisplacemen and ForceChain programed by MATLAB in this paper can realize the automatic processing of photoelastic experiment results.