Effective stress in unsaturated soils: Lessons from capillarity in regular sphere arrangements

Abstract

In this paper, capillary stresses and water retention of regular arrangements of rough spherical particles are analysed to shed light on the effective stress in unsaturated soils. These arrangements can be used to model soils with uniformly graded round particles such as sand as they are simpler to analyse due to their symmetries. Capillary stress determines the effective stress in unsaturated sphere arrangements and is highly dependent on the morphology of the soil voids.In general, it is shown that capillary stress and water retention dependence on suction and degree of saturation in regular arrangements of spheres is more complex than usually assumed with relevant implications for unsaturated soil mechanics and its effective stress. It is also shown that: (i) sphere roughness explains the observed disappearance of capillary stresses in sand with drying; (ii) the capillary stress evolution with suction (or RH) is non monotonic; (iii) it is possible to have significant capillary stresses at zero suction in regular and irregular sphere arrangements, and the Bishop’s effective stress cannot represent this or even the capillary stresses in regular arrangements; (iv) the water in the pores can assume diverse and complex configurations besides the simpler isolated menisci that prevail at larger suctions, and the alternative configurations for the same suction or degree of saturation support hysteretic response.