# Experimental Studies of Strain and Stress Fields in a Granular Medium Under Active Pressure Using DIC and Elasto-Optic Methods

## TL;DR

This study introduces a new method to observe strain and stress in granular materials under active pressure, revealing how shear bands form and how stress and strain are related.

## Contribution

The paper presents a novel experimental approach combining DIC and elasto-optic methods to study active earth pressure in granular media.

## Key findings

- Active failure occurs through narrow shear bands starting near the wall base and moving upward.
- Low-strain zones correlate with strong force-chain development, while high-strain zones show stress release.
- The findings offer new insights for calibrating DEM models and understanding active pressure in confined granular backfills.

## Abstract

This study presents a novel experimental methodology enabling the synchronous observation of strain and stress evolution in granular backfill subjected to active earth pressure. A physical model of plane deformation was used in which a rigid retaining wall was gradually moved away from the ground while simultaneously recording, at each step, both displacement-based images for digital image correlation (DIC) and photoelastic pictures of the force-chain rearrangements. The results show that active failure develops gradually through narrow shear bands, initiated near the wall base and propagating towards the ground surface. A consistent inverse relationship between shear-strain location and photoelastic stress concentration was identified: low-strain zones within the shear wedge in the shear and volumetric strain images correspond to strong force-chain development, whereas high-strain zones (strain localization) correspond to local stress release. These findings provide new experimental evidence regarding the micromechanics of active pressure and offer comparative data for calibrating DEM (discrete element method) models and interpreting the reduced active pressures reported in confined granular backfills.