Unconfined shear strength of compacted unsaturated plastic soils

Abstract

A central goal in unsaturated soil mechanics research is to create a smooth transition between traditional soil mechanics approaches and an approach that is applicable to unsaturated soils. Undrained shear strength and the liquidity index of reconstituted or remoulded saturated soils are consistently correlated, which has been demonstrated by many studies. In the liquidity index range from 1 (at wl) to 0 (at wp), the shear strength ranges from approximately 2 kPa to 200 kPa. Similarly, for compacted soil, the shear strength at the plastic limit ranges from 150 kPa to 250 kPa. When compacted at their optimum water content, most soils have a suction that ranges from 20 kPa to 500 kPa; however, in the field, compacted materials are subjected to drying and wetting, which affect their initial suction and as a consequence their shear strength. Unconfined shear tests were performed on five compacted tropical soils and kaolin. Specimens were tested in the as-compacted condition, and also after undergoing drying or wetting. The test results and data from prior literature were examined, taking into account the roles of void ratio, suction, and relative water content. An interpretation of the phenomena that are involved in the development of the undrained shear strength of unsaturated soils in the contexts of soil water retention and Atterberg limits is presented, providing a practical view of the behaviour of compacted soil based on the concept of unsaturated soil. Finally, an empirical correlation is presented that relates the unsaturated state of compacted soils to the unconfined shear strength.

The plastic limit of clays

“…The results of Cafaro (2002) and Marinho & Oliveira (2012) illustrate that the desaturation point lies close to the plastic limit. However, there is considerable evidence that the soil fabric and the mechanisms of aggregate development and crack propagation are also involved in brittle failure.…”

“…Several researchers have also found a wide range of suctions at the plastic limit (Rollins & Davidson, 1960;Black, 1962;Dumbleton & West, 1970;McBride, 1989; also Marinho & Oliveira (2012) at the optimum water content) and effective stresses at the plastic limit (Carrier & Beckman, 1984;Nagaraj & DeGroot, 2004).…”

“…At the plastic limit, wet of optimum (Gurtug & Sridharan, 2004;Marinho & Oliveira, 2012), the soil crumbles as a result of its aggregated and cracked state. Aggregation close to the plastic limit in the uncompacted state is demonstrated by Tarantino and De Col (Tarantino et al, 2009).…”

Remoulded shear strength at plastic and semi-solid states

“…While the paper has offered strong evidence of the exponential relationship between strength and water content in the plastic region, more evidence would be required to use the expression derived with confidence for the prediction of soil strength. While we have also expressed concerns with regard to the methodology used to determine the variation of strength for soils at water contents below the plastic limit, similar trends shown are also reflected in the data presented by Marinho and Oliviera (2012) for compacted soils. With additional testing, this paper may prove to have offered a valuable insight into the strengths of these brittle materials.…”

Use of fall cones to determine Atterberg limits: a review

“…Even PSL 25 can give a wide range of consistencies from brittle to plastic, sticky to non-sticky. Nevertheless, below the plastic limit the s u -I L relationship follows a significantly different path (Marinho & Oliveira, 2012;Vinod et al, 2013).…”