Shrink–swell capacity

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The shrink–swell capacity of soils refers to the extent certain clay minerals will expand when wet and retract when dry. Soil with a high shrink–swell capacity is problematic and is known as shrink–swell soil, or expansive soil. [1] The amount of certain clay minerals that are present, such as montmorillonite and smectite, directly affects the shrink-swell capacity of soil. [2] This ability to drastically change volume can cause damage to existing structures, such as cracks in foundations or the walls of swimming pools. [3]

Contents

Description

Cracked soil in Australia CSIRO ScienceImage 607 Effects of Drought on the Soil.jpg
Cracked soil in Australia

Due to the physical and chemical properties of some clays [4] (such as the Lias Group) large swelling occurs when water is absorbed. Conversely when the water dries up these clays contract (shrink). The presence of these clay minerals is what allows soils to have the capacity to shrink and swell. Some of these clay minerals are: smectite, nontronite, bentonite, chlorite, montmorillonite, beidellite, attapulgite, illite and vermiculite. [5] The amount of these minerals in a particular soil will also determine the severity of the shrink-swell capacity. [5] For instance, soils with a small amount of expansive clay minerals will not expand as much when exposed to moisture as a soil with a large amount of the same clay minerals. [5] If a soil is composed of at least 5 percent of these clay minerals by weight, it could have the ability to shrink and swell. [3]

This property is measured using coefficient of linear extensibility (COLE) values. If a soil has a COLE value greater than 0.06, then it can cause structural damage. [2] A COLE value of 0.06 means that 100 inches of soil will expand by 6 inches when wet. [2] Soils with this shrink-swell capacity fall under the soil order of Vertisols. [6] As these soils dry, deep cracks can form on the surface, which then allows water to penetrate to deeper levels of the soil. [7] This can cause the swelling of these soils to become cyclical, with periods of both shrinking and swelling.

Damage

Parched earth in Bourne Woods Parched-cracked earth in Bourne Woods - geograph.org.uk - 410994.jpg
Parched earth in Bourne Woods

Clay groups with a high shrink–swell capacity tend to damage crops during dry spells, as the soil contracts, by pulling roots apart. [4] Soils with shrink-swell capacity can cause engineering problems, or damage to existing structures. The swelling can cause structures to heave or lift, and the shrinking can uneven settling of sediment underneath foundations, potentially causing the structure to fail. [7] Some common structures that sustain soil damage are foundations, walls, driveways, swimming pools, roads, pipelines, and basement floors. [7] [8] [9] Roughly half of the houses in the United States are built on soils that are considered unstable, and half of those will sustain damage from soil. [8] This damage includes large cracks in walls and foundations, buckling of driveways and roads, and jamming of doors and windows. [10]

Shallow pipes, which are buried in the zone of seasonal moisture fluctuation, are put under stress by shrinking soils, [3] which can cause breakage of water or sewage pipes. Swimming pool shells can crack due to this pressure as well, and leaky pools can also introduce a lot of water into the surrounding soils over time, which could end up lifting pool decks and nearby foundations. [3] Annually, there is an estimated $7 billion in damage caused by clay shrink-swell soils. [11] All this damage is caused by the force exerted by expanding soil, or ground heave.

Expansive soils are the most problematic in regions with very defined wet and dry periods, as opposed to areas that maintain a certain level of moisture throughout the year, as this annual cycle causes the soils to expand and swell every year. [3] Water can also be introduced into the soil through people, or their infrastructure. Damage is quite often caused by differential swelling which is caused by pockets of wet soil situated right next to dry soil. [3] Examples of localized water sources include sprinkler systems, cesspools, leaky pipes, and swimming pools. [3]

Identification

Property owners and prospective buyers may check for expansive soils by consulting a soil survey, many of which are created and maintained by the United States Department of Agriculture-Natural Resource Conservation Service (USDA-NRCS). [2] A soil survey should list the coefficient of linear extensibility (COLE) value. [2] Professional soil scientists can also analyze samples of a soil to determine its shrink-swell capacity. [2] Expansive soils will form large cracks, in roughly polygonal shapes, on the surface of the soil during dry periods. [3] However, lack of these cracks does not mean a soil is not expansive. [7]

Mitigation

Many methods may be employed to mitigate or prevent the damage caused by expansive soils. When it comes to foundations, one solution is to extend building foundations to a depth where they are below the zone of water content fluctuation. [3] This supports them when the soil shrinks, and anchors them when the soil swells. Another solution is to remove the pre-existing expansive soil and replace it with a non-expansive soil, [7] but if the depth of the expansive soil is too deep, this option is very expensive. Maintaining a constant soil moisture is another solution, which sometimes may be achieved by allowing rainwater to properly drain away from the property, fixing areas around structures that have poor drainage qualities, fixing pipe leaks, avoiding over-watering nearby plants, and by planting trees some distance away from any structure. [7] Yet another solution is a process called soil stabilization, in which additional materials are added to the soil to limit its ability to shrink and swell. [7] Materials for stabilization include cement, resins, fly ash, lime, pozzolana, or lime-pozzolana mixture, [7] depending on the site conditions and the project goals.

See also


Related Research Articles

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Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, Al2Si2O5(OH)4). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impurities, such as a reddish or brownish colour from small amounts of iron oxide.

<span class="mw-page-title-main">Bentonite</span> Rock type or absorbent swelling clay

Bentonite is an absorbent swelling clay consisting mostly of montmorillonite which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-montmorillonite.

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<span class="mw-page-title-main">Montmorillonite</span> Phyllosilicate group of minerals

Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic crystals, known as clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite group, is a 2:1 clay, meaning that it has two tetrahedral sheets of silica sandwiching a central octahedral sheet of alumina. The particles are plate-shaped with an average diameter around 1 μm and a thickness of 0.96 nm; magnification of about 25,000 times, using an electron microscope, is required to resolve individual clay particles. Members of this group include saponite, nontronite, beidellite, and hectorite.

<span class="mw-page-title-main">Vertisol</span> Clay-rich soil, prone to cracking

A vertisol is a Soil Order in the USDA soil taxonomy and a Reference Soil Group in the World Reference Base for Soil Resources (WRB). It is also defined in many other soil classification systems. In the Australian Soil Classification it is called vertosol. Vertisols have a high content of expansive clay minerals, many of them belonging to the montmorillonites that form deep cracks in drier seasons or years. In a phenomenon known as argillipedoturbation, alternate shrinking and swelling causes self-ploughing, where the soil material consistently mixes itself, causing some vertisols to have an extremely deep A horizon and no B horizon.. This heaving of the underlying material to the surface often creates a microrelief known as gilgai.

Marine clay is a type of clay found in coastal regions around the world. In the northern, deglaciated regions, it can sometimes be quick clay, which is notorious for being involved in landslides.

<span class="mw-page-title-main">Smectite</span> Mineral mixture of phyllosilicates

A smectite is a mineral mixture of various swelling sheet silicates (phyllosilicates), which have a three-layer 2:1 (TOT) structure and belong to the clay minerals. Smectites mainly consist of montmorillonite, but can often contain secondary minerals such as quartz and calcite.

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Expansive clay is a clay soil that is prone to large volume changes that are directly related to changes in water content. Soils with a high content of expansive minerals can form deep cracks in drier seasons or years; such soils are called vertisols. Soils with smectite clay minerals, including montmorillonite and bentonite, have the most dramatic shrink–swell capacity.

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<span class="mw-page-title-main">Alkali soil</span> Soil type with pH > 8.5

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A gilgai is a small, ephemeral lake formed from a surface depression in expanding clay soils. Gilgai is also used to refer to the overall micro-relief in such areas, consisting of mounds and depressions. The name comes from an Australian Aboriginal word meaning small water hole. These pools are commonly a few metres across and less than 30 cm (12 in) deep, however in some instances they may reach several metres deep and up to 100 m (330 ft) across. Gilgais are found worldwide wherever cracking clay soils and pronounced wet and dry seasons are present. Gilgais are also called "melonholes, crabholes, hogwallows or puff and shelf formations".

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<span class="mw-page-title-main">Canal lining</span>

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<span class="mw-page-title-main">Soil aggregate stability</span> Property of soil

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References

  1. ↑ "Soil Properties Shrink/Swell Potential | NRCS". www.nrcs.usda.gov. Retrieved 2015-11-13.
  2. 1 2 3 4 5 6 Krenz, Jennifer; Lee, Brad; Owens, Phillip. "Swelling Clays and Septic Systems" (PDF).
  3. 1 2 3 4 5 6 7 8 9 Rogers, David; Olshansky, Robert; Rogers, Robert. "Damage to Foundations from Expansive Soils" (PDF).
  4. 1 2 Nagel, David. "Soil Science for Vegetable Producers." MSU: Coordinated Access to the Research and Extension System. 2001. Mississippi State U. 6 July 2008 http://msucares.com/pubs/publications/p1977.htm.
  5. 1 2 3 King, Hobart. "Expansive Soil and Expansive Clay". Geology.com. Retrieved 3 November 2015.
  6. ↑ "The Twelve Soil Orders". University of Idaho College of Agricultural and Life Sciences.
  7. 1 2 3 4 5 6 7 8 Mokhtari, Masoumeh; Dehghani, Masoud. "Swell-Shrink Behavior of Expansive Soils, Damage and Control" (PDF). Retrieved 3 November 2015.
  8. 1 2 "Problem Soils" (PDF). Retrieved 3 November 2015.
  9. ↑ British Geological Survey http://www.bgs.ac.uk/science/landUseAndDevelopment/shallow_geohazards/shrinking_and_swelling_clays.html
  10. ↑ "What Are Expansive Soils?". Property Risk. Archived from the original on 1998-05-24.
  11. ↑ "Clays, Clay Minerals and Soil Shrink/Swell Behavior" (PDF).
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