Sandy fat clay from a limestone quarry

[Don5716]

We have been purchasing a material from a limestone quarry in lower-central Alabama, that the owner describes as a Dark Orange Brown sandy fat clay. This material has worked well for us on several different projects. When compacted, it provides a good hard surface and it holds up well with very little change even when wet. There is another quarry 25 miles from this quarry with the same type of material and the owner swears it is the same material and it does look exactly the same but this material turns to mush when it gets wet. These are both limestone quarries with very similar clay, but they react very differently to water. We took samples of the material and had it analyzed with the good clay the plasticity index is 34, on the new material the index is 16

 

[Ron]

You need more classification index properties between the two. Soils that appear the same, obviously are not always so. Have full classification index run and do a CBR on both materials.

 

[killswitchengage]

you cannot take that man who provided you with the material seriously only by his words !. My primary guess is that the clay differs from the other because of its mineralogy . You should conduct Atterberg limits and classification just like Ron said

 

[Okiryu]

Along with what Ron has recommended, what are the natural water contents and proctors? Have they been compacted to similar compaction efforts?

 

[oldestguy]

Interesting subject. Up here in Wisconsin we have dolomite that weathers to a red fat clay, very high in montmorillonite mineral content. Forms in the upper reaches sometimes many feet thick and in crevices and unevenly, so you never know just where solid rock is present. Imagine down in a caisson excavated to solid rock and then you can probe clay filled variable width crevices many-many feet down. Sometimes found under beds of solid rock. This is vicious stuff and not usually any of use as compacted fill.

 

[fattdad]

how will a properly compacted sandy fat clay perform on an embankment slope or under shear load in general 15 years after placement? The soil will undergo multiple cycles of freezing and thawing, wetting and drying and other temporal changes. These dynamics will occur in the upper 5 or 10 ft and will release the cohesion over time.

Sure a mudpie is hard when it dries. It becomes wet and the capillary suction is released (i.e., the effective stresses are reduced). I'm not fooled by dry strength or cohesion. I'd be careful using peak strength on a sandy fat clay. At best, I'd require fully-softened strength, which means no effective cohesion. I'd run tests on a normally consolidated, reconstituted sample prepared at the liquid limit and then consolidated in the DDS (drained direct shear) device.

I wouldn't listen to either quarry.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

f-d, although the OP did not indicate the purpose of the fill (foundation support, embankment, etc), I agree with you about fully softened strengths (FSS) need to be considered for most of slope stability problems. However, what do you think about using undrained shear strengths for foundation design? Since settlement normally controls, I understand that undrained shear strengths are seldom used. But, just curious about your opinion of using FSS for foundation design. I assume that it depends also on the loading condition, though.