An explanation may be: At suction smaller then the aev it is supposed that the air is not entering the pores, there is no transport of air and the pores are fully saturated with water. Therefore, the slight desaturation observed during "boundary effect stage" is not because the expelled water from the pore system in the bulk material is replaced with air. The outer menisci at the boundaries are pulled causing suction increase and some mass of water may be lost . This is the motivation to use the term "boundary effect stage".

Thank you, Maria, for the explanation. It makes sense. Do you believe that the dissolved air in water (Henry's Law) can also be a reason for that? Is the saturated soil mechanics valid in this range (since the pore spaces are still filled with water)?

In the case of a large-scale problem (e.g. the tunnel construction in low-permeable rock), a slight desaturation has a considerable influence on the pore pressure development if the mass balance of the liquid is written for a partially saturated medium, i.e.

d( fluid density * porosity* saturation)/dt +div.q=0

Hi Dr. Khaledi -

This is a really interesting question. Perhaps you already know that aev estimate is not a very exact measurement, it is simply a tangent line from the desorption curve. Fredland and Xing (1994) describe needing a better method: https://www.soilvision.com/downloads/docs/pdf/research/cangeoequa.pdf

I agree with Maria Datcheva -- at very low suction, the moisture content is above the liquid limit. The desaturation you mention in the boundary effect stage (suction still lower than the air entry value) is water that is lost as the bulk soil as a whole (not just the fines) all begins to cohere and behave more like a plastic material would. Theoretically, the air entry value represents the point at which the force of gravity is equal to the matric suction in the very largest pores, or alternately, the amount of pressure you'd need to use if you were going to blow air into them. If you were working with a very fine plastic soil, then aev might also represent the pressure where soil moisture is at the plastic limit? I'm not really sure about this, though.

I am pretty sure that the saturated soil mechanics equations are still valid, because you need an air pressure term to calculate effective stress in an unsaturated soil, and (since it is still technically saturated) air pressure is still zero.

we may discuss the use of "unsaturated" soil mechanics for suction below air entry value + how to determine the air entry value correctly. These questions are more relevant for low permeable soils where we may need to change the experimental set-up close to aev.