Lateral earth pressure exerted by expansive soil
Lateral earth pressure exerted by expansive soil
(OP)
I obtained an undisturbed sample of desiccated, overconsolidated lacustrine clay from a depth of six to seven feet at a subdivision project site, and assigned a swell pressure test per ASTM 4546. I do not have the lab report yet, but I was informed by the lab tech that the swell pressure is about 1 tsf.
My question is, if we backfill the foundation walls with this soil, should I add 1 tsf to the design lateral earth pressure loading?
Would the entire 1 tsf act in a lateral direction? Why wouldn't some portion of this potential load act in a vertical direction? What if we slope the excavation face at say a 2H:1V to influence the expansion in a vertical direction?
Any thoughts would be greatly appreciated.
My question is, if we backfill the foundation walls with this soil, should I add 1 tsf to the design lateral earth pressure loading?
Would the entire 1 tsf act in a lateral direction? Why wouldn't some portion of this potential load act in a vertical direction? What if we slope the excavation face at say a 2H:1V to influence the expansion in a vertical direction?
Any thoughts would be greatly appreciated.





RE: Lateral earth pressure exerted by expansive soil
Some portion of the swell - whatever that is - will occur upward. The vertical portion will depend on the same factors I described above.
That will probably help, but you have to consider the "whole package."
Give us a more complete description, and perhaps we can give you a more definitive response.
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Lateral earth pressure exerted by expansive soil
RE: Lateral earth pressure exerted by expansive soil
BigH gave good advise and asks pertinent questions - and I forgot to ask that all-important question:
How was the swell test run?
This has a direct impact on the usefulness of the results.
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Lateral earth pressure exerted by expansive soil
LL = 49, PL = 17, shrinkage limit = 18, existing moisture content of this soil in the ground is between 19 to 25%, expansion index runs between 80 to 110.
The swell test result was 0.44 tsf per ASTM D4546, not 1 tsf as I heard earlier. I understand this test result is the pressure required to maintain constant sample volume during hydration.
I like the idea of low density "compaction".
I am suggesting non-expansive soil backfill, but hold little hope this will happen. For clay soil wall backfill I am proposing a slight (.2H:1V) slope on the excavation face, placing the soil wet of optimum, using pipe and stone drains inside and outside the footer with a geotextile filter on the outside between the stone and the backfill. I also show a poly moisture barrier above the backfill (and below the concrete basement floor slab). I am specifying a geocomposite drain layer on the outside wall. This geosynthetic material has a thickness on the order of .20". Do you think this will take up the expansion volume?
USACE says the Ko for expansive soil backfill may range from 1 to 2, but is normally about 1.3 to 1.6. (EFP up to 250 pcf!!) I come up with about 6,000 plf using the Corps Ko = 2, and 8,000 plf total load on the wall assuming 0.44 tsf lateral swell pressure. These seem like really big numbers for a eight foot high (inside) residential foundation wall if I did this right. I am no structural engineer, but this sounds like Fort Knox to me!
RE: Lateral earth pressure exerted by expansive soil
Also check out the other threads - again, we have a few that gave some pretty detailed discussions along with correlations.
RE: Lateral earth pressure exerted by expansive soil
A 1:5 (H:V) slope is pretty steep; you'd be better off with a 1:2 or flatter slope to cut back on the swell pressure. BigH's density guidelines look about right, but I have my reservations about how much movement even a loose sand can 'absorb'. I'd expect the lower density clay to handle most of it.
Another (off the wall) suggestion: use geofoam. It's "compressible" in the current context, extremely light, and would exert almost no pressure on the wall. It's hydrophobic (although you will still need a chimney drain along the back of the basement wall.) The only drawback: cost. 'In quantity' it's about $50 per cubic yard.
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Lateral earth pressure exerted by expansive soil
LL PL PI SL
49 17 32 18
49 17 32 17
Thanx for the direction, I have W&F and see now where that is discussed. I will figure out a volume for the expansion and take a look at the geofoam options to take this up. I will also spec 85% (modified Proctor?) on the wet side backfill.
Focht3, I think you are probabley right on that slope, the .5:1 is essentially a vertical cut, I will look at flattening this out.
RE: Lateral earth pressure exerted by expansive soil
RE: Lateral earth pressure exerted by expansive soil
Thanx again for your input
RE: Lateral earth pressure exerted by expansive soil
RE: Lateral earth pressure exerted by expansive soil