Residual Soils

[basto]

Hey Guys,

I work as a geotechnical technician training to be an engineer in Australia in the bush! Great job.

I seem to find myself in arguments with my boss about identifying alluvial and residual soils in the field. Its not a vital engineering property of soil i know, but i am interested in discussing this if anyone has some knowledge.

I have attached a photo of some clay i encountered at a recent job, which i have classified as residual. Does anyone else think this looks like residual soil? note the pieces of quartz and sandstone gravel contained within the clay. Below this clay profile i encountered weathered quartz-sandstone rock - orange and red in colour. This supports the residual classification i feel, as pieces parent material are evident in the clay.

Any info is appreciated!

 

[Guest262653]

From my professional point of view, it looks like a pair of pork ribs.
Bloody inside.

 

[basto]

thanks for the professional opinion, you must be on the structural team..

 

[Ron]

Easy there basto!!

Stratigraphy is often better defined in alluvial soils as compared to residual soils. Incomplete breakdown is one key to spotting residual soils (as you noted), pieces and parts of parent material can also be redeposited in alluvial soils. A key is where the pieces and parts occur...so check the underlying strata for comparison. If they are chemically and physically the same as the underlying unweathered material, my vote would go to residual.

 

[cvg]

basic knowledge of the regional geology would help. difficult to say with any certainty from just a photo.

 

[dirtsqueezer]

Just from the textbook, you might at least consider the lacustrian angle. Residual soil takes a little more time to degrade into significant layers, and with any decent weather in the region that clay would be downstream- at the bottom of a lake. Or maybe you have the lake bottom... I agree with cvg, though, what does your bedrock consist of? More info would help.

 

[dgillette]

Just left of center, that whitish piece of gravel looks pretty smooth, as if stream tumbled. Is that what it's really like? If so, did it happen before it became part of the sandstone that's now residual, or did it occur more recently, on its way to becoming the alluvium it is now part of? If you can answer that, well, I guess you wouldn't be asking.

DRG

 

[dirtgirlhawaii]

That looks like an alluvial material to me. Granted, I work in Hawaii and soils here are quite different. That being said, alluvial is very distinguishable from residual. Residual soil almost always does NOT have core stones (indicative of saprolite) or gravel in them. A Saprolite will. Alluvial soils usually are identifiable by rounded gravel particles and sand or a fat clay. I'm not sure where else in the world this may hold up, but if I were to do exploratory drilling elsewhere I'm sure that this generalization would hold up.