Visualization of Active Mode of Failure behind Flexible Walls under Pure Rotation Using Digital Image Correlation

Determining the shape of the slip surface is an important step in the calculation of earth pressure on a retaining wall. Fundamental assumptions are made regarding the geometry of shear bands and slip surfaces in methods that are widely used in the analysis and design of retaining walls. Experimental visualization allows observation not only of how the slip surface develops, but also how it evolves with wall movement and soil deformation. This paper presents results of experiments performed to study the formation of shear bands in sand retained by a flexible wall. Particular attention was paid to the effect of the initial sand density on the distribution of volumetric and maximum shear strains as a function of wall displacement. To achieve these goals, a small-scale test box was built to model a plane-strain condition for a retaining wall system. A model flexible wall was hinged to the bottom face of the test box. The wall was then backfilled with silica sand using the air pluviation method. The pluviator openings were such that different relative densities were achieved (Dr = 15% and 77%). A displacement controlled mechanism was employed to apply outward displacement on the crest of the wall away from the backfill material to model an active-pressure mode of failure. The digital image correlation (DIC) technique was used to observe shear band evolution with progressive outward movement of the wall. The results highlighted the suitability of this technique to visualize and characterize the formation of slip surfaces in laboratory-scale experiments.