Lateral earth pressure exerted by expansive soil

[jimbo2]

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.

 

[Focht3]

My question is, if we backfill the foundation walls with this soil, should I add 1 tsf to the design lateral earth pressure loading?

This will depend on a lot of things, like the problem geometry, construction materials, sequencing, drainage, what will be placed on top of the zone within a 45[°] projection of the bottom of the cut, etc. There is no pat answer. The best I can do is give you a "qualified maybe."

Would the entire 1 tsf act in a lateral direction? Why wouldn't some portion of this potential load act in a vertical direction?

Some portion of the swell - whatever that is - will occur upward. The vertical portion will depend on the same factors I described above.

What if we slope the excavation face at say a 2H:1V to influence the expansion in a vertical direction?

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?"

 

[BigH]

Focht3 has good advice - this is sort of a specialty in his area. The only other thing I can suggest is that you put a cushion layer of loose sandy soil agains the wall between the expanding fill and the wall. This should provide some relief to the expansion - One other aspect is not compact the soil to too high a compaction value - it is basically landscaping isn't it? You won't be supporting anything on it - so I would compact it to a uniform but "low" density. This should decrease the swelling pressure. A question, though, what are the Atterberg properties, the % clay size, etc. for this soil, just so we know - i.e., where does it plot on various graphs of degree of expansiveness?

 

[Focht3]

Hmmm,

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?"

 

[jimbo2]

thanx for the info:

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!

 

[BigH]

Jimbo2 - shrinkage limit isn't higher than the plastic limit - are you sure that what you reported the PL to be isn't the PI?? I checked out Fang (formerly Winterkorn and Fang and they show for a highly plastic clay soil that if the PI is between 15 and 28, there is a moderate degree of expansiveness (10 to 20% volume change); if the shrinkage limit is greater than 15 and PI is less than 18, it is low degree of expansiveness (<10% volumen change)(USBR Earth Manual was referenced). It seems to me that the natural moisture content of 19 to 25 percent is a number that wouldn't likely change much given that equilibrium conditions can be maintained - if this is the case, then how much water (how much swell) would be induced. My suggestion again is to compact to a low density (say 85%) on the wet side of optimum and use a loosely placed sand layer between the clay and the wall so that much of the volume change pressure goes into compacting the sand.
Also check out the other threads - again, we have a few that gave some pretty detailed discussions along with correlations.

 

[Focht3]

Hmmm,

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. [blue]BigH[/blue]'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?"

 

[jimbo2]

BigH, I have Atterberg limit results for two samples of the stiff upper OC clay as follow:

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.

 

[BigH]

Good direction - I'd still be a bit skeptical of Shrinkage limit higher than the plastic limit.

 

[jimbo2]

I will keep that in mind. Now that you mention the SL question, I am not really happy with the consolidation testing that was done, and I was a bit surprised at a few other results.

Thanx again for your input

 

[lallatin]

I must be missing something here. Why would you use the material for backfill at all? If you have to use it, use it at a mosture content and a percent compaction that precludes appreciable swell if the material is subjected to increased moisture. Typically, the spec for using such clays as an engineered fill or subgrade would be, say "at least 3% wet-of-optimum moisture content and 88% relative compaction." It's pretty fundamental that expansive clays are not considered if post-construction changes in moisture content cannot be prevented. On this basis, lateral pressures are not a issue.

 

[jimbo2]

why use expansive soil for backfill?? good question and the answer appears to be inertia and $$$$. Hauling away the clay soil and bringing in off-site soil or free draining material is expected to cost between $4,000 and $7,000 or more, and most contractors will not add that cost to the foundation construction. These are residential structures built in a competative market where most of the developers have not had significant problems in the past, many not believeing the clay soil is really a problem.