The mechanical behaviour of synthetic, poorly consolidated granular rock under uniaxial compression

Abstract

In order to isolate the effect of grain size and cementation on the mechanical behaviour of poorly consolidated granular rock, we prepared synthetic rock samples in which these two parameters were varied independently. Various proportions of sand, Portland cement and water were mixed and cast in a mold. The mixture was left pressure-free during curing, thus ensuring that the final material was poorly consolidated. We used two natural well-sorted sands with grain sizes of 0.22 and 0.8 mm. The samples were mechanically tested in a uniaxial press. Static Young's modulus was measured during the tests by performing small stress excursions at discrete intervals along the stress–strain curves. All the samples exhibited nonlinear elasticity, i.e., Young's modulus increased with stress. As expected, we found that the uniaxial compressive strength increased with increasing cement content. Furthermore, we observed a transition from grain size sensitivity of strength at cement content less than 20–30% to grain size independence above this value. The measured values of Young's modulus are well explained by models based on rigid inclusions embedded in a soft matrix, at high cement content, and on cemented grain-to-grain contacts, at low cement content. Both models predict grain size independence in well-sorted cemented sands. The observed grain size sensitivity at low cement content is probably due to microstructural differences between fine- and coarse-grained materials caused by small differences in grain sorting quality.