Expansive Soils

[Xcenterline]

I have a single-story commercial project that is on expansive soils. Heavy exterior walls but the interior supports are relatively lightly loaded. Recommendation from the geotechnical engineer is for under-reamed piers. No problem here.

Recommendation for the slab though is just "slab on grade." I am assuming for now the client is aware of the risks and is looking for cheap instead. There is no further information about the slab being structural, additional thickness, added reinforcing, etc... nothing.

Anyhow, assuming I went down this path, how on earth would one mitigate uplift on the grade beams and pier caps when the use of void spaces under these elements would compromise the slab on grade?

Seems to me there is an incompatibility here.

 

[Xcenterline]

I forgot to add that the exterior walls are tilt-panel and will likely be tied to the slab. The remainder of the interior slab would be free-floating.

Also, there is language in the report about replacing the upper 1 to 1.5 feet of soil with appropriate fill. Even if this helps mitigate some of the expansive soil issues relative the slab, it is not enough for the grade beams and other foundation elements that are somewhat deeper. The expansive soils, from what I recall in reading the report, persist to much greater depths.

 

[emmgjld]

I am presently the owners consultant for remediation of a structure with similar conditions and the later complications. As the foundation designer, you must get a real handle on the owners expectations, knowledge of the required construction details and knowledge of the long-term building & site upkeep. Also, what limits the geotechnical report has really put on everything.

This calls for a bit of questioning of the Geotechnical and the owner. I suggest, STRONGLY, that the discussion be placed in writing.

--You need a definite statement from the Geotechnical regarding the potential differential heave of the slabs (interior & exterior) with respect to the foundation system.
--How much soil heave is being mitigated by the replacement with structural fill, compared to the original subgrade section.
--Does the building use require that some slabs should or must be tied to the foundation?
--What provisions for the 'floating slab', to include non-bearing wall voids is the client willing to accept?
--Who is going to make sure that all prudent drainage provisions are incorporated into the Civil Drawings and then actually, properly constructed?

As a final comment, 1-1/2 ft of Structural Fill is not much but, you have not provided any info on the soils. For use with a deep foundation system I typically have to recommend between 2-1/2' to 10', depending on the nastiness of the clays and the owners' expectations. Usually, my recommendation starts with a structural slab over a crawl-space. Your conditions may not be that severe.

 

[hokie66]

Agree with emmgjld, and I also have had the experience of dealing with differential movements in buildings where the superstructure is on piles and the ground slab is supported on moisture sensitive clay. The concept is fraught with problems. Are the expansive soils in their natural state or has the site been reclaimed at some stage? If the latter, deep seated settlement can still be an issue.

How much confidence do you have in the geotechnical engineer? If there are problems at a later date and the geotech advice is shown to be faulty, you will nevertheless be in trouble as well.

 

[Xcenterline]

Thanks everyone. I appreciate the feedback and agree. My first order of business is a documented phone call.

The client has used this approach before, providing soil retaining boards to avoid slab collapse due to use of voids under foundation structure. I, personally, am not in favor of this sort of system - so I will have to heavily disclaimer the work if I am asked to go down this path.

This building does have conditions wherein the slab will need to be tied to the foundation elements in one way or another. There just isn't any good way to avoid it.

The fill soil is only 1.5 feet deep as a buffer. The soil below is lean clay up to 20 feet. Geotechnical clearly states "a moderate potential for expansion/contraction and therefore the slab can be supported on the ground surface once the subgrade is prepared."

Seems to me that drainage around the site is the critical link in this approach. Quite risky, but cheap.

If you don't mind, I would like to tag on another question...

What is the typical diameter of drilled piers that contrators prefer to work with on low-rise commercial buldings?

I have previosuly used 30" diameter, but this project has some rather large interior loads due a mezzanine structure. Either I shift to rather large pier (4 ft plus) or I design a pier cap (pile cap per se) on multiple piers.

Thanks again.

 

[MiketheEngineer]

Cut it out and replace it. Drainage won't help when it dries out in the summer and then needs drainage in the Spring unless you constantly "water" your footing. Shrink - collapse - wet - grow ad infinitum.

Fix it now - somehow or pay for it later.

 

[JAE]

Xcenterline,

I've worked with expansive clays quite extensively in South Texas some years ago. We successfully used soil retainers on almost all our structures with great success. This is a very common detail.

We typically called out 1 1/2" thick concrete planks - cast on site - and reinforced with WWF. The planks were typically 18" wide and were set against the perimeter grade beams, tilt panels, etc. on each side with a 6 to 9 inch void below. The bottom of the planks were set 6" into the earth to avoid lateral sliding under earth pressure.

I agree with emmgjld above - you need to get a handle on the owner's desires - and perhaps educate the owner on the rammifications of various alternatives.

Having "lean clay" for 20 feet doesn't sound too bad as "fat clay" is usually the term for highly expansive clays. But I'm not a geotech.

Also - with an interior floating slab on expansive clays, having vertical slip joints at the intersections between exterior and interior walls is a must.

 

[Xcenterline]

Thanks JAE. I came across the plank soil retaining solution today actually. Makes sense. This site is actually in south Texas!

I talked with to geotech today and supposedly he is very experienced overall and with the local area. He was very confident in the approach he recommended... that the nature of the soils over which a previous building had existed for some time, with a foot or two of fill, would not exhibit significant expansion or contraction issues as long site drainage was handled appropriately.

Regardless, I will have a conversation with the design team and client and make sure everyone is aware of the risks (inherent uncertainty). If the push is to continue forward with this solution, I suppose some reasonable conservatisms in the design would be prudent insurance.

Thanks again everyone.

 

[TXStructural]

In north Texas, I have a fair amount of experience with expansive/fat clays. We repaired a tilt wall building where the walls were lifted several inches off the piers by expansive soil after void retainers failed. The soil expands in all directions, not just up. A floating slab might be adequate, but over time, it will give serviceability problems.

Also, make sure the under-slab fill is a lean clay or other material that will not allow movement of water. Any kind of granular or pervious fill will increase moisture in the soils under this "bathtub" and make the problem much worse. If you intend to drain the fill, make sure the drain permanently goes to daylight and there are no low spots where water will stand. If the area has persistently wet soils, be sure you reinforce the slab sufficiently to bridge voids as the soil dries.

If the soils and the site are not persistently wet, it sounds like a structural slab on void cartons might be useful.
--
I worked for five years (at a manufacturing plant) in a 897,000 sq ft building where the interior had risen about 6 inches in 20 years, and they would simply fill in the collars around columns and a strip along the edge of the slab at walls every so often. Where there was floor to ceiling fenestration with vent registers in the floor, they installed handrails to keep people from tripping or falling into the foot-wide low spot along the wall. Interior partitions had to be cut loose and allowed to rise independent of the structure, after several trusses were damaged.

In one high school we repaired, the interior partitions lifted the concrete double-T's off the supports due to soil expansion under the slab. It was a little exciting when we cut the partitions loose.

And finally, a slab under a church I worked on sloped up from the perimeter, to a maximum of about 6 inches over 4-5 feet. This slab is on 18 inches of granular fill (sand) put there as remediation for fat clay (per the drawings.) What seemed to be unlimited amounts free water drained out when we dug under the grade beam.

 

[Xcenterline]

Thanks TX. This site is great. I have searched here before for tips. Glad to be "on board" now (this is my first thread). So, I thought I would keep this thread going to share my experience for future people who visit this thread.

Got my hands on a bunch of projects done by the same client in the same area. Same basic design... piers ranging from 18" shaft to 30" diameter shaft with bells out to about 72". Fairly shallow depths overall. Perimeter is tilt wall with panels spanning between piers (no grade beam). The slab is however tied to panels. The slab is usually 5" thick (common for tilt construction), but reinforced slightly more than is normal in areas where slab-on-grade is not subjected any real expansion (heave) potential. Where I see isolated foundation elements, some use voids spaces, some do not. Otherwise, construction is fairly "normal."

Anyhow, what I see matches fairly well with the recommendations of the geotechnical engineer. I did have a conversation with the client also, who turned out to be very knowledgeable about these issues. It was a comfort to have that conversation. I believe that all the involved parties at this point know the risks and understand fairly well how the structural system operates relative to the soils. I will of course still disclaimer the work accordingly in the drawings. While in a perfect world, the slab and structural system would be completely isolated from the soils, it seems that in this case the risk is low enough and acceptable to the client to proceed with a design that is not as intensive to construct.