This paper described the use of photoelasticity and digital image analysis to determine horizontal stress (σh) and to estimate the lateral earth pressure coefficient at rest (Ko) in 2-dimensional particulate assemblies. Square 25.4 by 25.4 mm (1.0 by 1.0 in.) glass plates with 12.7 mm (0.50 in.) circular holes at their centers served as the photoelastic force sensors. Digital image processing and analysis quantified the intensities of the light colors at key locations of high stress concentration in the loaded sensors. The light colors were separated into red, green, and blue (RGB) components. It was shown that the R-intensity was the most sensitive to the applied stress and thus was used to determine the induced stresses. Calibration charts between relative red intensities and applied stress were developed. The calibrated sensors were attached to the sidewalls of the test box so that horizontal stress in 2-dimensional particulate assemblies could be measured. The relative red intensities from the sensors at the sidewalls of the test box were converted to horizontal stress values from which Ko could be estimated. The results showed that quantified intensities of light colors can be used for estimating stress in experimental photoelasticity.
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