How to understand if a clay is overconsolidated 2

[ONENGINEER]

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.

 

[BigH]

Well, I would surmise that if it is "real" soil and it was as hard as you indicated, compare an "estimate" of the Su (must be "hard" if you can't insert torvane) with what a normally consolidated Su would be (0.23 or so times effective Overburden pressure) - you will find it is overconsolidated.

 

[fattdad]

I'm on board with BigH. I'd also contrast the liquid limit to the natural moisture content. I'd expect for your undrained shear strength, you'd find the natural moisture content is pretty close to the PL.

Nothing beats a consolidation test though. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[ONENGINEER]

The PI = 3.7 for a PL of 19 and a moisture content of 13% to 24%.

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.

 

[fattdad]

Well, it doesn't seem like much of a clay if the LL is 23 and the PI is 3 - more like a silt. What's the sand content?

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!

 

[aeoliantexan]

Normally consolidated soils at ordinary depths are pretty soft. Consider a bay or lakebottom deposit normally consolidated under its own buoyant weight. At a depth of 100 feet the overburden pressure is probably less than 6000 psf (300 kPa). Su would be roughly 0.23*(6000) = 1380 psf. The unconfined compressive strength would be roughly twice Su, about 2760 psf or 140 kPa. This is a "stiff" consistency, readily indented with your fingers. The soil you described is probably "hard"; can't be dented with the fingers, with an unconfined strength greater than 8000 psf (400 kPa).

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.

 

[ONENGINEER]

aeoliantexan,

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!

 

[aeoliantexan]

In its present condition, yes. I would still be concerned about what will happen to the strength if it gets wet. The low PI indicates there is not much clay there. I am used to dealing with loess, which has a porous structure of silt grains held together with small amounts of clay at the contact points. Water destroys the "cementing" action of the clay, and the structure collapses. There is a lot of room in the pores for more water. We would expect a silty till to be dense and get most of its strength from internal friction, so you may be OK, but I would try to expose a confined sample to saturation and see what happens to the strength.

 

[ONENGINEER]

Please what do you mean by confined sample? If you mean triaxial testing?
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?

 

[ONENGINEER]

Corrections:

natural unit weight = 2.0 g/cm³.
When the oven dried sample soaked, it crumbled by hand easier.

 

[BigH]

If this is till, there is no problem for bearing. Where is the till located?? I would surmise in Canada. In the Ontario region (Toronto, Mississagua area) the tills would be good typically for 350 kPa bearing pressures. The tills usually have a PI of about 10, and if I remember correctly a plastic limit of about 20. Good material.

 

[aeoliantexan]

If the formation is below the water table, doesn't soften much in water, and is still hard, you don't need to worry about collapse.

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.

 

[fattdad]

If it's glacial till, it's overconsolidated. If we knew where the site was located and if you had reviewed the local geology, we'd/you'd be in a better position to answer the question.

f-d

¡papá gordo ain’t no madre flaca!