Settlement of Cohesive Soil

[Nr Ahmad]

[pre]Hi Dears.
In our Region most of the soils encountered at foundation levels are Unsaturated Clay and silts (CL, CL-ML, GC) The ground water table is usually (10 - 40+ meter) below the foundation level, So if the soil is not saturated can I assume that there is no consolidation settlement of the soil and only elastic settlement to be calculated.?

 

[EireChch]

If have read on this forum that many engineers only consider consolidation settlement if the CLAY layer is below the water table.

Personally I consider consolidation in CLAY soil regardless of water table location. This is the only approach that I have seen implemented. I have worked in several companies in several countries and everyone did it the same way. But obviously, from my first line, people have different approaches.

 

[dirt-nerd]

I believe the most appropriate approach is a little more specific than "above vs below groundwater".

Consolidation occurs where there is an additional load, it increases the PWP within the soil, and PWP dissipates from expelling the water. The PWP may increase where the soil is saturated or nearly saturated (can happen above or below GW).

I recommend checking the clay using moisture, density, Gs to determine the saturation. If the saturation < 70%, you'll be dealing with elastic settlement. If saturation > 90%, you'll be dealing with consolidation settlement. If 70% < saturation < 90%, you should look into it further. Then the question becomes, based on the elastic settlement, will I reach a point where consolidation settlement may kick in?

 

[Nr Ahmad]

First of all thanks for you time and comments on this issue.
As I stated earlier in our area the soils at foundation level and well below that are mostly found in an unsaturated state (I think the saturation is significantly below 70%) termed as the Vadose Zone, saturated soils are rarely found, I studied some references on the behavior of unsaturated soils at Vadose Zone, which states that with De-saturation as the water is expelled from the soil the void spaces are filled with free air the remaining moisture bounds to the soil particles.
I haven't found any good reference to the settlement of unsaturated soils however the book (Geotechnical-Engineering-Unsaturated-and-Saturated-Soils_Jean-Louis.B) contain a chapter on consolidation test of unsaturated soils, it concludes that unlike for the saturated soils measuring the effective stress for unsaturated soils is difficult with a +-40% error vid comparison to the correct value. And except for the collapsible soils, a soil in the saturated state with the water in compression is more compressible than the same soil in the unsaturated soil with the water in tension.


Considering that air is 20K times more compressible than water Why the soil with water in tension is less compressible than the same soil with water in compression?

 

[Nr Ahmad]

Once for an Unsaturated CL-ML soil the lab reported only elastic settlement and consolidation settlement as 0; I have asked the reason, The lab clarified that Consolidation means expulsion of water from a saturated soil sample due to an increased load and since the soil is not saturated there is no consolidation in the soil. @nerd explained the consolidation well.

The question raised from there, When the soil is not saturated what happens then? can we rely solely on elastic settlement? as I posted above in vadose zone soils when the soil is de-saturated the void spaces are filled with free air, and some moisture remains well around the soil grains. what happens with the air and the remaining moisture when the soil is loaded with an excessive load since the fine grained soils have low water and air permeability.

@EireChch; I think in conventional geotechnical practice all fined grained soils are assumed saturated maybe that's the reason why many labs consider Sc irrespective of phreatic level, Maybe that's one of the solution .

 

[geotechguy1]

The conservative approach is to assume saturated; if you do this, and the answer is that settlements are tolerable, no need to investigate further

 

[EireChch]

Dirt Nerd - while I like the thought process of your response re saturation level, the minute you put that in a report a reviewer will say "why not 60% or 80% saturation". How to you prove what you proposed is correct. tbh I think you probably are correct but proving it is the hard part.

Also, i would say many clay layers are in the 70-90% region and then you are screwed as you have to look at unsaturated soil mechanics. I have never even heard the word "unsaturated soil mechanics" spoken in a geotech office. Engineers just dont deal with it, excluding the very niche problems that have to be sovled using it, like some dam stability assessments.

Hence why, in my experience most just assume saturated.

 

[LittleInch]

Does it not rain where you are?

Water table might be low, but surface water needs to go somewhere, unless this is the Gobi desert.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.