Percent Swell of Soils, ASTM D4546 1

[ninetofive]

Hello,
I have spent some time searching the forum for a discussion regarding this and did not find anything so I figured I could ask..

I have received a soil test result for a proposed foundation design on some potentially expansive soils. I am familiar with relating the Atterberg and %silt & %clays results to estimate the potential for swell, but, this report also included the ASTM D4546 test for percent swell. I have done some research and found that anything above 1.5% is very high and this soil actually had the test come back at 2.61%, but what I am struggling with is relating the % swell to any sort of industry standard and/or any published research or tables that can break down the percent swell.

Any wisdom on this would be greatly appreciated.

 

[MTNClimber]

Where is the site located? I ask because I've found that different regions (or even firms within the same region) have different approaches for expansive soils.

 

[ninetofive]

Thanks for the response MTNClimber. The soil is from the SE corner of Arizona.

 

[MTNClimber]

I haven't worked in that region so I can't speak to as what the standard of practice is for dealing with expansive soils. I've worked with expansive soils in central and western Colorado. The standard of practice varies slightly from firm to firm but they all use 1D Swell/Consolidation, sometimes paired with soil suction, to determine the expansion potential of onsite soils. I've read some papers from Arizona State that mention using the same method for computing heave.

Using the 1D swell/consol results can assess the risk of poor slab and foundation performance. Keep in mind, poor performance is subjective. Some owners in Colorado are willing to accept risk of basement slabs moving several inches while owners in other parts of the country would not accept that level of performance.

The older recommendations from the Colorado Associate of Geotechnical Engineers (CAGE) indicate that 1D swell results with a initial surcharge loads (before flooding the sample) of 500 and 1000 psf have the following slab performance risk category:

The 1D Swell test is considered somewhat conservative since you're allowing the soil to fully saturate and swell. In reality this will not happen throughout the entire stratum which will be exposed to moisture increase (also known as Depth of Wetting). Depth of wetting is also hotly contested. Some people say 12', 24', 29', ect... some nutty people say 90'. It depends on many factors including suction force, material type, source of wetting (home owner overwatering their garden and lawn vs. medical office building that has well maintained pavement and xeroscape surrounding the structure), among other things.

The soil stratum within the depth of wetting can change between non-expansive material to highly expansive material which will affect the risk of movement and damage.

The swell pressure can be calculated by slowly applying load back onto the swelled sample during the 1D test. The load to get the sample back to its pre-swollen height will be the load-back pressure. This is helpful when figuring out the uplift pressure on a footing or deep foundation from heave.

If tolerance of movement for the owner is low, a building constructed on deep foundations with a structural slab is recommended. If the soils allow, you could get away with a PT slab. For owners that don't have deep pockets, a portion of the expansive soil is replaced with non-expansive soils or sometimes (it's debated if this is a good idea) the expansive soil is moisture conditioned to optimum and then put back (think of it as pre-swelling the soil before the structure is in place). The moisture conditioning method is used often for housing developments along the Front Range in Colorado.

I hope this helps. This area is still very much a developing/contested subject. I apologize for any grammar or spelling mistakes. I have to jump on a conference call.

 

[ninetofive]

No this is extremely helpful, and thank you for your insight!
Yes, I know all about the 1D test and that is exactly what was completed for the site, ASTM 4546, and it came back at 2.16% but that is where I had to stop. That number really does not mean anything to me, is that a lot or a little? My educated guess would be that is on the low side, therefore an assumption of low-moderate swell potential for this soil will need to be designed for. Soil reports are not required for permitting in the rural areas (this is for a PEMB) and the owner would prefer not to spend the extra amount for a geotech to take a look, so I am attempting to learn a few things about this field. I know a fair amount but do not feel comfortable with relating the swell % from the ASTM 4546 to any real tangible data that can be used in a foundation design. And from the research I have done, it seems like what you have said, this area is developing and not an exact science. Some assumptions and estimating is definitely industry standard.

 

[MTNClimber]

Is there a boring? How thick is the expansive soil deposit? Is the expansive potential variable with depth or is 2% an average?

Since I don't know the standards of practice in the area where your site is and there's limited geotechnical information, I cannot say if it is of concern or not. You would have to contact a local geotechnical engineer.

I can tell you if there was a site in the areas where I practice, an average 2% swell for the entire depth of wetting would be concern for sensitive structures, say for machinery that must stay level but less of a concern for less sensitive structures, say for an average homeowner. A lot of the potential swell can be taken out by excavating a portion of the expansive soil and replacing it with non-expansive soil. A PT slab or a structural slab founded on shallow foundations would be less risky.

It is up to the geotechnical engineer on the project to help the owner put the risk into perspective and up to the owner to accept risk if they decide not to go with the least risky (but practical) foundation solution. If the owner doesn't want to hire one, it's up to them to assume all risk.

 

[ATSE]

If the building slab cracks, or the footings heave or settle, will the owner thank you for saving him money on geotechnical services during construction? Of course not.
You might be acting a little too brave and too mindful of the owner's budget and not your own future heartburn.
Get an experienced dirt engineer involved now. The owner will probably need to over-ex 2 ft min and import clean backfill, along with a robust drainage plan.

 

[MTNClimber]

If the building slab cracks, or the footings heave or settle, will the owner thank you for saving him money on geotechnical services during construction? Of course not.

But there are plenty of people that would be ok with the cracking if it saved them money during construction. I’ve met plenty of them. This is why the owner needs to understand the risk/reward with the type of construction they select. And, as you said, this is where a local geotech can help them make that decision.

 

[ninetofive]

Thank you for your insight MTNClimber & ATSE. I have spoken to the owner and requested we have a geotech take a look and give his recommendations before I can proceed.

 

[Steve3125]

Great choice ninetofive!

Even a small movement at the base of a pre-engineered metal building can result in high stresses at the ridgeline of the building.

I'd be very leery of adding moisture into the clays to 'pre-swell' them then building on top of them, Especially in SE Arizona. What happens when the desert air dries the soil back? Yes, here in Colorado a standard practice is to add water in the soil to pre-swell the soil then compact it. I see a problem in that as our droughts worsen the soil will then start to shrink causing the reverse of swell damage.

Another option for PEMB's might be to have a flexible connection at the base of columns which allow adjustment should alignment and/or stresses become an issue.

 

[MTNClimber]

Steve - Valid points. Another reason why a local geotech must be contacted. The moisture conditioning to "pre-swell" may only work for projects where there's landscaping that requires watering and the owner is OK with some amount of movement.

Heck, the building might be sitting on 1 foot of expansive soil which should only produce 0.31" or less of heave if fully saturated. That may be within tolerance for the PEMB and the owner's acceptance of movement. You don't know until you get some type of explorations done out there.