Critical state of polymer-coated sands

The water-repellent behaviour of polymers such as polydimethylsiloxane enables them to be utilised as coatings in soil particles to control wetting, with potential applications in barriers and fills. However, little information exists on the mechanics of coated soils. Here, the mechanical behaviour of a coated sand subjected to two mass ratios of dimethyldichlorosilane (0.05% and 3%) was investigated by triaxial tests. Dimethyldichlorosilane was mixed with the sand to produce polydimethylsiloxane coatings. Damage to the polymer coatings was investigated by interferometry. The results revealed that for a low mass ratio of dimethyldichlorosilane (0.05%), the shear strength and critical state locus are similar to those of natural sand; whereas for a high mass ratio of dimethyldichlorosilane (3%), there is a marked reduction of the shear strength, with the critical state locus showing a significant difference compared to that of natural sand. The quantification of roughness prior to and after the triaxial tests confirms the damage to the polymer coatings, with the effect of the coatings diminishing with the rise of stress level. The findings highlight the importance of the thickness of the coatings, with the 0.05% coated sand sufficient to manipulate the soil's hydraulic behaviour and the 3% coated sand opening new possibilities to manipulate the soil's mechanical behaviour.