A Definition

Swelling soils are soils or soft bedrock that increase in volume as they get wet and shrink as they dry out. They are also commonly known as bentonite, expansive, or montmorillinitic soils.


Swelling soils contain a high percentage of certain kinds of clay particles that are capable of absorbing large quantities of water. Soil volume may expand 10 percent or more as the clay becomes wet. The powerful force of expansion is capable of exerting pressures of 20,000 pounds-per-square-foot (psf.) or greater on foundations, slabs or other confining structures. Subsurface Colorado swelling soils tend to remain at a constant moisture content in their natural state and are usually relatively dry at the outset of disturbance for construction on them. Exposure to natural or man-caused water sources during or after development results in swelling. In many instances the soils do not regain their original dryness after construction, but remain somewhat moist and expanded due to the changed environment.

Aggravating Circumstances

Design and construction of structures while unaware of the existence and behavior of swelling soils can worsen a readily manageable situation. Where swelling soils are not recognized, improper building or structure design, faulty construction, inappropriate landscaping, and long term maintenance practices unsuited to the specific soil conditions can become a continuing, costly problem. Design problems might include improper foundation loading, improper depth or diameter of drilled pier, insufficient reinforcing steel, and insufficient attention to surface and underground water. Miscalculating the severity of the problem for a particular clay soil can result in damage although some mitigating measures were taken.

Construction problems related to swelling soils include lack of reinforcing steel, insufficient or improperly placed reinforcing steel, mushroom-topped drilled piers, and inadequate void space between soils and grade beams. Allowing clays to dry excessively before pouring concrete and permitting the ponding of water near a foundation during and after construction also are contributing factors in swelling-soil related construction problems. Building without allowance for basement or ground floor movement in known swelling soils areas is a very common source of property damage. Improper landscaping problems include inadequate management of surface drainage and planting vegetation next to the foundation so irrigation water enters the soil.

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 ¾ 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 as shown in the simplified diagrammatic sketch below.

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 swell ore shrinkage may cause damage to streets and buildings, but swell 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 ½ times their original volume (a 50 percent volume increase) (Jones and Holtz, 1973). A small load may decrease the actual swell to less than 1 ¼ times the original volume (a 25 percent volume increase). However, as 25 percent 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.

Severity of problem

Swelling soils are a nationwide problem, as shown by Jones and Holtz (1973):

Each year, shrinking or swelling inflict at least $2.3 billion in damages to houses, buildings, roads, and pipelines – more than twice the damage from floods, hurricanes, tornadoes, and earthquakes…Over 250,000 new homes are built on expansive soils each year. 60 percent will experience only minor damage during their useful lives, but 10 percent will experience significant damage-some beyond repair…one person in 10 is affected by floods; but one in five by expansive soils.

Swelling is generally caused by expansion due to wetting of certain clay minerals in dry soils. Therefore, arid or semi-arid areas such a Colorado with seasonal changes of soil moisture experience a much higher frequency of swelling problems than eastern states that have higher rainfall and more constant soil moisture.

Rocks containing swelling clay are generally softer and less resistant to weathering and erosion than other rocks and therefore, more often occur along the sides of mountain valleys and on the plains than in the mountains. Because the population of Colorado is also concentrated in mountain valleys and on the plains, most of the homes, schools, public and commercial buildings, and roads in the state are located in areas of potentially swelling clay. Swelling clays are, therefore, one of the most significant, widespread, costly, and least publicized geologic hazards in Colorado.

Criteria for Recognition

Although several visual methods for identification of potentially swelling clays exist, only a competent, professional soil engineer and engineering geologist should be relied upon to identify this potential hazard. Some warning signs for swell might include: a) soft, puff, “popcorn” appearance of the surface soil when dry; b) surface soil that is very sticky when wet; c) open cracks (desiccation polygons) in dry surface soils; d) lack of vegetation due to heavy clay soils; e) soils that are very plastic and weak when wet, but are “rock-hard” when dry.

Engineering soil tests include index tests and design tests. Rapid, simple index tests are used to determine whether more complex design tests are necessary. Some index properties that may aid in the identification of probable areas of swelling clay include Atterberg limits, plasticity index, grain size determination, activity ratio, dry unit weight, and moisture content (Asphalt Institute, 1964). The primary design tests for swelling soils are the consolidation swell* test for buildings, and the California Bearing Ratio* swell test for roads (Asphalt Institute, 1964).

Consequences of Improper Utilization

Damage from swelling clays can affect, to some extent, virtually every type of structure in Colorado. Some structures, such as downtown Denver’s skyscrapers, generally have well engineered foundations that are too heavily loaded for swelling damage to occur. At the opposite extreme are public schools and single family homes, which are generally constructed on a minimal budget and which may have under-designed lightly loaded foundations that are particularly subject to damage from soil movements. Homeowners and public agencies that assume they cannot afford more costly foundations and floor systems often incur the largest percentage of damage and costly repairs from swelling soil.

In 1970, the state of Colorado spent nearly $1/2 million to repair cracked walls, floors, ceilings, and windows caused by swelling-clay damage at a state institution near Denver. In 1972, a state college library in southern Colorado required $170,000 to repair swelling-clay damage. A six-year old, $2 million building on the same campus was closed pending repairs to structural components pulled apart by swelling clay. A college building in western Colorado and a National Guard armory near Denver are among the other state buildings severely damaged by swelling clays. These examples of damage to public buildings do not include the hundreds of thousands of dollars spent for repairs by local school districts. One school district near Denver is attempting to circumvent these expensive repairs by spending an additional $42,000 per school on structural floors. No figures are available for the total damage to homes in Colorado from swelling clays. However, several examples are known where the cost of repairs exceeded the value of the house. Cracked and heaved sidewalks, patios, driveways, and garage and basement floor slabs are very common indicators of swelling clay throughout Colorado.

Highways in some areas of Colorado have required frequent and very expensive reconstruction or maintenance due to damage from swelling clay. As much as one foot of uplift from swelling clay forced the repair of two concrete lanes of interstate highway in eastern Colorado only six months after completion of paving. In the same area, additional right-of-way had to be purchased, and the highway design had to be revised to eliminate cuts and fills in order to prevent similar problems with the two remaining lanes.