Porosity–Height Relationship of Unsaturated Soil–Rock Mixtures Based on Field and Laboratory Investigations

The stripping ratio increases as the surface mine gets deeper, causing an increase in the height of waste dump slopes. This threatens stability since dumps are composed of soil–rock mixtures, mostly in an unsaturated state. In this research, field and laboratory studies were carried out on soil–rock mixtures to estimate in situ porosity, a key factor influencing shear strength of these materials, and consequently, the stability of waste dumps at various dump heights. Sand replacement methods in the field and standard Proctor test and consolidated drained triaxial shear tests in the lab were performed to establish the porosity–height relationship curves. It was found that the porosity decreases with height, as described by a negative exponential distribution with an exponent termed the “pseudo-pore mechanical compaction coefficient,” which determines the shape of the curve. The proposed relationship was then utilized to evaluate the compaction of soil–rock mixtures at a preresidual state. The parameters of the porosity–height relationship were also assessed for lightly, partially, and fully compacted dump slopes.