Bentonite, the mineral responsible for the expansive soils common along the Colorado Front Range, has wreaked havoc on the home building industry. However, this pesky material is actually a beneficial material in many industries.
Sodium bentonite, the expansive form of this material, is used in industries as far flung as cosmetics, toothpaste, soap, paint, insecticides and water purification, according to a report by the Wyoming State Geological Survey (WSGS).
The expansive nature of bentonite makes it desirable for sealing earthen dams, as well as lake and pond bottoms. It has become popular as drilling mud in the oil and gas industry.
Health spas offer baths and facials using the absorptive nature of bentonite to draw toxins and heavy metals from the body.
Oh, yes — cat litter often contains it. So do the “speedy dry” agents used on highways to mop up oil and chemicals after a crash. Who knew the bane of Colorado builders is a commodity in such high demand?
Despite the headaches it causes for builders, this protean material isn’t without its uses in construction. It is actually useful to stop moisture in concrete construction. My crew has used bentonite strips between slabs and vertical walls as a water stop in the course of our business. (Yes, we had to pay for it.)
Bentonite’s volatile genesis
Bentonite, a montmorillonite-type clay, or hydrous silicate of alumina, is formed by the aging and decomposition of volcanic ash where water is present, according to WSGS. (Did you know that most of Colorado was once at the bottom of an ocean?)
Its name derives from Fort Benton in Wyoming, where huge deposits were found in the Benton Shale formation. Large mines are located in South Dakota and Wyoming, as well as in India, Russia, Ukraine, Greece and Turkey, according to the Wyoming Mining Association.
Over geologic time, the bentonite has migrated around the area, driven by winds and washed away by water. Other strata have likewise sifted over the layers of clay, resulting in the various depths at which we find bentonite.
WMA notes that when wetted, sodium bentonite can expand up to 16 times its volume, and absorb 10 times its weight in water. So those 4% soils are actually only part bentonite clay — imagine how the pure stuff would react.
This expansive property can lead to broken foundations, heaved floors and cracked drywall in homes built over bentonite- rich soils. In extreme cases, buildings have been completely destroyed by the heaving and wracking associated with soil movement.
As home building professionals, we have avoided bentonite by building on gravel, granite and anywhere the “bad clay” is rare or nonexistent. However, as much of the riverbeds and mountain backdrop have been used up, or protected as open space, builders have had to move onto the more expansive soils across the plains.
Over time, structural problems arose. Some were small, others rather profound. Lawsuits were filed and litigated. Solutions were tried; some highly successful, some not so much.
In future posts, we will discuss the structural issues associated with building on expansive soil, the legal history, and various solutions to expansive and compressive soils.