Dear Ebrahim,

I'm not sure in which context you mean, but are you familiar with Soil-water retention (curves) for example?

At saturation the forces acting on the water content, i.a. adhesion of water on the solid surfaces and cohesion within the water body, will be 0. As soon as the water content becomes lower, the "pressure" becomes negative (called matrix tension). In practice this means the water attracts the solid face to each other. That's why it's easier to build a sandcastle with "wet sand" instead of saturated or completely dry sand.

You can imagine that pore space is important for this, as in smaller pores the surface tension is relative more important than the cohesion forces.

Hello Ebrahim,

I agree with Jeroen. A mathematical approach to describe the partially saturated state is the approach after Bishop.

A nice publication on the effective stress in a partially saturated soil/rock is:

Vlahinić, I.; Jennings, H. M.; Andrade, J. E.; Thomas, J. J. (2011): A novel and general form of effective stress in a partially saturated porous material - The influence of microstructure; Mechanics of Materials (43/1): S. 25–35

I strongly agree with the answer by Dr. Jeroen (J.H.T.) Zethof . Dr Christian Günther has referred good paper.

Dear Ebrahim,,,, the pressure in the pores can be resulted either from water or air, anymore, there is a pore pressure when the soil saturated or partially saturated. for saturated soil , the pore pressure termed as pore water pressure whereas for partially saturated soil there is air pressure and water pressure in the pores. furthermore, the water content depends on soil condition and type, if you provide me with these two conditions i can send you approximate values. 

Yes. That is depends on soil conditions