Settlements in unsaturated granular soils induced by changes in saturation and suction

Abstract

In this contribution the hydro-mechanically coupled behaviour of a sand is experimentally investigated with the focus on settlements induced by changes in degree of saturation and suction. This phenomenon, referred to as collapse behaviour, is attributed to rearrangements of the grain skeleton due to changing capillary effects on wetting of the soil. For the experimental investigation of the collapse behaviour of a medium coarse sand cyclic dryingwetting tests are performed under oedometric conditions. In the test setup a sand specimen, subjected to a constant small vertical stress, is cyclically dried and wetted by volume control of the pore water, whereas matric suction is measured using a tensiometer implemented to the specimen loading plate (topcap tensiometer). The test procedure, originally designed to investigate the hysteretic nature of the soil-water characteristic curve of the sand, allows to measure the one dimensional volume change of the specimen as evoked by the applied hydraulic paths under constant net stress. By varying the specimen void ratio the impact of density on the collapse behaviour can be assessed. The test data is important for the development of a mechanical constitutive model which can take the volume change behaviour due to suction changes into account.

Theoretical and experimental investigations on the role of transient effects in the water retention behaviour of unsaturated granular soils

“…This represents a link between the hydraulic and mechanical behaviours of unsaturated soil where changes in s and S r may provoke capillary collapse. 28,36 Also, the above-mentioned effect of possible water redistribution is visible within the drift of suction stress at constant S r = 0.8 for the last cycle in Fig. 15.…”

“…26, 27 to study the hysteretic SWCC of a medium coarse-grained sand and then it was further refined to study hydro-mechanical coupling and capillary collapse in sand due to drying and wetting cycles. 28 The setup is based on a simple shear apparatus for the investigation of unsaturated sandy soils presented in Ref. 29.…”

An automated testing device for continuous measurement of the hysteretic water retention curve of granular media

“…The method has already been applied to investigate different hydraulic paths, especially scanning paths, of different sands at cyclic drainage and imbibition [27] and to investigate non-equilibrium WRCs by applying different flow rates of pore water [30,33]. With a suitable set-up, allowing to control axial stress and to measure the axial displacement of the specimen top, also capillary collapse during cyclic drainage and imbibition could be investigated [32]. The mentioned method, that was originally applied within a modified simple shear device, described in [29], has now been redesigned in a miniaturised and simplified version with the aim to place the whole set-up in a CT-scanner.…”

Transient Two-Phase Flow in Porous Media: A Literature Review and Engineering Application in Geotechnics

“…Using the same device from Milatz and Grabe [254], the shear loading was switched off, and the normal loading was preserved so that the suction-controlled oedometer could be achieved. Eventually, according to their experimental outcomes, Milatz, Törzs, et al [256] replicated the wetting-induced soil collapse. This shearing and consolidating apparatus demonstrated its advantages in studying the hydro-mechanical coupling behaviour of unsaturated coarse-grain soil.…”

“…Later, Milatz, et al [256] investigated the settlement behaviour of unsaturated granular soils by varying soil suction and water content. Using the same device from Milatz and Grabe [254], the shear loading was switched off, and the normal loading was preserved so that the suction-controlled oedometer could be achieved.…”