Home & Garden Columns

About the House: Mother Nature and Our Best Laid Plans

By Matt Cantor
Thursday September 17, 2009 - 09:40:00 AM

It always amazes me how nature manages to foil our best laid plans. Nothing is predictable, even the ground we build our houses on. And I’m not just talking about faults or landslides. 

Soils can be troublesome. Especially the ones that expand all by themselves. That’s right, some soils are like carnival rides, tossing you up here and dropping you down there and when they get done with you (and the house you rode in on) they don’t even give you a paper bag. 

What we’re talking about is expansive clay and it’s all around the United States, including plenty in our own East Bay. Clays are hard enough to build on successfully due to properties that we all take for granted, the most notorious being that they don’t drain particularly well.  

Soils that don’t drain well can cause a wide range of problems including a tendency to hold water around the beneath our building. This increases settlement over time since the soils sit wet long enough to allow the natural weight of the building to push into the soft smooshy stuff below, while houses that sit on well draining soils will be far less apt to do this. Of course, the constitution of the soils plays a role in this as well. A house that sits on perennially damp soils that are stiff and rocky will be far less likely to destabilize and settle but, interesting, that’s far more rare since harder materials are usually ones that drain better. 

Clays are, after all, what we make water vessels out of and have for at least 12,000 years. It’s a natural because even when damp, it holds water (actually, we later learn that when damp, certain clays are one of our world’s more stalwart moisture barriers). 

When we hold water below our houses there are other problems, though they needn’t be overwhelming. When water sits in a sufficiently warm environment, it evaporates and evaporation is a transport mechanism that puts it inside our houses if we lack a means to prevent it. This is, in fact, the way in which I am most apt to see molds and mildews growing in houses, far more often than actual leakage through ceilings and walls. 

Though it’s off topic, I would like to say that, for most situations, the means of prevention is a vapor barrier laid upon the soil (never on the framing above) and complimented by improved ventilation (usually passive, though powered ventilation is justified in some cases). 

So clay beneath our homes does this doubly nasty duty of becoming soft, when wet, and causing settlement, while also damping the insides of our homes through evaporation of moisture that cannot percolate downward. What to do? 

But this isn’t the bad news (he said while twisting his mustache). The bad news is that some clays (and you have to guess where they are) are more expansive than others. 

Clay has the potential to be expansive but varies depending on its chemistry (mostly the amount of sodium, but it’s far more complex than that). While some clays tend only to expand slightly when wet, others can expand many times normal volume (I seem to hear ranges anywhere from four to 20 times for highly expansive Sodium Bentonite clays).  

These highly expansive clays are like fields of tiny jacks that can push houses up and down with enormous force and can take weaker foundations and bust them to pieces. In fact, these clays are sometimes used as non-explosive alternatives to bust apart huge sections of rock by inserting them dry into cracks or holes and then adding water. A very slow explosion. 

A funny aside at this point is that many years ago a client of mine at an inspection was listening to me give a description of what was happening to her home when she jumped in any said that she was well are that there was “Clay-Jacking” going on in her home and that it was true all around her neighborhood. I was genuinely flummoxed by her familiarity with the subject and the use of this most apt term (which I had never heard). I asked her where she learned it and she said that many of the “ladies” she knew used it to describe this condition. I assume that a certain amount of neuro-chemistry and linear algebra was also discussed at these knitting bees. Never underestimate anyone. 

I was inspecting a house in Lafayette recently and Lafayette is a place where we find lots of highly expansive clayey (that’s not a typo; it’s how we spell it) soils. 

It was clear from the first walk across the floor that something was not quite right. When I finally got under the house two things were clear. One was that the foundation has been broken in many places and what was once level had been converted into that carnival funhouse ride I mentioned earlier. The other was that the soil was expansive. This showed itself clearly right on the soil surface in the form of surface deformations and deep fissures. 

When soils expand and contract and are protected from the weather (so that their surface chances are not obscured by rain or activity), one can see at least one common effect, and that is deep cracking. This deep cracking is caused by the fact that when soils expand they puff up with water, and when they dry out, they are so full of water that the simple horizontal component of compaction isn’t adequate to address the rate of shrinkage, and they pull into clumps divided by cracks which can, at times, be more than an inch wide. I’ve seen some that were two or three inches wide. 

The other effect that is less common is a sort of ballooning of soils that I see periodically and, in fact, saw that day. The literature that I (the lowly home inspector) have been able to cull isn’t showing me much, but it’s clear that the lumpy or, if you will, “bubbly” soil appears to be a function of expansion so rapid that it forms semi-spheroidal arcs across the tops of clumps. With the help of a geotechnical engineer I could have learned the specific make-up but it’s clear that this was clay soils expansion and likely that these contained significant amounts of Sodium Bentonite. 

Sodium Bentonite is amazing stuff. It’s made mostly of a mineral called Montmorillonite for the French locale of its identification. Sodium Bentonite is so expansive that we use it to seal wells and to create (oddly enough) moisture barriers below houses (though we use it is very thin and uniform layers. When wet, Sodium Bentonite absorbs many times its mass by bonding with water molecules and can form a tight barrier that will keep more water from passing through. We use this to line land-fills where we want to prevent toxins or pollutants from entering ground water and we use it in earthen dams. It’s also used as a desiccant (moisture absorber) for these same qualities. The CV of this stuff goes on and on and curiously includes many medical applications (some of which were know by ancient peoples including treatments from various dyspepsias) as well as a range of mechanical applications. 

It’s important to realize that if your house is being affected by this fascinating (or troubling) phenomenon, it needn’t ruin your day. There are solutions, the first being to control the water that is so essential an element in these equations. Without water, the most expansive clay soils remain essentially static (but for other geological effects), but how do we achieve this. The short answer is through drainage. If your property drains well, and this may involve significant modifications, it should be able to tolerate the presence of, even highly expansive soils. 

Of course, better and more appropriate designed foundations are a big part of our answer today. These often include Mat or Raft style foundations that, as the names indicate, float on top of the soil, disregarding a range of soils behaviors that lesser foundations will tend to get all upset about. 

Now this is all well and good but it you have a 1920s foundation, as many of us in the east bay do, it’s not much help to hear about mat foundations. What you need to do in these cases is to install drainage and keep that clay just as dry as you can. If the foundation has already been damaged, talk to an expert and see what intermediate fixes are available. Low cost “hand-dug” piers or mechanically installed “helical” piers can fix many foundation or, at least, decrease their rates of movement without depriving of you of limbs (well may be one limb) are often an adequate fix. 

I’m not really much for the tilt-o-whirl or the roller-coaster but if you are, go to Great America. Houses make lousy rides and it’s hell on property value. Actually, I like things pretty quiet. You’ll find me with my knitting group. I think we’re discussing particle physics today.