Failure, instability, and the second work increment in loose silty sand

Abstract

Triaxial tests have been performed to demonstrate the conditions for stability and instability in loose silty sand. Drucker (1951) and Hill (1958) stability conditions in terms of the sign of the second work increment were employed in the design of the stress paths used in the triaxial compression and extension tests performed with quasi-constant shear stress while the mean normal stress was reduced until failure occurred. It is shown that the sand is completely stable under drained conditions for any stress path and irrespective of the sign of the second work increment. This is demonstrated by completely stopping the change in stresses and observing the stable behavior in the range of stresses where the sand contracts and where it dilates. Once the effective stress failure surface is passed, the sand becomes unstable, and the sign of the second work increment is always negative. Run-away instability can occur inside the failure surface for loose silty sand under undrained conditions for which the sand tends to contract, pore pressures continue to develop, and the second work increment is negative. Liquefaction may follow if the loose silty sand is sufficiently loose.

Hydromechanical behaviour of a silty sand from a steep slope triggered by artificial rainfall: from unsaturated to saturated conditions

“…This should be a valuable tool for identifying impending collapse, as shown by Daouadji et al (2010) and Chang et al (2011) on saturated clean sands, by Monkul et al (2011) on a saturated silty sand, and by Casini et al (2010) on the saturated natural samples of Ruedlingen silty sand. These authors have focused mainly on the behaviour of the soil under saturated conditions, whereas the same criteria is applied to both saturated and unsaturated specimens in this paper by using a Bishop stress approach to analyse the data.…”

Instability of unsaturated soils: A review of theoretical methods

“…1a). Such instabilities are generally observed to occur when the measured second-order work takes negative values [41,81,83], which under constant deviatoric stress imply a transition from volumetric expansion to compaction (Fig. 1b).…”

“…Although the connection of these ideas with the notion of stability in Lyapunov's sense [78] is still an open question [27], the linkage between controllability, sustainability and bifurcation has favored the combined use of mathematical methods and physical concepts in the description of the failure modes of soil specimens [85]. An interesting example is the case of constant shear drained tests in fluid-saturated sands, often used to model the effects of rainfall events [4,5,25,41,83]. These experiments consist of two stages: drained shearing up to a given value of deviatoric stress and a subsequent constant-shear path (during which the effective confinement is reduced by pressurizing the pore fluid).…”