Abstract
The water retention curve is fundamental for a comprehensive description of the hydro-mechanical behaviour of unsaturated soils. The water retention testing system developed at Durham University allows direct and continuous measurement of suction using a high capacity tensiometer, water content determined from mass readings of a digital balance and measurements of volume change. The system was modified to accommodate an additional tensiometer to measure suction at the top besides the existing one at the bottom of the soil specimen. Soil specimens were subjected to drying and wetting following two procedures: discrete measurements carried out in stages to ensure equalisation and continuous measurement at different rates. All suctions measured during continuous and discrete measurements were very close at high saturation degrees. At lower saturation degrees, the suction values from the top and bottom of the specimen deviated from suctions observed in discrete measurements. This deviation in suction values was more evident in accelerated drying and wetting patterns. This can be explained by the fact that water permeability reduces with the decrease in saturation levels.
Highlights
The relationship between the amount of water stored in soil pores and suction, known as Soil Water Retention Curve (SWRC), is essential to understand the hydromechanical behaviour of soils under unsaturated conditions
The discrete measurements carried out in stages (DS and wetting test (WS)) represent suction and water content values at hydraulic equilibrium, the state of equilibrium for the SWRCs obtained from the continuous measurements needs to be assessed due to continuous variations of water content
The predicted curve facilitates the comparison between the SWRC from drying test (DS) and the SWRCs obtained from the continuous measurements
Summary
The relationship between the amount of water stored in soil pores and suction, known as Soil Water Retention Curve (SWRC), is essential to understand the hydromechanical behaviour of soils under unsaturated conditions. The most common suction-controlled techniques used for water retention testing are axis translation, vapour equilibrium and osmotic suction [5, 6]. All these techniques suffer from the long testing time that is required to ensure water equalisation is achieved across soil specimens upon imposed suction levels. The high capacity tensiometer provides rapid and direct measurements of suction while the soil specimen is subjected to water evaporation or continuous wetting through water droplets, thereby allowing the SWRC to be obtained much faster and more representative of the natural mechanism (true drying) than other standard methods. The accuracy of measurements during continuous drying and wetting and the state of equilibrium of water and suction within the soil specimen have not been fully addressed and need further investigations
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