Foundation in underconsolidated clay
Foundation in underconsolidated clay
(OP)
I'm dealing with a mat foundation to be constructed at 3m below grade level. Dimensions are 20m x 20m. The loading from the structure will be 50 kn/m2. The removed load of soil is estimated around 60 kn/m2 (3mx20kn/m3). Normally I wouldn't expect settlements to occur, however the oedometric tests from the geotech lab indicate that the preconsolidation pressure of the soil is 35 kn/m2. Theoretically, it means that (without construction) the soil is still undergoing consolidation (ocr less than 1). soil is composed of clay with degree of saturation>95%. Modulus of elasticity is 5000 kn/m2.
Will the structure undergo consolidation settlements after the finished grade with the structure because of the ocr value?
Will the structure undergo consolidation settlements after the finished grade with the structure because of the ocr value?





RE: Foundation in underconsolidated clay
RE: Foundation in underconsolidated clay
It is a relatively undisturbed sample. I believe they used rotary drilling sampler to obtain it, not a Shelby tube.
So bottom line, I shouldn't expect settlement since the oedometer test fully submerges the sample on water while that's not the case in field conditions. Is that what you mean oldestguy?
RE: Foundation in underconsolidated clay
RE: Foundation in underconsolidated clay
The soil is composed of 51% clay, 23% silt, 23% sand, and 3% gravel. LL = 52, PL = 29, PI = 23, and LI = -0.5. The soil is classified as CH. - 1 grain size distribution and classification.
The unit weight is 20 kN/m3. The field engineer did not report they've faced the water table (drilled to a depth of 35 m). SPT N values range from 20 to 30.
2 consolidation tests (oedometer tests) were done for this clay, samples obtained at 4m and 7m. The preconsolidation pressure resulted to be 35 kN/m2 and 41 kN/m2 respectively. The virgin curve starts at around 40 kPa as loading is increased (at both samples).
For those samples, the overburden stress is 80 kN/m3 and 140 kN/m3 respectively. Based on this, calculating the OCR, i believe I'm facing underconsolidated soils (OCR less than 1).
I hope i've given a clearer picture of the problem.
Now, If we excavate to a depth of 3m, we remove a the load of 60 kN/m2. The building load will be 50 kN/m2. Even though I know the load of the removed soil is more than the load of the structures, should I expect settlements now that I know the soil is underconsolidated?
Or should I not believe the oedometer data and compute the OCR based on Mayne and Kemper 1988-Profiling OCR based on SPT and CPT? Based on Mayne and Kemper's correlations, with SPT N value from 20 - 30, the OCR will be greater than 1. If I can assume that the OCR is greater than 1 and the soil is overconsolidated, the settlements will equal 0 since we are removing more load than actually adding.
RE: Foundation in underconsolidated clay
RE: Foundation in underconsolidated clay
The company who performed the borings indicated that water level was not present at all.
RE: Foundation in underconsolidated clay
I'll post a chart that I have used many times with confidence.
Note how low your liquidity index is.
This indicates your effective consolidation pressure due to low moisture content gives very high load capability. That is why looking at the consolidation test plot may also show something wrong with the test. Errors are very possible there!!! That is something that seems fishy about the interpretation of that curve plot as meaning the correct value. Before getting too involved with the data you have, I'd have an experienced geotech look at EVERYTHING.
RE: Foundation in underconsolidated clay
RE: Foundation in underconsolidated clay
RE: Foundation in underconsolidated clay
Considering the low moisture content, possibly at the whole site (two tests might be representative, who knows), I suspect soil shrinkage cracks affected the consolidation test. Not showing the plot doesn't help me. Anyhow, take a look into the crack situation by a Google search. Two of many papers found are listed here which indicate there might be an explanation for the apparent assumed partial consolidation results of the consolidation test on low moisture sample.
I know of some other affects of shrinkage that are not apparent by just looking at classification tests. One being permeability.
http://users.ictp.it/~pub_off/lectures/lns018/39Ta...
https://www.researchgate.net/publication/260812537...
RE: Foundation in underconsolidated clay
- SPTs ranging from 20 to 30
- Unit weight of 20 kN/m3
- Low W% and no even close to the LL
- No groundwater
I would not expect underconsolidated conditions. I think that there was some sampling disturbance (looks like they did not use the proper tools), that affected the consolidation test results. Do you have any other strength test results? If so, you can double check your consolidation test results following BigH's suggestion.
Also, you are reporting the preconsolidation pressures in kN/m3 units (I assume this is a typo).
Anyway, talk with the drillers and the lab and verify with them the sampling and testing conditions.
RE: Foundation in underconsolidated clay
Yes, I do not trust the reports from the lab either.
I did not receive any unconfined compression results, but Im calculating Cu based on the equation Su/Pa = 0.06*N. For N = 20, Su = 120 kPa. With an Su of 120 kPa, using BigH's equation, the estimated preconsolidation pressure is around 500 kPpa.
I will not be designing, the foundations, my supervisor will, Im a young engineer and just wanted to know out of curiosity for future reference on what to do in cases like this.
Also, for you that have had much greater experience than myself, are underconsolidated clays common, and how have you proceeded with settlement calculations if you've faced them?
RE: Foundation in underconsolidated clay
RE: Foundation in underconsolidated clay
One question: how did you get the soil modulus of 5000 kPa?
RE: Foundation in underconsolidated clay
RE: Foundation in underconsolidated clay
Do you have any unconfined compressive strength data (i.e., pocket penetrometer) data?
I'd also look at the water content in light of the LL and I'd look at the undrained shear strength in light of SHANSEP, considering some Su/P of 0.2 or thereabouts.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Foundation in underconsolidated clay
I was under the impression that natural deposits of underconsolidated soils may be found in rapidly-depositing deltas or bays. Man's activities, such as placing fill on soft, fat clay can produce an underconsolidated condition for a few decades. I have been practicing almost 50 years in a variety of geologic conditions and don't recall ever encountering underconsolidated soil.
RE: Foundation in underconsolidated clay
It's easy to know if the soil is underconsolidated. Just install a well or use a piezo-cone!
f-d
p.s., I carry a pocket penetrometer to the drill site to see if there is a disconnect between n-values and unconfined compressive strength. We have marine soils with N of 8 or 10 that will return PP of 3.5 to 4 tsf. Wouldn't want to think they are too soft!
ípapß gordo ainÆt no madre flaca!