How to understand if a clay is overconsolidated
How to understand if a clay is overconsolidated
(OP)
A layer of grey clay was encounted at the end of a test pit. It was hard enough that it did not allow inserting the Torvane into the soil (as if it is an extremely weak rock).How could one undrstand if the clay is overconsolidated by such observation without an oedometer test. Thank you.





RE: How to understand if a clay is overconsolidated
RE: How to understand if a clay is overconsolidated
Nothing beats a consolidation test though. . .
f-d
¡papá gordo ain’t no madre flaca!
RE: How to understand if a clay is overconsolidated
I have no reason to dout if it is real soil. It should have been a perfect compaction process used to make such a deposit, otherwise.
My estimation of Su is 300 kPa. (used 200kPa for bearing capacity calculation, though). According to ISRM (extremely weak rock) it may be up to 1 MPa.
The site is overlain by 2 m of fill. Let's assume there was 6 m of overburden. That makes 120 kPa overburden pressure. 0.23x120= 280 kPa.
What is that 0.23? Is it the limit under which the soil would be normally consolidated? Thanks.
RE: How to understand if a clay is overconsolidated
the value 0.23 represents the typical Su/P ratio, where P represents the effective overburden stress. For normally-consolidated soils this ratio is about 0.23. So, if your pocket penetrometer/tor-vane values are much greater than this ratio as you go down the hole, you are likely overconsolidated. Now you could get to some depth where the undrained shear strength and the effective overburden pressures re-establish this relationship. If this occurs, you have an overconsolidated "crust" that's present in the near surface. This often occurs from surface effects like wetting and drying, desiccation, etc. Not all "overconsolildation" is related to prior fill or prior erosion.
f-d
¡papá gordo ain’t no madre flaca!
RE: How to understand if a clay is overconsolidated
However, as it is a silt, a dry silt can be hard, but subject to softening and collapse upon saturation. I suggest determining the dry unit weight and checking the Navy Manual, DM-7.1, Figure 5 for collapse susceptibility.
In your calculation, I think 0.23*(120) = 28 kPa.
RE: How to understand if a clay is overconsolidated
I had the samples examined by an experienced local geologist who believed that the materials are indeed silty clayish till. This is supported by inclusion of some gravels floating within the silty clay materials in another test pit about 100 m distance. Will you agree that an assumed Cu value of 100 kPa for bearing pressure calculation would be well underestimated?
PS: Many thanks for correcting my awful math!
RE: How to understand if a clay is overconsolidated
RE: How to understand if a clay is overconsolidated
I have soaked the samples of materials (1" cubes) for 5 weeks already and there is no sign of any collapse or significant stress loss.
The materials were located below the groundwater already.
I also put an oven-dried sample in water - it has not collapsed after one day but can crumple by hand easier.
The measured unit waeght of soil is 2.0 g.cm3 and the texture is very compact.
If we assume it is silt, what sort of soil parameters do you advise?
RE: How to understand if a clay is overconsolidated
natural unit weight = 2.0 g/cm3.
When the oven dried sample soaked, it crumbled by hand easier.
RE: How to understand if a clay is overconsolidated
RE: How to understand if a clay is overconsolidated
I would run unconfined compression and assign a bearing pressure equal to the unconfined strength, then check it using Phi = say 32 to 34 degrees and a nominal cohesion, say 100 psf (5 kPa). If that approach results in unreasonably large footings, run triax and oedometer tests.
RE: How to understand if a clay is overconsolidated
f-d
¡papá gordo ain’t no madre flaca!