I am potentially looking at building in a spot that is black clay. Specifically, Frelsburg and Latium clay. Just looking for some opinions on what I am getting into here. I am thinking I am going to have to pay an engineering company to pull soil cores and design it. Does anyone have a ballpark on what that engineering work would cost (say 2500sqft foundation)? Also, what can be expected in terms of cost increase per sqft? Just curious also in what modifications are usually done to address this (haul in several feet of road base, add piers into the design, increase rebar size, increase number and size of beams in the design, etc). Thanks in advance.

Color has no bearing on the properties of a soil. In clays bark color is often related to weathering. That said the dark clays, especially Texas, do tend to have common traits. They are often prone to shrinking and swelling with changes in moisture. Off the top of my head I'm not familiar with yours specifically

No idea on cost. But drilling a hole, running some tests, and getting a report will be beneficial. Their answer will likely include compaction (vibratory, and greater than just the weight of a skid steer on tracks), and proper drainage design around the home.

If the soil is prone to shrink swell with changing moisture then drainage keeps the moisture away and compaction reduces the ability for it to get in.

You may also need some carton forms to provide some relief. Personally if there were a better soil type, I'd move the build. If you are set in this spot, as the Previous post suggested, have a soil test run then have the compaction tested. We do on our build and the added cost of the tests were minimal compared to repairing a foundation.

We are building a pier and beam home so we went a little bigger and deeper with our piers.

An Engineering firm will calculate vertical lift of the soil and design a foundation.
Cost is about $5K. Requires two soil sample bore holes 20 feet deep.

Depending on calculated vertical lift they may require excavation, soil replacement, and possibly piers.
No guarantee or warranty.

I'll add to my earlier post. If soil movement is shown to be significant I'd seriously look into pier and beam. With all of the improvements in insulations (especially closed cell spray foams) and moisture barriers i think you can easily enjoy the air tightness and insulating properties of a slab on grade with an elevated pier and beam. You can set the piers deep enough to get beyond depths of seasonal impacts of moisture variation. Additionally you can use this to add some elevation without fill if there is any concern regarding flooding.

For what it's worth, my last house was built up about 15' out of the flood plain. Not only was it built up, it was built up with soil dug out of a stock tank, which was pure 100% Brazoria clay. The foundation was designed as slab on grade but they had added 30 bell bottom piers to it somehow and those were 20' deep. This was a two story house, 4700 square feet total. Needless to say, the entire soil could have been washed away and the slab would have sat atop it's piers. I don't know how they do it now, but in 20 years, there wasn't so much as a hairline crack in that slab and no issue whatsoever with foundation settling. I'd think they have even better ideas now.

I require foundation engineering on the homes that I build in the Brazos Valley, as are most other builders. As far as cost it can vary by engineering firm, but I had a 2,300 SF slab engineered a few months back and for soil cores, slab design, and pre-pour inspection the total ran $2,940.00.

Construction cost increase, if any, is much harder to predict. This is 100% dependent upon the engineers recommendations. this could be anywhere from bring a couple loads of select fill to level the pad with a slab on grade penetrating XYZ inches into native soil with XYZ sized grade beams to a structural slab on carton forms or decking. Engineering recommendations will depend on the plasticity index and the potential vertical rise of the soils.

Kenneth_2003 said:

---

I'll add to my earlier post. If soil movement is shown to be significant I'd seriously look into pier and beam. With all of the improvements in insulations (especially closed cell spray foams) and moisture barriers i think you can easily enjoy the air tightness and insulating properties of a slab on grade with an elevated pier and beam. You can set the piers deep enough to get beyond depths of seasonal impacts of moisture variation. Additionally you can use this to add some elevation without fill if there is any concern regarding flooding.

---

Second this. I always wanted to build one.

Frelsburg and Latium…….Brenham
/Bellviille?

All, thanks for all the replies. TXAG17, thanks for the ballpark engineering cost. I figured the engineering work would be fairly reasonable and the real costs would be in the design changes to the foundation.

To the other comments above regarding pier and beam, yes, I might also look into that. Maybe they have gotten things to where the perils of pier and beam can be managed. I have always associated pier and beam houses with temperature and humidity control issues, crawl space pest issues, etc. However, maybe modern day fixes like closed cell foam underneath can change the game in terms of that.

B-1 83, yes, this is Austin County.

Those type soils are moderate to very plastic = add water, then much force and rise
ergo erases any doubts a STRUCTURAL engineer in your locale manage the design & inspections
Fees range but begin at $1.20/sf of foundation from borings to blessing

Good sleep insurance

One potential solution is to have piers drilled and poured, then pour a slab over them like a giant pier cap. No need for a crawlspace.

Another is to remove the clay around your chosen location and replace it with something else (some kind of other base material). If you elect to do that, you'd need to go pretty deep and over build (make your hole and backfill wider than your floor plan) to make sure you don't have any issues next to your house or with any clay beneath your base.

Talk this over with an engineer first though.

After living in a few houses with a concrete slab foundation and now one that's pier and beam it would be hard for me to go back. Having the ability to run new utilities underneath the house, or just to easily move them has enabled me to do a lot that I couldn't have done previously.

Bonfire97 said:

---

All, thanks for all the replies. TXAG17, thanks for the ballpark engineering cost. I figured the engineering work would be fairly reasonable and the real costs would be in the design changes to the foundation.

To the other comments above regarding pier and beam, yes, I might also look into that. Maybe they have gotten things to where the perils of pier and beam can be managed. I have always associated pier and beam houses with temperature and humidity control issues, crawl space pest issues, etc. However, maybe modern day fixes like closed cell foam underneath can change the game in terms of that.

B-1 83, yes, this is Austin County.

---

I lived in that area for a while and every new build that I saw always had pad material brought in. Almost everything was post-tension slabs as well. We had some hairline cracks in our slab but no moving or heaving.

My dad always asked for the homeowner get a engineered foundation. If they did not want any of the suggestions or basic the homeowner had to sign a waiver. I had Gessner here in College Station engineer mine. They will generally provide you with 3 to 4 options of design and all increasingly more expensive with less movement possibility. He built in Washington and surrounding counties all his life.