Where the bedrock stands on end.

Each grain of montmorillonite is a stack of thin sheets. Water slips in between the sheets and pushes them apart. According to the Colorado Geological Survey, pure montmorillonite can expand up to 15 times its original volume. Natural soils are mixtures and rarely swell beyond about one and a half times their size, but engineers consider an increase of only 3 percent potentially damaging.

The force is the real problem. The Survey says swelling clay can push on foundations and slabs with pressures reaching 30,000 pounds per square foot. A house weighs nothing by comparison. On the eastern plains the culprits are the Denver-Dawson, Laramie and Pierre shales. On the Western Slope it is the Mancos and Lewis.

In a wet climate, clay stays close to saturated and has little room to swell. On the Front Range it spends most of its life dry. Build a house on it, plant a lawn, run the sprinklers, and the ground beside the foundation gets more water than it has seen in centuries. The Colorado Geological Survey’s 1978 guide to building on swelling soils names the semi-arid climate as one reason Colorado is among the most severely affected states, and notes that more than 90 percent of its people live on the eastern plains or in Western Slope valleys, right where these soils are common.

Shrinkage works the other way. The same guide warns that trees and shrubs can dry clay out near a foundation and pull support from under it. Damage often comes slowly, and houses on the same street can be hit years apart, depending on how each one has been watered, planted and drained.

Geologists Noe, Higgins and Olsen mapped what this does near Denver in a 2007 study. They describe Mesozoic bedrock tilted from about 30 degrees to vertical, even overturned, and claystone weathered and fractured to depths of roughly 64 to 73 feet. When water reaches it, each clay band swells while the layers beside it do not.

The result is a set of long, low ridges running parallel to the mountain front. The team measured heave features up to about 28 inches high, up to 58 meters wide and as long as 1,067 meters. They describe severe damage near the Denver metropolitan area. A house built across one of those bands gets lifted along a line, not evenly.

Local governments responded with maps and rules. Douglas County created a Dipping Bedrock Overlay District, Jefferson County regulates dipping bedrock areas, and the Colorado Geological Survey published a heave susceptibility map for Colorado Springs.

In 1973, engineers D. E. Jones Jr. and Wesley Holtz published an article in the American Society of Civil Engineers’ magazine titled “Expansive Soils: The Hidden Disaster.” They estimated that shrinking and swelling soils caused at least $2.3 billion a year in damage to American houses, buildings, roads and pipelines. That was more than twice the damage from floods, hurricanes, tornadoes and earthquakes combined.

The Colorado Geological Survey adds its own ledger: more than 250,000 new homes built on expansive soils each year nationally, about 60 percent with minor damage and about 10 percent with significant damage, some beyond repair. In eastern Colorado, up to a foot of uplift forced the repair of two interstate lanes only six months after they were paved.

The Survey’s guide for homeowners recommends drilled pier and grade beam foundations for moderately to highly swelling soils. The piers extend below the zone where seasonal moisture changes and are anchored in ground that does not swell. The grade beam spans between them with a 3 to 6 inch gap left beneath it, so the clay can swell into empty space instead of pushing on concrete. An ordinary slab on grade is not recommended on natural swelling soils.

Inside, the same logic continues. Structural floors span above the soil instead of resting on it. Where a floating slab is used, it is isolated from walls, columns and stairs, non-bearing walls can be hung with a gap above the floor, and utility lines get flexible connections so a moving slab does not tear them apart.

For an existing house that has already moved, the question is the same as anywhere: what is the mechanism, how deep is the active zone, and what does a support have to reach to get below it.