ABSTRACT
Currently, the photoelastic measuring method is the only way to obtain field information on the stress distribution in plane strain tests with granular material. In this technique, sand is replaced by crushed glass. The mechanical properties of random shaped glass particles are similar to those of sand. The assembly of crushed glass can be made transparent by submerging the pores in a liquid with a refractive index equal to that of glass. The transmission of the forces through the sample can be made visible with a circular polariscope (Dantu, 1957). An automated optical device and a calculation procedure have been developed at the University of Delft (Allersma, 2001) to digitize the stress distribution in plane strain tests with crushed glass. In fact the grains themselves are used as a sensor in this technique. They transform the contact forces into optically measurable stress components. Thanks to this technique two stress components can be measured in any point of a plane strain sample. If the normal stress at some points at the boundary is known the equations of equilibrium can be used to calculate the stress distribution in the sample. Because the sample is transparent, normal light can be used to digitize the co-ordinates of black markers during a test.
