Consolidation Behavior of Clay Under Different Saturation Ratios
Consolidation Behavior of Clay Under Different Saturation Ratios
(OP)
My question is related to consolidation test behavior under different saturation ratios.
By standard, common oedometer tests are performed under 100% (approximately) saturation. They wet the soil for a period, then conduct the test. This will give full consolidation behavior.
However, in site, this is not true.
So, what will be the result, if I perform a consolidation test under a sample that is directly taken from the UD, not saturated in lab? Maybe you came across some graph in a book or paper, that shows in same graph, consolidation behavior of a clay in different saturation ratios?
What I am expecting? I expect that in less saturated clays most of the settlement will occur instantly.
Did you see any graph like that, or have you ever performed consolidation test in a specimen that is not saturated under lab conditions?
Regards.
bdbd
By standard, common oedometer tests are performed under 100% (approximately) saturation. They wet the soil for a period, then conduct the test. This will give full consolidation behavior.
However, in site, this is not true.
So, what will be the result, if I perform a consolidation test under a sample that is directly taken from the UD, not saturated in lab? Maybe you came across some graph in a book or paper, that shows in same graph, consolidation behavior of a clay in different saturation ratios?
What I am expecting? I expect that in less saturated clays most of the settlement will occur instantly.
Did you see any graph like that, or have you ever performed consolidation test in a specimen that is not saturated under lab conditions?
Regards.
bdbd





RE: Consolidation Behavior of Clay Under Different Saturation Ratios
RE: Consolidation Behavior of Clay Under Different Saturation Ratios
Thanks. So, did you just assume only elastic settlement will occur, for example, in a clay that has saturation around 70%.
I want to see the oedometer test behavior of these clays. I wonder if we have it in literature.
RE: Consolidation Behavior of Clay Under Different Saturation Ratios
RE: Consolidation Behavior of Clay Under Different Saturation Ratios
In the consolidation test, you inundate and load. You measure strain and time and you get various measurements of Cv. Cv is related to load increment and influenced by stress history and also degree of saturation. As the load increments increase and the strain accumulates, you eventually drive out all the air, rather than dissolve all the air. When saturation is found, you then get Cv for the saturated condition.
Maybe, I'm not being clear, but it's incorrect think that a soil under water table conditions by definition.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Consolidation Behavior of Clay Under Different Saturation Ratios
Thanks for explanations, I understand your point.
But, aside from this discussion, do you have an idea about my question? Can you imagine the oedometer results of 6 clay samples with 0, 20, 40, 60, 80, 100% saturation ratios?
Should there be same amount of settlement with different time-settlement behavior? Or, should there be less settlement for less saturated. Latter one justifies the OG's method of calculation..
If there is no study like this, I will perform it.
RE: Consolidation Behavior of Clay Under Different Saturation Ratios
In the below-80-percent range, I expect you could base the analyses on modulus. Problem is after the first rain (or the rising water table), the negative water pressure would go away, the stress history would remain and the presence of water would trump the elastic behavior - for a clay that is. . .
f-d
ípapß gordo ainÆt no madre flaca!
RE: Consolidation Behavior of Clay Under Different Saturation Ratios
RE: Consolidation Behavior of Clay Under Different Saturation Ratios
aeoliantexan, if you can expand more about your observations about secondary compression, it will be appreciated. I did not get it very clearly.
RE: Consolidation Behavior of Clay Under Different Saturation Ratios
You both mentioned the same thing. So, do we have oedometer proval of accepting NO CONSOLIDATION for saturation ratio below 85%? It is over-simplification, no doubt. But for practice, we should do that. I see people calculating consolidation settlements no matter if clay is saturated or not, and we have to make it clear.
I wonder what you guys, aeoliantexan and okiryu, do in practice? A year ago, I asked the same question but couldn't get a good answer. Imagine a soil profile, lets say 20m. GW is at 10m. Soil below is fully saturated, and soil above lets say 50% saturated. It is not possible to have same Eu and mv values but lets say it is.
So what do you do here?
a) Elastic at upper, elastic+consolidation at lower
b) elastic and consolidation at both.
This is one question I couldn't find answer for a long time. My current practice is, if GW is not changing or there is no risk of changing until the building is completed, I do not calculate consolidation for unsaturated or partially saturated layers.
RE: Consolidation Behavior of Clay Under Different Saturation Ratios
For a), the main issue is how you can calculate the appropriate modulus for elastic settlements. If you are off, say 30% in you modulus value, you will be getting +/- 30% for elastic settlement values. In my area, for our medium overconsolidated clays, people normally use unconfined compression tests or correlations with SPT for getting modulus (perhaps not the best way to get the modulus parameters, but it is the common practice here). There are also some correlations between Eu and PI from Mike Duncan.
Also, even for saturated clays, if you have reliable modulus values, you may use an elastic approach (Hooke's law) dividing the layer in several sublayers and then integrate all them to get the total settlement. Perhaps this is more applicable for overconsolidated clays. For normally consolidated clays, I will use values from the oedometer.
I am sure that more experienced engineers in this forum will provide better inputs and am also curious to hear from them...
RE: Consolidation Behavior of Clay Under Different Saturation Ratios
If the loess was moist enough (say, above optimum) and sigificant earth fill and/or structure loads were planned, we often preloaded with an earth surcharge. There was considerable immediate settlement and some settlement with time, and we would monitor with settlement plates to determine when settlement was essentially finished so we could remove the surcharge. This was nearly always less than 3 months.
We founded a grain elevator on collapsible loess after overexcavating and replacing about 10 feet of it. We monitored the settlement during the first loading, maybe the second season too. I forget how much it settled, probably between 8 and 12 inches. I went back out of curiosity several years later, and it had settled a few more inches. I took that as secondary compression. Later, we drilled more borings for a second structure, and found the water table had risen up into the loess because the farmers nearby had started irrigating the fields. So---who knows?
bdbd, I would rarely put footings on clay at a pressure that would cause virgin consolidation. For your example, yes, I would calculate consolidation settlement in the deep saturated soil if virgin consolidation appeared likely, and elastic settlement in the shallow soil.
I seldom considered recompression settlement using consolidation tests because it seemed to overestimate the settlement greatly.
RE: Consolidation Behavior of Clay Under Different Saturation Ratios
RE: Consolidation Behavior of Clay Under Different Saturation Ratios
On well-drained hilltops, especially with trees, water contents might be roughly 8% to 15%. In areas with some infiltration, usually in the high teens to mid-twentys. Where underlain by glacial till or other clay, it might be saturated - around 30%.
We did not run a lot of Atterbergs, but PL either side of 20 and LL 30s and 40s.
Based on 20-year-old memories.
RE: Consolidation Behavior of Clay Under Different Saturation Ratios