achieving more dead load for expansive clays

[stevenl]

I have 1700 min, 3500 max soil parameters. Since its a smaller home, and I typically apply structural fills (piers would require drill rigs driving from 4 hours away). We have a crazy market this year, and the str. fills are bid to cost more than the foundation!

I can meet the parameters without fills, via void forms and a technique I've never tried - much wider footings to catch the soil backfill loading. I would have a "heel" of 24 inches or so. Looking for feedback...

 

[emmgjld]

If you would, post where this is and a quick description of soils/formation. Some formations and some climates present very interesting challenges. It would also be helpful to know which swell test method was used (Consol apparatus [ASTM D-4546 method A, B or C], the FHA-PVC Meter or what??).

1st The minimum/maximum range is too tight to do this without soil improvement (structural fill) to increase the Maximum Allowable. How committed is the geotech on these numbers? As the geotech has provided numbers which do not lend themselves to design of shallow foundations, I assume they provided alternative foundation systems.

2nd Wide footing (voided, I assume) and using the soil weight could present some interesting issues.

3rd If this is a crawl space, does the eccentric load on the wide footing cause a problem?

4th Would 1 foot of soil reworking allow a little decrease in the required minimum bearing and result in enough increase of the Maximum bearing to allow you to properly balance the contact pressure? This is what I commonly do, near daily.

 

[stevenl]

Thanks! The site is by Steamboat Lake, north of Steamboat Springs, CO. Soils-"Clays:slightly sandy to sandy with occasional gravels, moderately stif to very stiff, moist and brown to gray in color". Swell-consolidation and gradation testing done (CH class). Soils engineer called back to approve my soil dead load use provided I spec compaction there.

1) Yes, piers are recommended option. Soil report offers max,min option, and is client's requested option.
2) Yes, voided. 3) Slab is present thoughout.
4) I like the idea of reworking 1' of soil. (i.e. 1' str fill). Not sure how to arrive at a new min bearing better than 1700-110=1590 min. THe max is already pretty high too.

 

[COEngineeer]

steven,1700 3500 max min is pretty hard to work with especially if you dont have tall basement wall. Have you look at the cost for helical piers? You dont need concrete delivery. all you need is the helical pier installer (pretty small crew). Just look at the TL, and specify the pier for that TL and call it good. I would try to sway the owner not to put slab on grade.

http://www.krmconsultants.com

 

[emmgjld]

The risk of excessive differential movement between the Slab and a foundation increases greatly if a deep foundation (drilled piers or helical piers) is utilized. As COEngineer suggests, the slab on grade may not be the best choice. I would agree that helicals may be a very good choice, as long as the type and installation is understood and properly specified.

If you go with a shallow foundation, such as a voided wall, with some structural fill (reworked native or imported nonexpansive). The amount of differential movement is decreased, usually dramatically.

Now for the armchair engineering.
a] The minimum dead load of 1700 psf is not real high for shallow foundations, even single story residential. My question is how that number was arrived at. The reason for my request of the test method.

b] The description of a moderately stiff to very stiff clay indicates the maximum bearing may be conservative, to very conservative. The different test methods arrive at a swell pressure by different methods and using different assumptions. Some methods are better suited for deep foundations and others are better for shallow types.

c] A shallow foundation on expansive soils works best when the true maximum bearing is approached by the applied foundation loads. You are best to get the foundation balance quite a bit above the minimum and start pushing the maximum. The minimum is the value to stay away from. Keep in mind the helicals tend to really push the limit to the maximum bearing, oftentimes with very little factor of safety. Also note the are virtually no failures due to overload.

 

[stevenl]

You guys are a great help.

The owner, a young architect, is savy to his slab and soil issues. There are NO interior pads, the fnd is a very simple, inherently rigid, "L" shape. The fnd is 50% 9' conc walls, 25% 6' walls and 25% 4' walls. The slab level will have maybe 3 slip jointed walls, all non-bearing.

I will forward these ideas on to him. We have discussed helicals and piers, and he is looking at those options again.

a)b) I'm not used to seeing much higher soil max values in our region, across several engineering sources over the years, so I'm reluctant to press this one to go higher. 4,000 psf would be a rare max. I believe the F.S. is 2.0 at their end.

c) I like this advice. Have yet to see expansive movement complaint over the years, but this is a smart prompt not to balance my max and min on a small margin.