Posted by Johnny F Flores, P.E. (TX).   If you are building a bridge, a road, a parking lot, a school, an office building, an apartment complex,  or a  high-rise you need to watch out for swelling clays.  Swelling clays, also called  “expansive soils”  can cause severe and costly damage,  by exerting jacking forces that crack concrete and bend steel.

You need to take steps to tame them or design for the eventual movements they will cause.  But designing for these forces comes with significant added cost. Figuring out how much the above ground parts of your project cost are usually straightforward.  The prices of materials are predictable and the design is known in advance.  It’s the underground, hidden parts of the world which supports the foundations, the floor slabs and the pavements, the parts that never win architectural awards,  that can add to the uncertainty and surprise both in terms of cost and performance, when it comes time to build.

Here are three proven ways to tame these swelling soils.

Option Number One – Avoid Them if You Can

If you have an alternate site for your project which does not have expansive soils, you should consider it, all other things being equal.  This may mean that within a site, your should locate your building  strategically  to lessen the impact of these soils, or you should select a completely different site, if you have a choice.  Public infrastructure projects rarely if ever have the choice, but building developers and corporate site selection teams can at time exercise choice or use the added cost  associated for dealing with the expansive soils as a negotiation point with land sellers.   So with your geotechnical reports in hand – (see my earlier post), all other things being equal, you can pick the better site.

Option Number Two –  Remove and Replace 

If your site has a thin layer of swelling soil, you might be able to scrape by, literally.  Sometimes the most economical thing you can do is to scrape away the clay and build on the stable competent ground below.  After you remove the clay (and put it somewhere away from your structure), you may have to replace the removed clays with non-swelling material.    If you are lucky and work with an innovative earthwork contractor, the non-swelling material could be mined from a non-critical part of your project site.  Otherwise, you may wind up paying to have non-swelling material, crushed stone or derivative of crushed stone, hauled to your site from a quarry or pit.

Option Number Three  – Bridge Over Them

If you are risk averse or your structure requires a high level of performance – from an aesthetic or functional perspective,  you can bridge over these soils and  isolate your structure from the most destructive expression of these clays, that of swelling in slow motion against a floor slab or some other foundation element. In this case, performance trumps first construction cost and you  need a high  performance foundation system.  For this approach,  shafts are drilled  down past the swelling soils into a stable and competent supporting stratum and filled with concrete and steel.  The connecting beams and or floor slabs, supported entirely by the drilled shaft,  are built over a  crawl space or air gap, out of reach of the swelling clays.  Done right, the  clays shrink and swell harmlessly without touching parts of the building sensitive to movement.  Not all movements is eliminated, but generally, this is the option chosen by risk averse owners.  If you think you may need this approach, you also need to make sure that water will not accumulate under the slab.

Every project has  soil features and performance requirements.  The choice of a method to deal with swelling clays is best made with the advice of an experienced geotechnical engineer.