Innocuous as it sounds, Colorado’s most significant geologic hazard is expansive or swelling soil — that is, soil laced with layers of various clays. These clays cause more property damage than any other natural hazard. Bentonite and montmorillonite (weathered volcanic ash) clays—in the form of soils or soft bedrock—underlie many populated areas of Colorado. They can expand up to 20% by volume when exposed to water and exert a force of up to 30,000 pounds-per-square-foot, more than enough to break up any structure they encounter. One Denver suburb has the dubious distinction of suffering more annual monetary loss from expansive soil than any other in the nation.
Expansive soils are one of the nation’s most prevalent causes of damage to buildings and construction. Annual losses are estimated into the billions of dollars. The losses include severe structural damage, cracked driveways, sidewalks and basement floors, heaving of roads and highway structures, condemnation of buildings, and disruption of pipelines and sewer lines. The destructive forces may be upward, horizontal, or both.
Expansive 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 or greater on foundations, slabs or other confining structures. Subsurface expansive soils on Colorado tend to remain at a constant moisture content in their natural state and are usually relatively dry at the outset of the disturbance that construction causes on them. Exposure to natural or human-sourced water during or after development results in expansion. In many instances the soils do not regain their original dryness after construction, but remain somewhat moist and expanded due to the changed environment.
Soil with a high clay content exhibiting typical desiccation-cracking when drying out after natural or human-caused wetting. Photo credit: US Department of Agriculture.
Design and construction of structures without an awareness of the existence and behavior of expansive soils can significantly worsen what might have been a readily manageable situation. Where expansive 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 and costly problem. Detrimental engineering design issues encountered 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 even though some mitigating measures were taken.
Construction problems related to expansive 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 contribute to expansive-soil related construction problems. Building without allowance for basement or ground floor movement in known expansive 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.
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.
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 0.75 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 period 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 period dot is flat and two-dimensional.
The clay minerals in rocks and soils are responsible for their expansion. This expansion 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 expansion, occurs throughout the mass of soil that is being wetted, and causes increased volume and significant expansive pressures within the mass. The opposite effect, called shrinkage, may occur if a previously wet expansive clay is dried. Although no large positive pressures are exerted, shrinkage will cause a volume decrease of the soil mass. These processes of expansion and shrinkage may occur any number of times for a single soil mass. Either swell or shrinkage may cause damage to streets and buildings, but expansion accounts for nearly all such damage in Colorado.
The clay mineral responsible generally for expansion is montmorillonite, often called bentonite. A sample of pure montmorillonite may expand up to 15 times its original volume. However, most natural soils contain considerably less than 100% montmorillonite, and few swell to more than 1.5 times their original volume (a 50% volume increase). A small load may decrease the actual swell to less than 1.25 times the original volume (a 25% volume increase). However, as 25% increase can be extremely destructive because volume increases of 3% or more are generally considered by engineers to be potentially damaging and require specially designed foundations.
Severity of problem
Expansive soils are a nationwide problem: each year, shrinking or expansion inflict a few billion dollars worth of 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. Sixty percent will experience only minor damage during their useful lives, but 10% will experience significant damage-some beyond repair. One person in ten is affected by floods; but one in five by expansive soils.
Since expansion is generally caused by the wetting of certain clay minerals in dry soils, arid or semi-arid regions such a Colorado with seasonal changes of soil moisture experience a much higher frequency of expansion problems than in the eastern half of the US that experiences higher rainfall and more constant soil moisture.
Rocks containing expansive 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 rather 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 expansive clay. These 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 expansive 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 expansive clay include Atterberg limits, plasticity index, grain size determination, activity ratio, dry unit weight, and moisture content. The primary design tests for expansive soils are the consolidation swell test for buildings, and the California Bearing Ratio swell test for roads.
Consequences of Improper Utilization
Damage from expansive 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 expansion 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 expansive soil.
In 1970, the state of Colorado spent nearly $0.5 million to repair cracked walls, floors, ceilings, and windows caused by expansive-clay damage at a state institution near Denver. In 1972, a state college library in southern Colorado required $170,000 to repair expansive-clay damage. A six-year old, $2 million building on the same campus was closed pending repairs to structural components pulled apart by expansive clay. A college building in western Colorado and a National Guard armory near Denver are among the other state buildings severely damaged by expansive 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 expansive 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 expansive clay throughout the state.
Highways in some areas of Colorado have required frequent and very expensive reconstruction or maintenance due to damage from expansive clay. As much as one foot of uplift from expansive 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.