Compacted soil behaviour: initial state, structure and constitutive modelling

Abstract

The paper explores the behaviour of compacted soils throughout the (dry density-water content) compaction plane by means of a conceptual framework that incorporates microstructural information. The engineering properties of compacted soils are described by an initial state in terms of a yielding stress, soil suction and a microstructural state variable. Microstructure is defined by the ratio of microvoid volume to total void volume. The pattern of variation of the microstructural parameter within the compaction plane has been determined, for some compacted soils, by analysing mercury intrusion porosimetry data. The microstructure of wet and dry compaction conditions can then be quantified. To ensure consistency, the framework is cast in the form of a constitutive model defined in terms of an effective suction and a constitutive stress that incorporate the microstructural variable. The model is shown to be consistent with a number of experimental observations and, in particular, it explains the intrinsic collapse potential of compacted soils. It predicts, for a common initial suction, a higher collapse potential for dry of optimum conditions than for wet compaction. It also predicts in a natural manner the observed evolution of soil compressibility during drained or undrained loading. Model capabilities are illustrated by application to a testing programme on statically compacted samples of low-plasticity silty clay. The compression behaviour of samples compacted wet and dry of optimum and the variation of collapse strains with confining stress have been successfully reproduced by the model.

A unified mechanical and retention model for saturated and unsaturated soil behaviour

“…Sivakumar [41] observed the same qualitative pattern of behaviour in his experimental tests, with initial wetting-induced swelling followed by wetting-induced collapse compression and then finally more wetting-induced swelling. Conventional models expressed in terms of net [53]) or with both s and Sr (Alonso et al [3]). The evidence presented in Figure 19b suggests that such complexity may be overcome by developing models that use * p as stress state variable and fully account for the coupling between mechanical and water retention behaviour.…”

One-dimensional compression and swelling of compacted fly ash

“…Comparing the behaviour of saturated soils under oedometric compression, it is possible to conclude that the aggregates remain even at high stresses (Otálvaro et al, 2015). According to Alonso et al (2013), if water accesses to an initially compacted dry soil, the microstructure will saturate first, because of the strong affinity between water and clay platelets. As soon as the microvoids in clay aggregates are saturated, any excess water will enter the macropores.…”

Challenges in treating earthen construction materials as unsaturated soils

“…In particular, UNSAT many geotechnical studies have shown that compacted clay forms aggregated structures which behave like large particles inside which water is trapped, being separate to water held in the pendular regime between the particles (both aggregates and larger solid particles). A recent example of such a modelling framework can be found in [14] in which the microstructure is quantified by the ratio of microvoids (i.e. voids within the clay aggregates) to total void ratio, a state parameter on which the stress and suction behaviour can be based.…”