Lean Clay and Residential Foundation 1

[Benson329]

Regarding Residential Foundation. For some reason I have asked several contractors in my area (South Texas) regarding the need to excavate expansive clay soils and replace with select fill. They all say this is required for commercial applications but not for residential foundations. We have obtained soil bore measurements of the proposed residential build and the first 6' of ground is Lean Clay with LL% of 36 and PI% of 19. Water content was 12% and 9% on each bore sample. I have searched for threads in this forum and have found that that lean clay is less riskier of expansion than fat clay which doesn't appear until 7' - 17'. Obviously I can go with what the geotech report states and dig out 2' and replace with compacted select fill. However, given the previously noted, does the "excavate and replace with select fill" exercise substantially mitigate the expansion risk and thus causing foundation problems? The cost differential between excavating/replace and is considerable and I would like to know how much risk I am mitigating for the cost to determine if its acceptable.

Thanks in advance.

 

[MTNClimber]

This would be a question for the geotechnical engineer who provided the report. We simply don't have enough information of the onsite soils or proposed construction to provide you with an estimated magnitude that excavation and replacement will mitigate. I do not practice in Texas but in Colorado there are some sites that will excavate up to 18' below finished grade if the site warrants it. Yes, even for residential developments with lean clay. When it comes to individual lots we may recommend excavation and replacement of 3 to 4 feet of soil or require deep foundations. It all depends on how much risk the owner wants to accept.

 

[oldestguy]

I'd call a soil scientific from the nearest office of the US Dept of Agriculture if they can advise as to the probable soil situation in your area. What you describe would not warrant any excavation and replacement unless it was soft, etc. You might also bite the bullet and hire another geotech from your area since they most likely can advise as to what to do. I'd question not taking some precautions other than playing with the shallow clay zone.

 

[GeoEnvGuy]

I would recommend you give a quick google on building on expansive clay. The general options are.

Option 1 piles
Option 2 excavate and replace
Option 3 excavate to remove equivalent structure weight
Option 4 rigid concrete slab to withstand movements

 

[EireChch]

With a plasticity index of 19%, you are just in the low volumne change potential, based on BRE 1993a. (The UK standard method for assessing expansive soil risk). Furthermore, if you use the modified PI, which is based on the percentage of fines, you may have a lower PI, but only slight I suspect.


In the UK it is common to found footings that take structural loads on non expansive soils so at several feet deep and then support your slab on grade. Beneath your slab you would include a void former (essentially some load bearing expanded polystyrene that can support the slab load, however if there is any swelling pressure from beneath, the void former can compress and relieve the expansive forces). You are in the low category, just barely, which indicates you do not actually need a void former. However you could adopt the recommendations for medium expansive soil risk (PI 20-40), which estimates 25mm of expansive and therefore requires a void former of 50mm in thickness. See below info from a supplier:

GeoEnvGuy - I wouldnt recommend Option 3, if anything, an increase in structural load would be recommended to counteract the upward expansive pressures. If you excavate material, say 0.5m to remove your slab load of 10kPa (assuming a unit weight of 20kn/m3), you have to backfill with something. If you use well compacted granular material, the expansive pressures may still be transferred to the slab. However, in saying that, if you remove enough expansive soil and replace with well compacted granular material, any expansive forces from beneath could be "lost" within the compacted material.

 

[aeoliantexan]

I agree that you need to talk with the geotech to understand the reasons for his recommendations and discuss alternatives. The water contents are well below the PL, so the lean clay probably has some expansive potential. His experience may indicate that 2 feet of replacement sufficiently mitigates that. However, I would want to know what amount of movement he anticipates. I would also discuss the water contents in the deeper fat clay are; if they too are low, the fat clay may swell and heave with time.

 

[oldestguy]

For what it may be worth, my experiences in Wisconsin with "fat clays" (with high LL and high water content) is that moisture changes easily go to 20 feet depth. That means those moisture changes are reflected by surface soil elevation changes, affecting those structures. Playing with that better clay above 6 feet does not eliminate effects from below.

 

[MTNClimber]

Depth of wetting, a major component of magnitude of heave in expansive soil and bedrock, is a hotly contested topic and varies from location to location. So what happens in one area doesn't necessary mean its going to happen in another. This is why its important to talk to the project geotechnical engineer. They should understand the soils in that area better than anyone on this forum.

 

[fattdad]

I think Steven Wright shows sites with softening to depths of 20 ft in Texas. He looked at the loss of cohesion in fine-grained soils owing to softening - mostly related to slope stability, or so I recall?

It seems moot to replace some interval of lean clay with engineered fill. It's not the lean clay that's the bad actor - it's the fat clay below 7 ft.

To what extent would the loss of cohesion affect your foundation performance? Doesn't seem likely to me?

To what extent would changing moisture in the 7 to 17-ft fat clay affect shrinking or swelling pressures and foundation performance? Maybe? It'd just have to overwhelm the dead foundation loads and the overburden pressure. Maybe?

I just don't see how an engineered subbase to replace the first few feet of bearing soil would have much consequence to the fat clay - heck, it may even direct more water into that lower layer?

Agree. Call a local dirt squeezer.

f-d

ípapß gordo ainÆt no madre flaca!

 

[MTNClimber]

I think Steven Wright shows sites with softening to depths of 20 ft in Texas. He looked at the loss of cohesion in fine-grained soils owing to softening - mostly related to slope stability, or so I recall?

Texas is a huge state. The depth of wetting will likely vary across the state. It will also vary based on land use. A house constructed on undeveloped land and is heavily irrigated for a nice green lawn would theoretically have more heave than a site for a building where they have xeriscaping.

While fat clays typically swell more, lean clays in the Front Range can still swell 5-8 percent. I'm not sure what Texas is like.

There was some research by Walsh, Colby, Houston & Houston that showed the probability distribution for depth of wetting for homes in Denver Metro Area was 95% for 25.9', 99% for 29.9', and 99.9% for 35.1'. There are professors in the Denver area that will dispute those numbers, and have done so in court. One professor claims its something like 50 or 70 feet. Then as soon as you start going west, some engineers are using lower numbers for depth of wetting, 12-18 feet.

Other construction options not discussed in this thread are:
PT Slab foundations
Basement versus No Basement
Deep footings
Moisture Conditioning
Creating a stiff footing and using a structural floors
Minimum deadload and void form

Its a complicated subject with many differing opinions which is why its best to talk to a local geotech.

 

[Okiryu]

How the other buildings in your neighborhood were built? Are they performing OK?

 

[Benson329]

For the most part, we don't hear too often of foundation cracking problems. In fact,the genesis of my question is because most contractors that I have spoken to say it's not necessary to dig out condition and repack and that they haven't experienced any issues. As a follow up question, would increasing the PSI on the concrete from say 3,000 to 4,000 make difference or would the physics be the same?

 

[charliealphabravo]

Increasing the compressive strength of the concrete will have no effect since conventional residential foundations are not designed for much in the way of primary structural action. They mostly rely on direct load pass-thru, tolerance of differential settlement through flexibility, and crack control.