As shown in Chapter 9, Section 9.5, a complete description of a material’s behaviour necessitates an effective stress approach with the pore pressures (or pore
Number of load pulses Number of load pulses
Fig. 10.3 Typical results from permanent deformation testing where the accumulated axial strain is shown as a function of the number of load pulses in the permanent deformation tests (Erlingsson, 2000) suctions) being separately controlled, or monitored, from the applied pressures. Because most road materials are coarse grained and partially saturated and/or above the ground water table, it usually proves impossible — and certainly it is impractical in most situations — to monitor the pore suctions during each transient pulse. For this reason, almost all testing programs determine parameter values for resilient and incrementally-developed plastic strain models in terms of total, not effective, stresses. Instead, test procedures typically seek to control the moisture or suction conditions so that data is collected at representative conditions.
As described above, unsaturated soils exert an attraction on water, either by capillarity in the pores, between grains of the soil, or by the physico-chemical effects. The energy necessary to extract water from the unsaturated soil, for example from a dense compacted swelling clay used for engineering barriers in nuclear waste, can be higher than 10 MPa. However, a granular soil will have a suction that is
several tens of kPa (Delage 2001). The following techniques are commonly used
for suction/moisture control:
i) Axis translation system
This system is based on ceramic low porosity stone, called High Air Entry Value (HAEV) porous stone. The principle of the system is that the pores of the ceramic are too small to de-saturate, even when a high air pressure is applied. In other words, the capillary air-water menisci located at the surface of the ceramic can resist an air pressure applied to it, thereby maintaining a continuous water column from stone to specimen. The system has been employed for determination of the water retention curve (i. e. the soil water characteristic curve) (as in the Richards cell) as well as for suction control in oedometer and triaxial testing (Peron et al., 2007; Cuisinier & Laloui, 2004). The retention curve can be determined as follows. A saturated sample should be placed in the cell and the air pressure increased in a step-by-step progression. For a given air pressure, suction equilibrium should be achieved in a time period of a few days (typically 3-10). At the end of equilibrium phase, the sample should be quickly withdrawn and weighed to determine the water content. The sample is then placed into the cell and the air pressure increased to bring about a new equilibrium with a higher suction and lower water content and lower degree of saturation. The process is then repeated as desired.
ii) Osmotic technique
This was initially used by Kassif and Ben Shalom (1971) in an oedometer test to study expansive soils. Several authors extended this technique to a standard triaxial apparatus (Laloui et al., 2006; Komorik et al., 1980; Delage et al., 1987). The technique is based on drainage of the specimen caused by the process of osmosis. The soil sample is placed in a tube-shape cellular semi-permeable membrane, which is immersed in an aqueous solution of PolyEthyleneGlycol (PEG). Since the PEG molecules are too big to cross the semi-permeable membrane, the difference between concentration of the PEG solution and that of pore water results in an osmotic pressure. Thus, the suction value depends on the concentration of the solution: the higher the concentration of the polyethylene glycol (PEG) solution, the higherthe suctionof soil specimeninthe semi-membrane after the equilibrium state is reached (Dineen & Burland, 1995; Delage et al., 1998).
iii) Vapour phase method
Usually, this method is used to determine the water retention curve of soils with high suctions, though it can be used to bring a triaxial or other test specimen to a particular moisture-suction condition prior to mechanical testing. The method is based on the theoretical standpoint that water potential (i. e. suction) is related to a particular relative vapour pressure of the water in the soil-water system (Qian et al., 2006). The relative vapour pressure of water in equilibrium with the system is characterised by its relative humidity. Therefore, suction can be established by creating the relative humidity that is related to the concentration of a solution identical with the composition of the soil water. The soil specimen should be placed in a desiccator containing an aqueous solution of a given chemical compound. Depending on the physico-chemical properties of the compound, a relative humidity is imposed within the desiccator. Water exchange occurs by vapour transfer between the solution and the sample via the vapour and a given suction is imposed when vapour equilibrium is reached. It should be noted that the technique generally requires a very long equilibrium time that can be a few months (1-2 and up to 6). Marcial et al. (2002) introduced an air circulation technique to reduce the equilibrium time from 6 months down to 2-4 weeks. Also, it should be noted that either the same products at various concentrations or various saturated saline solutions may be used.
In conventional practice, osmotic and vapour phase methods have quite often been combined to bring soil specimens to any desired point on the soil-water characteristic curve. The osmotic technique should be used to control low matrix suction (less than 2 MPa) and the vapour phase method for measurements of higher suctions (> 2MPa).
