This relationship differs for different soils and clays, as shown by a "pF curve" giving the exact relationship. The water potential (or suction tension) declines with decreasing water content, but the exact relationship will have to be measured for the material of interest.

I cover the principles of this in Chapter 2 of Lambers, H., Chapin, F.S. III & Pons, T.L. 2008. Plant Physiological Ecology. 2nd edition. Springer, New York.

My opinion is the same

Shear strength of unsaturated soil is a very important topic in unsaturated soil mechanics. Shear strength is dependent on a number of variables and is highly amenable to probabilistic treatment. Probabilistic analysis of shear strength is used as an effective tool to evaluate uncertainty so prevalent in soil variables. In some researches the jointly distributed random variables method is used for probabilistic analysis and reliability assessment of Fredlund’s shear strength equation. The selected stochastic parameters are effective angle of shearing resistance for saturated soil (’, angle of shearing resistance with respect to matric suction (b and effective cohesion of saturated soil (c’), which are modeled using truncated normal probability distribution functions. The stress variables i.e., net normal stress and matric suction on the plane of failure are regarded as deterministic parameters. The resultant probability density functions are compared with the Monte Carlo and Point Estimated methods. Comparison of the results indicates superior performance of the proposed approach for assessment of reliability of the shear strength model for unsaturated soils.

For more information check this:

https://link.springer.com/chapter/10.1007/978-3-642-31343-1_25

Regards

MJM

https://link.springer.com/chapter/10.1007/978-3-642-31343-1_25

Dear Hossein

You can find your answer in page 3 of this paper:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=10&cad=rja&uact=8&ved=0ahUKEwjMi6__pvjXAhWiC8AKHR-kCzIQFghbMAk&url=http%3A%2F%2Fwww.ijens.org%2FVol%252011%2520I%252003%2F115603-3939%2520IJCEE-IJENS.pdf&usg=AOvVaw0Z-prkxf6WgF3N7BOpFfrH

Luck