Expansive soils, also known as soils with high shrink-swell potential, are common in various geographic regions, especially the central portion of North America. These soils are fine-grained clay minerals comprised of illites, kaolinites, or montmorillonites. The net effect of their constituency is such that they have the unique ability to absorb relatively large volumes of water and therefore expand. The extent of expansion is influenced by the percentage and type of clay mineral present and the amount of water available for absorption.
When expansive soils are subject to a compressive load, the ability of the clay mineral to increase in volume is diminished. Upon removal of the applied load and in the presence of water, these soils will continue to expand. In effect, clay minerals with expansive constituencies are prone to an increase or decrease in volume depending upon the relative change in water content of the soil, or, more specifically, the amount of free water available for absorption (swell) or desiccation (shrink). Over time and without due consideration for the shrink-swell potential of a soil, structures built on or in close proximity to expansive soils are prone to damage from movement as the water content of the soil increases or decreases.
The extent of expansion in a clay soil is primarily dependent upon three factors:
- the type or constituency of the clay mineral,
- the density of the soil, and
- the change in moisture content of the soil.
Given that the magnitude of volume change is highly dependent upon the change in water content of the soil, engineering properties of the soil become important considerations when evaluating the potential for structural damage.
For clays, these properties, which are expressed as the percent moisture content of the soil, include the following:
- Plastic Limit (PL) – the moisture content of the soil at which the consistency changes from a semi-solid to a plastic
- Liquid Limit (LL) – the moisture content of the soil at which the consistency changes from plastic to liquid
- Plasticity Index (PI) – the range over which the soil is plastic; calculated as the difference in moisture contents between the liquid limit and the plastic limit (PI = LL – PL)
Collectively, these properties are referred to as the Atterberg limits. Atterberg limits are determined in accordance with standard laboratory testing methods such as those published by ASTM International or the American Association of State Highway and Transportation Officials (AASHTO). The larger the PI, the greater the tendency for a soil to shrink or swell.
Arguably, and from a numerical perspective, a clay soil with 0 < PI < 20 is considered to have a low swell potential. In contrast, a soil with 35 < PI < 55 is considered to have a high swell potential. The preceding range in PI reflects the size, percentage and type of deleterious clay mineral constituency, the variation in the size of the clay particle, and the specific chemical characteristics of the soil.
In addition to the PI, load influences the extent of volume change in an expansive soil – the lower the confining load, the higher the potential to swell. Similarly, and given the inverse relationship between void content and density, low-density soils are less susceptible to swelling as compared to high-density soils. In general, low-density soils with a low PI are less prone to expansion as compared to high-density soils with a high PI.
For expansive clays, two additional soil properties are available to assess and evaluate the volume change: the potential for vertical rise (PVR) and expansion index (EI).
PVR test results quantify the latent ability of a soil to undergo vertical rise (swell) when subjected to additional water (e.g., water in addition to a specific moisture content present at the time of initial construction). Determination of the PVR is especially useful for pavement designs where the applied loads are variable or for slab-on-grade designs where the applied slab loads are typically much less than that of an adjacent heavier column load.
Analogous to the PVR, determination of the EI provides a means to quantify and compare the volumetric expansion of a moisture-conditioned soil. Essentially, the EI is an indication or measure of swelling potential such that soils with a higher EI are more prone to swelling than soils with a lower EI.
Building Code Requirements
The adverse effects of expansive soil are addressed in the International Building Code (IBC). Specifically, the IBC requires that, “In areas likely to have expansive soil, the building official shall require soil tests to determine where such soils do exist.”
Soil properties that are consistent with the potential for swelling include the following:
- A plasticity index (PI) of 15 or greater
- More than 10 percent of the soil particles pass a No. 200 sieve (a No. 200 sieve has a nominal mesh/screen opening size of 0.003 inches)
- More than 10 percent of the soil particles are less than 5 micrometers in size (5 micrometers is equivalent to 0.0002 inches)
- An expansion index (EI) greater than 20
In general, smaller clay particles are more prone to swelling as compared to larger clay particles. With respect to the preceding, the IBC further states that tests associated with Items 1, 2 and 3, “Shall not be required if the test prescribed in Item 4 is conducted.” In other words, if the EI of a soil is less than 20, then additional testing of the soil may not be required by the building official. Analogous to the PI of a soil, the expansive properties of a soil with EI > 35 are considered very high. Quantitatively, the IBC states that highly expansive soils typically exhibit volumetric changes between 7 and 10 percent – under abnormal conditions soils can expand 25 percent.
Relative Moisture Content
Although a positive relationship exists between PI and the swell potential of a soil, it is important to recognize that an inherent volume increase is not always synonymous with soil that has high swell potential. The magnitude of expansion or PVR is dependent upon the moisture content of the soil at the start of construction. An increase or decrease in moisture content corresponds to an increase or decrease in the volume of the soil relative to a starting point.
Likewise, if the moisture content of a soil remains in a state of moisture equilibrium (i.e., no measurable short- or long-term increase or decrease in the amount of free water available), then little or no increase in soil volume would be anticipated. However, the expectation of maintaining an equilibrium moisture content in soil that provides support for a building or structure is seldom realistic due to factors such as seasonal variations in the amount of rainfall and snowmelt.
Many pavements, slabs-on-grade and foundation support systems throughout the Midwest have been successfully constructed in areas with expansive soils, indicating that the shrink-swell potential of the site-specific soil has been identified and addressed during design and construction.
After determination and evaluation of the appropriate engineering properties (e.g., particle size and PI), several options are available to mitigate the detrimental effects of volume change:
- Replacement of the expansive soil – for clay minerals that exhibit moderate to high shrink-swell potential, and when these clays are located at shallow depths, an economical solution may involve removal and replacement of the expansive soil with a compacted structural fill material comprised of impermeable non-swelling soils.
- Chemical stabilization of the expansive soil – chemical stabilization involves the addition of various products that alter the shrink-swell potential of the soil: portland cement, fly ash, or lime.
- Installation of waffle or collapsible forms – when cardboard forms are placed beneath shallow foundation elements such as slab-on-grade and grade beams, swelling of the underlying expansive soil results in collapse of the forms without vertical rise of the element.
- Moisture maintenance – although the moisture content of soil changes with seasonal variations in the amount of precipitation and snowmelt, if an equilibrium moisture content of the soil can be maintained via the utilization of geosynthetic liners or bentonite mats, then the expansive characteristics of the soil can be mitigated.
- Utilization of deep foundation elements – foundation elements such as piers have the capability to transfer structural loads through an unstable or expansive soil layer to a deeper incompressible non-swelling soil strata.
While damage associated with expansive soils is frequently discussed in the context of vertical movement, foundations and retaining walls can also be subjected to significant lateral earth pressures due to swelling soils. The incorporation of controlled backfills facilitate a reduction in lateral pressures due to swelling soils. Although reinforced concrete walls provide restraint to lateral movements, unreinforced concrete masonry units (CMUs) should not be used as foundation elements in regions with known expansive soils.
Damage Assessment and Evaluation
Damage associated with expansive soils may occur within a few months of substantial project completion or become progressively noticeable over a period of years. The type and extent of damage may take many forms but almost always starts with differential movement in a structure’s foundation. In general, the damage due to swelling soils is assessed based upon the extent of floor cracks, movement (vertical or lateral) in floors and walls, and misalignment of building components supported by the foundation. Cracks often propagate from the corners of openings in walls as well as separation between adjoining walls and ceilings and walls. For basement floors and slabs-on-grade, level measurements provide a means in which to quantify the net and adverse effect of expansive soil conditions.
The evaluation of structural distress in regions with expansive soils must delineate between foundation movement due to expansive soil conditions verses long-term settlement or consolidation of poorly compacted soil. Sampling of site-specific soils and testing (e.g., determination of LL, PL, PI, soil density and PVR) facilitates a determination of damage causation.