Experimental Examination of the Arching Mechanism on the Micro Level

Abstract

The arching phenomenon is manifested through the reduction of stresses experienced by underground structures. Arching plays an important role in geotechnical engineering construction such as; excavations, retaining structures, pile groups, tunnel boring machines, culverts and underground facilities. The arching mechanism is intrinsic to granular material/rock mass independent of scale effect. Its fundamental mechanism relates to the ability of discrete units to transfer loads through interaction in a preferable geometry and, thus, to bridge between the zone (or point) of load application to the zone (or points) of reaction. The testing results of an advanced experimental technique is presented and analyzed. A model of granular material made of photoelastic particles is utilized. The model and the sophisticated image and global data acquisition system allow to track the development of the arching within ideal granular material during a trap door experiment by following the motion of each particle and the contact forces between the particles. Visual and quantitative analyses are presented demonstrating the relationship between the global arching phenomenon and the particle interaction on the micro-level. The obtained information allows one to observe the changes associated with the arching mechanism and the stress variation resulting from it. The arching mechanism is observed in details that previously could not have been achieved. Photoelastic Discrete Simulation (PDS) De Josselin de Jong and Verruijt (1969) suggested that the interparticle contact force magnitude could be determined as a function of the relative size of the isochromatic fringes at the contact, and the corresponding contact force direction followed a line connecting the center of gravity of isochromatic fringe near the contact. The isochromatic fringes can be observed through a circular polariscope, which consist of quarter -wave plates and polarizers. Further development and testing of the above method has been presented by Paikowsky et. al. (1993). A calibration process establishing the relationship between the photoelastic isochromatic fringes and the contact force magnitude and direction were developed, allowing to accurately monitor the interparticle contact forces. Independent digital images acquired in parallel, enable to follow markings on the particles. These images allow to monitor the motion of each particle (translation and rotation) as presented by Paikowsky and Xi (2000). The two techniques were combined into an experimental system that enable s the investigation of both, kinematic behavior and interparticle contact force variation of photoelastic particles. This experimental system was termed Photoelastic Discrete Simulation (PDS). Experimental Setup The PDS was used to construct a granular mass made of photoelastic particles, and conduct trap door experiments. A trap door testing system has been developed to study the