In the consolidation theory of unsaturated soils proposed by Fredlund, the excess pore-water and -air pressures will dissipate with time, and will eventually return to their values prior to loading. We all know that the pore water pressure in unsaturated soils is often negative. If the pore water pressure is still negative after the surcharge is applied, will the excess pore water pressure caused by the surcharge dissipate? I am so confused. Can anyone explain this?
My infirmation in this subject is limited but I think the excess pore water pressure of unsaturated soil dissipate even remain negative after applied load due to presence of air pressure in such case.
Zheng Chen
I think unsaturated soil mechanics is also not only contentious but also confusing and misleading issue. As you can see in your example journal part given by you show that almost same results and there is not much difference from the conventional saturated soil consolidation settlement logic anyway.
In the Unsaturated soil theory, in addition to the saturated soils, it is assumed that in the area above the water table (vadose zone), there is “air” with “water” in the inter-grain space of soil. The water between these grains is formed by the assumption that the attraction (affinity) between the water molecules increases and the “Capillary suction” effect on the fine grained soils. Therefore, the water pressure in this area is negative (because of suction effect).
In the condition that you put a strip load on the ground, an additional “excess pore air and water pressure” occurs under this external loads effect and this excess pore air and water pressure is dissipated in time (meanwhile the air pressure dissipation is faster). Immediately in the first condition, as in saturated soils state, while total stress conditions prevail, in the long-term permeability condition, as long as positive excess pore water pressure decreases, effective stress increases and soil body go through drainage condition.
I hope it can be helpful and clearly for you
Best regards.
Yazgan Kırkayak Thank you for your answer, it helps me a lot. Maybe I should do a consolidation test to better understand the characteristic of unsaturated soils. After all, the theories proposed by different people are very different, even contradictory.
Hİ Zheng Chen
I am very pleased to help you.
Your opinion totally true, there is a currently not fully understood theory. I also think like you, it is going to be useful for evaluate and understanding the theory if you provide comprehensive, appropriate physical condition and can do laboratory test. I would be very want to study test results and experiments that is obtained by you if you going to share.
Best regards.
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