Heaving Bedrock

Calling the phenomena heaving bedrock, CGS geologists determined there is a bedrock equivalent of swelling soils. Just as certain kinds of clays in soil can attract and absorb water and can swell when they get wet, the same phenomena may be seen in clay-rich bedrocks such as claystones or shales.

Because rock layers just east of the Front Range are tipped up at an angle, the edge of any one rock unit tends to crop out in a more or less linear fashion, parallel to the mountain front. Claystone layers follow this pattern too. Changes in drainage after construction and watering of new grass around a building allow more water than usual to soak into the claystones. The claystones respond by doing what comes naturally: they swell. As long as extra water is provided, the claystones will stay expanded. Rocks on either side of the claystones that contain little clay do not swell. The linear ridges mark the tipped-up edges of claystones that swell more than the surrounding rocks do.

Sangre road damage caused by heaving bedrock

This “roller-coaster road” is the result of uneven swelling and heaving of steeply dipping bedrock layers. Photo credit: Dave Noe.

Legal definition

H.B. 1041, 106-7-106 (6): “Expansive soil and rock” means soil and rock which contains clay and which expands to a significant degree upon wetting and shrinks upon drying.

Descriptive definition

Sedimentary rocks and surficial soils are composed of gravel, sand, silt, and clay particles. In order to visualize the relative grain sizes of these particles, an example using familiar objects can be given. Although the average diameter of a gravel particle is approximately 3/4 in., suppose an average gravel particle were the size of a basketball. An average sand particle would then be the size of a baseball and a silt particle the size of a pea. The average clay particle, however, would be almost invisible, with a pencil dot representing a large clay particle. These clay particles may consist of a variety of minerals—quartz, feldspar, gypsum, and clay minerals. Common clay minerals in Colorado are montmorillonite, illite, and kaolinite. To return to the previous analogy, gravel, sand, silt, and some clay particles are often round, three-dimensional objects. Clay minerals, however, are generally flat, nearly two-dimensional plates just as the above mentioned pencil dot is flat and two-dimensional.

The clay minerals in rocks and soils are responsible for their expansion, or “swell”, as it is generally called. This swelling is caused by the chemical attraction of water to certain clay minerals. Layers of water molecules can be incorporated between the flat, submicroscopic clay plates. As more water is made available to the clay, more layers of the water are added between the plates, and adjacent clay plates are pushed farther apart.

This pushing apart, or swelling, occurs throughout the mass of soil that is being wetted, and causes increased volume and high swell pressures within the mass. The opposite effect, called shrinkage, may occur if a previously wet swelling clay is dried. Although no large positive pressures are exerted, shrinkage will cause a volume decrease of the soil mass. These processes of swelling and shrinkage may occur any number of times for a single soil mass. Either swelling or shrinkage can cause damage to structures, but swelling accounts for nearly all such damage in Colorado.

The clay mineral responsible generally for swelling is montmorillonite, often called “bentonite”. A sample of pure montmorillonite may swell up to 15 times its original volume. However, most natural soils contain considerably less than 100 percent montmorillonite, and few swell to more than 1-1/2 times their original volume (a 50% volume increase) (Jones and Holtz, 1973). A small load may decrease the actual swell to less than 1-1/4 times the original volume (a 25 percent volume increase). However, as 25% increase can be extremely destructive because volume increases of 3 percent or more are generally considered by engineers to be potentially damaging and require specially designed foundations.

Damages

Heaving bedrock is a distinct geologic hazard in certain areas in Colorado near the base of mountains where the sedimentary bedrock layers are steeply dipping (upturned and tilted). In such areas, the bedrock layers may swell unevenly to form linear heave features along the ground surface. Houses and structures built over such heave features may be subjected to extreme amounts of vertical and lateral stress, and the resulting damage can be severe. Offset foundations, driveway slabs, roads, and sidewalks are common disruptions from heaving bedrock.

Map

Case Histories

Colorado State Prison

In 1976 at the site of the new maximum security facility for the Colorado State Prison in Fremont County, swelling soils and bedrock were shown on geologic maps. Field investigations and soils tests resulted in a remedial plan by the geologic and soils engineers, architect, builder and others on foundation design, drainage and landscaping. Millions of dollars in potential damages were avoided.

Fairway Vista

Fairway vista bentonite bed 200

Surface view of a near-vertical bentonite layer in the Upper Cretaceous Pierre Shale in Jefferson County, Colorado. The layer heaved with a differential displacement of three inches within 24 hours after a rainstorm at this construction site. Note the hump in the fence aligned with the trend of the bentonite layer. Heaving bedrock damage is occurring in the subdivision in the background. Photo by Dave Noe.