Colorado’s Sucking Soil Worth Exploring

By Christopher E. Carter, P.E.

Colorado soil sucks water , that is – sometimes a lot, and sometimes a little. Our local geotechnical engineers are familiar with this concept, and refer to its measurement as soil suction. In Colorado, our geology and climate combine to create some very challenging soil conditions. We have everything from rock, to clay, to silt, to sand, which interacts with our semi-arid climate to produce soil reactions that can swell, shrink, collapse, and/or settle.

Trying to understand why these things happen, however, can be just as challenging. Let’s look at expansive soils, for example. If you took a straw poll on the street, and asked passersby why foundations heave in Colorado, they would most likely say, “It’s due to all that Bentonite.” If you were to then ask them, “What’s Bentonite?,” you’d get a wide array of answers from, “soil that swells” to “rock that hurts Superman.”

However, if you were to ask a soils engineer that question, you’d better have 45 minutes for the answer. Eventually, you’d learn about the vadose (pronounced “vay-dose”) zone within a soil profile. This zone contains soil found between the ground surface and the top of the water table. In wet climates, the vadose zone is very shallow or non-existent because the water table is close to, or at the surface. In drier climates, such as in Colorado, the vadose zone can be very deep. This is important because soil engineering properties can change within this zone as soil moisture changes.

In wet climates, soil engineering properties are generally more reliable since their soils tend to remain saturated. In drier climates, soil moisture content tends to fluctuate above the water table with seasonal climate changes – often altering initial engineering properties. In fact, moisture equilibrium always seems to be out of balance in the upper vadose zone (closer to the surface), where soil is either trying to suck in more water to reach chemical equilibrium, or to expel excess water after reaching chemical equilibrium in response to the seasons.

That’s where measurement of soil suction can come in handy. Geotechnical engineers can quantify soil suction in their lab to determine its existing tendency to attract or repel water, and to predict maximum or remaining potential movement within the zone.

This information is especially valuable in foundation design and repair. In foundation design, soil suction can help determine foundation type once likely behavior is predicted. In foundation repair, soil suction can help explain why the foundation has moved, what is likely to happen next, and how it should be stabilized. And if you want to impress your soil engineering friends, just tell them you think that, “unsaturated soil mechanics really sucks!”