Collapsible Soils

By Jonathan White, Senior Engineering Geologist, Emeritus

At the end of the 19th and beginning of the 20th Century, some of the first settlers of the plateau region of western Colorado along the Colorado River, and the Uncompahgre and North Fork of the Gunnison river basins, looked to fruit crops for their livelihood. The semi-arid but moderate climate was well suited for fruit orchards once irrigation canal systems could be constructed.

But serious problems occurred when certain lands were first broken out for agriculture and wetted by irrigation. They sank, so much in places (up to four feet!) that irrigation-canal flow directions were reversed, ponding occurred, and whole orchards, newly planted with fruit trees imported by rail and wagon at considerable expense, were lost. While not understood, fruit growers and agriculturists began to recognize the hazards of sinking ground. Horticulturists with the Colorado Agricultural College and Experimental Station (the predecessor of Colorado State University) made one of the first references to collapsible soil in their 1910 publication, Fruit-Growing in Arid Regions: An Account of Approved Fruit-Growing Practices in the Inter-Mountain Country of Western United States. They warned about sinking ground and in their chapter, Preparation of Land for Planting, made one of the first recommendations for mitigation of the hazard. They stated that when breaking out new land for fruit orchards, the fields should be flood irrigated for a suitable time to induce soil collapse, before final grading of the orchard field, irrigation channels excavation, and planting the fruit tree seedlings.

So, what are these soils? Why do they collapse? The reference cited above briefly stated that, “The tendency to settle appears to be due to the porous conditions of the subsoil.” Such soil properties are diametrically opposite from the better-known swelling problems that are found in the “fat” plastic clay soils of the Front Range. Collapsible soils are generally dry, low density, silty soils with high void space or air gaps between the soil grains where the soil particle binding agents are highly sensitive to water. These micro-pores can sometimes be seen by the naked eye. When exposed to and weakened by water, the binding agents break, soften, or dissolve such that the soil grains shear against each other and re-orient in tighter, denser, configurations. This reconfiguration causes a net volume decrease in the soil mass that, in turn, results in settlement of the ground surface. This condition can occur just by the weight of the soil itself, called the overburden, or the weight of a structure, such as a home foundation or dam abutment.

The binding agents of the collapsible soil structure can be very strong while the soil is in a dry state, and may possess high bearing capacities able to support heavy structures. When water is introduced, the soil fabric’s skeletal structure quickly weakens and fails. Collapse rate is also dependent on saturation rate of the soil. Because the introduction of water causes this collapse, the terms hydrocompactive and hydrocompressible are also used to describe these soils.

There are other types of soil collapse. One is piping and formation of soil caverns in dispersive and erodible soils, caused by active suspension and removal of soil particles by flowing water. Another is soil with a high evaporite-mineral or gypsum content, where actual dissolving of mineral grains and the cementation matrix (soil mass loss) can result in volume loss and settlement at the surface.

Structures and underground utilities founded on these types of soils can suffer from distress because of differential settlement. Because of the differential between two rates of settlement, strain can build until the structure bends, distorts, or breaks. The shifting and settling of the structure can be seen in a number of ways: 1) settlement, cracking, and tilting of concrete slabs and foundations; 2) displacement and cracking in door jams, window frames, and interior walls; and 3) offset cracking and separation in rigid walls such brick, cinderblock, and mortared rock. The damage can be similar to that caused by expansive or swelling soil. In fact, where both types of soils occur, usually in complex interlaying, it becomes difficult to initially determine what soil property is the cause of damage.

So, why do these soils form and where are the locations? The soils are derived from a number of different types of sediment deposition, but the key is really geology, climate, and resultant geomorphology.

Many regions of Colorado, outside of the crystalline rocks that form the major mountains, are underlain by poorly indurated (soft), clay and silt rich, sedimentary bedrock. The bedrock weathers easily and forms residual soils and is susceptible to rapid erosion.

It has been shown that semiarid areas are more prone to high sediment yields (expressed as tons of soil per acre lost by erosion, per year), which is to say that deposition of new sediments eroded from poorly vegetated hillsides is quick. Semi-arid regions have less vegetation and sufficient runoff of intense thunderstorms to transport large amounts of sediment. Sediment yields peak within the range of 12–20 inches (0.3-0.5m in annual precipitation that is typical for most of western Colorado, the intermontane valleys, and the high prairies next to the Front Range.

Numerous studies and case history compilations that include soil engineering properties (see map below) have shown that certain types of recent sediment deposits and soils can be susceptible to collapse. Those soils include windblown deposits of dust, silt, and fine sand called loess, hillside gravity deposits called colluvium, rapid deposition of unsorted waterborne material (mud and debris) in alluvial/debris flow alluvial fans and hillside slope wash, and recent overbank deposits called alluvium (silt and clay laid along tributary streams and gently sloped mud flats) (see block diagram, below). With few exceptions, soil collapse appears to occur in areas that have less than 20 inches (0.5m) of annual precipitation.

The common characteristic of these soils is recent and rapid deposition, depositional dynamics that result in an inherently unstable internal structure. The generally dry environmental conditions of the area cause these deposits to quickly desiccate (dry out) in their original condition, without the benefit of further re-working or packing of the sediment grains by water. Local ground-water levels generally never rise into these mantles of soil so they never become saturated. Only through human development and land use do local ground-water levels rise. The soils are introduced to moisture, through combinations of field irrigation, lawn and landscaping irrigation, capillary action under impervious slabs, leaking or broken water and sewer utilities, and altered drainage.

There are available engineering techniques to mitigate collapsible soils. They are grouped broadly into 1) ground modifications that mitigate the collapse potential of the soil, 2) structural reinforcement techniques, and 3) deeper foundations to transfer building loads through the collapsible soil horizon to a competent soil or rock layer below.

The most important thing to remember is that collapsible soils are dry in their natural state, and it is important that they remain so where structures have already been constructed without mitigation. Water and drainage management is always important for new-site development but is even more so with maintenance of existing structures. Certain restrictions for lawn-irrigation systems are also recommended. To reduce possible water introduction into the subsoil, xeriscape landscaping, requiring lower water usage, is suggested.

The CGS has been studying collapsible soils in Colorado for a number of years and has compiled case histories in Colorado on sites studied by the CGS, cited in published references, or supplied by other government agencies and private consulting firms. The data have been analyzed with respect to local geology, geomorphology (landforms), soil formation, and climate.

---

Further Resources:

ON-006-04 Collapsible Soils of Colorado — The CGS published both a regional susceptibility map and state-wide report on susceptibility of collapsible soils in Colorado in 2002.

This map identifies locations that may be susceptible to collapsible soils and subsidence related to dissolution of evaporite minerals. Meant as a guide for landowners, planners, municipal and county land-use regulators, and the geotechnical and civil engineering community, the map is a tool for formulating appropriate and proper types of investigation in the Roaring Fork River Corridor. 1 color plate (1:50,000).

Citation: White, Jonathan L. “MS-47 Collapsible Soil Susceptibility Map of the Colorado River Corridor in the Vicinity of Rifle, Garfield County, Colorado.” Soil and Karst Hazards. Map Series 47. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 2008.

This map identifies locations that may be susceptible to collapsible soils within the broad, semi-arid valley from the towns of New Castle to Parachute (formerly Grand Valley) along the Colorado River. Collapsible soils are dry, low-density, high-porosity soils that can spontaneously compact when they become wet. Also known as hydrocompaction, this phenomenon manifests itself as ground settlement and has been responsible for damage and distress for structures in the towns of New Castle, Silt, Rifle, and Parachute within the corridor. Digital zip download.

Citation: White, Jonathan L. “MS-34 Collapsible Soils and Evaporite Karst Hazards Map of the Roaring Fork River Corridor, Garfield, Eagle, and Pitkin Counties, Colorado.” Soil and Karst Hazards. Map Series 34. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 2002.