News

By: Mark Clippinger, P.E.

Expansive clays, often called high shrink-swell soils, are soils that exhibit significant volume change or impose swell pressure as a result of changes in moisture content. The American Society of Civil Engineers estimates that 25% of all U.S. homes have some damage from these types of soils. They cause billions of dollars in damage annually to foundations, pipes, and roads, are causing more financial loss than floods, tornadoes, and hurricanes combined with estimated losses reaching $15 billion annually!  This includes cracked foundations, bowing basement walls, and “stair-step” cracks in brickwork. Movement caused by expansive clays often leads to misaligned doors and windows that stick, and gaps between walls and ceilings.  They often heave floor slabs and pavements.

While they are found in all 50 states, the severity and prevalence vary by location, with some states having much higher concentrations than others. I began my geotechnical career along the Front Range of the Rockies in Denver and Colorado Springs.  The expansive clays along the Front Range are derived from claystones and clayey shales and are considered moderately to extremely expansive due to their minerology and the semi-arid climate.  Typically in these areas, mitigation consists of over-excavation and replacement with granular, non-expansive structural fill. The degree of over-excavation is a result of the expansion potential of the clays as determined by laboratory testing.

There are two primary methods used to determine the expansion potential of clays in Colorado; the Denver Consolidation Test and the FHA Swell Test.  The Denver consolidation-swell test is an abbreviated version of the consolidation test. The undisturbed sample is subjected to a small surcharge (150 psi), then inundated with water and allowed to swell for about 24 hours.  The percent of volume change is noted, then incremental loads are applied to determine how much load is required to return the sample to its original volume. This applied load is considered the swell pressure, although it doesn’t account for pore pressure that develops due to the short duration of the test. The FHA swell test is performed on recompacted (disturbed) samples.  It requires an enclosed swell “pot” that keeps the sample from swelling (without volume change) after it is inundated with water, which allows the swell pressure to be determined.

In Northern Virginia, shrink-swell potential is based on the Swell Index test, or more commonly, the degree of plasticity as determined from the Atterberg Limit test.  The Swell Index is determined by placing clay in graduated cylinders, one with distilled water and the other with kerosene.  The ratio of swell in water vs. kerosene determines the Swell Index. Counties such as Loudoun, Fairfax and Prince William, have very strict policies regarding shrink-swell soils.  In the humid, continental climate of Virginia, the moisture fluctuation zone is generally regarded as approximately 4 feet below the surface.  Below this depth, the moisture in the soils is more constant and the clays are considered to be more stable. Mitigation is not considered necessary below the moisture fluctuation zone. For comparison, in the semi-arid areas along the Front Range, the moisture fluctuation zone typically extends to depths in excess of 10 feet.

Because of climate, effective ways to mitigate expansive clays in one region are often not effective in other regions.  The natural resources available in different regions also play a part in the form of mitigation. This has resulted in each region having its own method of mitigation of expansive soils.  Areas in the Midwest often use lime treatment of clays to reduce the shrink-swell potential.  In South Texas, instead of over-excavation or lime treatment of the clays, the foundations consist of post-tensioned slabs designed to resist uplift from expansive clays.  The most common form of mitigation of expansive clays in the U.S. is removal and replacement.  The clays are removed to a specified depth below foundations or pavements and replaced with non-expansive granular structural fill. This is effective for two reasons; it removes a portion of the clay, which is the underlying source of the swell pressure, and it increases the overburden pressure on top of the clays.

The most extreme version of removal and replacement that I am aware of is in the “Steeply Dipping Bedrock” zone along the Front Range of Colorado.  The upturned bedrock (dipping from 30 to >90 degrees) consists of a wide variety of sedimentary rock, including shale, claystone, siltstone and sandstone.  Occasionally, there are layers of pure transparent montmorillonite, 3 to 6 inches thick.  Back in the 1980’s, in order to mitigate the expansive dipping rock, cast-in-place concrete pier foundations were used.  In several residential subdivisions, the swell pressures, estimated at 10,000 psf, raised most of the concrete piers up to 12 inches, resulting in severe foundation and structural damage.  Consequently, a new approach was needed.  The local engineers went back to over-excavation, but not the standard method of over-excavation. The various rock beds are over-excavated to a depth of 10 feet below bearing elevation.  In addition, the foundation excavation extends a horizontal distance of 10 feet from the foundation.  On sloping sites, this resulted in foundation depths in excess of 25 feet!  The various rock materials excavated from the foundation are thoroughly mixed, moisture conditioned well above optimum moisture and recompacted in place with large pad foot rollers to a depth of 5 feet.  This method reduces the swell potential of the expansive layers and the horizontally placed fill significantly reduces moisture from getting to the undisturbed expansive beds. A perimeter drain is installed around the entire excavation, and the remaining 5 feet is backfilled with non-expansive granular structural fill.  An additional perimeter drain is placed around the entire foundation at footing elevation.

It is very interesting and challenging to relocate to a new region where you are exposed to unfamiliar ways to deal with some familiar geotechnical issues such as expansive soils.  Engineers in each region have experimented with various methods over the years or, sometimes decades, through trial and error until consistent success has been achieved.