Concept
The thermo-hydro-mechanical oedometer (THM Oedometer) is an advanced laboratory test and refers to an oedometer test with temperature and suction control [1]. The THM oedometer
allows characterizing the soil
stress-strain behavior during one-dimensional compression or swelling under controlled thermal and saturation condition. It allows testing of soil samples at different degrees of saturation and different temperatures. The THM oedometer is considered as an advanced laboratory tests main used for research or special projects in geotechnical engineering practice.
Soils in many cases are unsaturated in their natural state. It means
that the pores are filled not only with water (liquid) but also partly
with air (gas). The co-existence of the these two phases in unsaturated
soils generates the soil suction. In simple words, the total suction in
the soil refers to the potential of soil water and it can be measured in
terms of the partial vapor pressure of the soil water [2,3]. Total soil
suction has two components: (i) osmotic suction, (ii) matric suction.
Osmotic suction is linked to the water solute potential and usually is
neglected in unsaturated soil mechanics. Matric suction represents the
liquid tension tension generated through the interaction of soil water
and the soil matrix and it is defined as the pressure difference between
the soil water and the surrounding gas. Soil suction influences the
mechanical behaviour of unsaturated soils, including the shear and the
compression properties. Accordingly, mechanical behaviour of unsaturated
soils undder triaxial stress-states shall be assessed under
controlled-suction condition.
Description and test procedure
The thermo-hydro-mechanical oedometer apparatus is similar to oedometer test apparatus with the exception that it allows controlling/measuring the soil suction and temperature. The test is designed to
investigate the effect of the temperature and the suction on the
mechanical response of the soils. There are three main methods to control suction for unsaturated soil testing :
Custom-designed
thermo-hydro-mechanical oedometer cellls cells use one or a combination of the above
methods to control the soil suction. The vapour equilibrium method, however, appears to be an approriate candidate in combination with temperature-controlled condition. In the vapour equilibrium method, total suction is controlled through the control of relative humidity. A salt or acid solution regulates the
relative humidity of the atmosphere within a desiccator. A convection
circuit forces this regulated atmosphere to circulate through the sample
or on its boundaries. Once the equilibrium reached, the soil
water potential is equal to that of the vapour phase which itself is
in correlation with the relative humidity of the surrounding atmosphere.
The temperature is usually controlled by means of a heating device consisting of a
ring-shaped chamber surrounding the soil sample and filled with
circulating heated water or oil. The liquid itself is heated to the desired temperate through an electric cryostat. An approriate thermal insulation of the oedometer cell is required in order to avoid the influence of ambient room temperature. This can be done by placing the oedometer cell system in a isulated box or by using thermal isulating covers around the cell.
The mechanical load, or the axial stress can be applied by means of a classical hydaulic jack. Depending on the controlling systems and methods, different ranges of suction, temperature and axial stress can be applied to the sample.
The test is carried out on a cylindrical specimen of saturated or unsaturated soil. As in oedometer test,incrementals of vertical static load are applied to the sample and the vertical displacement of the sample is measured at different loading steps. The difference here is that the loading can be a combination of vertical stress, suction, and temperature variation. The results are then used to characterize the consolidation and the stress-strain behaviour of the soil under the influence of suction and temperature.
References
François, B. & Laloui, L. (2010),
An Oedometer for Studying Combined Effects of Temperature and Suction on Soils, Geotechnical testing journal, vol. 33, num. 2, 2010, p. 112-122
Richards, B. (1965). Measurement of free energy of soil moisture by
the psychrometric technique using thermistors. In Moisture equilibria
and moisture changes in soils beneath covered area, pages 39–46,
Australia, butterworths.
Fredlund, D. and Rahardjo, H. (1993). Soil mechanics for unsaturated soil. John Wiley & Sons, Inc.
Test Standards
Not available
Thermo-Hydro-Mechanical Oedometer test results on Database
GD002: Temperature and suction controlled oedometer tests on reconstituted sandy silt, François B., Laloui, L. Access
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