Concept
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 [1,2]. 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 Procedure
The suction-controlled triaxial test apparatus is similar to conventional triaxial test apparatus with the exception that it allows controlling the soil suction. There are three main methods to control suction for unsaturated soil testing
- Axis translation method
- Osmotic method
- Vapor equilibrium method
Custom-designed unsaturated triaxial cells use one or a combination of the above methods to control the soil suction. The axis translation method [1], however, appears to be the most commonly used method in triaxial cells. It involves imposing and controlling positive air and water pressure in the soil sample (higher than atmospher pressure) and allows controlling the matric suction in the soil.
In the axis translation method for tiaxial testing, the cylindrical soil sample in the flexible membrane is placed on a high air entry value (HAEV) ceramic disc which allows passage of water and not air below a given air pressure. Accordingly, air and water pressure are independenpty controlled in the sample. The axial and lateral stress are applied using the axial loading ram and the confining water pressure as in conventional triaxial cells.
Application of air pressure results in diffused air bubble collecting beneath the ceramic disc. The diffused air should be flushed out and be used for accurate water change volume measurement. Volume change of the sample is usually measured using the double wall cell system. The double wall triaxial cell provides the same pressure inside and outside the inner cell allowing the accurate measurement of the total volume change of the sample. from a volume change transducer inserted in the cell pressure line.
Standards
- NA
References - External Links
- 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.
- Hilf, J. (1956). An investigation of pore-water pressure in compacted cohesive soils. PhD thesis, Technical Memorandum 654, University of Denver, Colorado.