Expansive Soils

Expansive clay is a clay soil that is prone to large volume changes (swelling and shrinking) that are directly related to changes in water content. Soils with a high.
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This common phenomenon is a perfect illustration of how the location and magnitude of soil expansion will be greatest where the confining pressure is the least. Differential heave of expansive soil is also a common occurrence for pier and beam foundations. The differences in loading are often between interior isolated piers and continuous footings which usually carry heavier loads. As with the slab-on-grade foundation, uniform wetting of foundation soils can result in a mounding pattern where interior floors have heaved more than the building perimeter.

In evaluating damage which may have been caused by expansive soil, one must always consider patterns of wetting and drying of the soil. Soil moisture changes may be due to a rise and fall in the ground water table with the seasons. Soil moisture changes may also be due to periods of unusual rain, changes in humidity or unusual drought. These kinds of changes would be most likely to produce more uniform soil moisture conditions and patterns of foundation movement.

There are also moisture conditions which are caused by other factors such as plumbing leaks, site drainage, and irrigation practices. These conditions can cause differences in the volume of moisture which is being adsorbed by the expansive clay crystals, influencing the behavior of expansive soil and bringing about a variety of foundation movement patterns.

How Many Buildings are at Risk?

For example, if there is a slow drip or leak in the plumbing system, foundation heave surrounding the leak may be more pronounced. This will show up on the manometer plot as an anomaly which can lead to the location and repair of the leak. Perhaps because of poor site drainage, the crawl space at one time became saturated due to heavy rains and flooding. Later, because of the effects of sun and wind, the perimeter soils dried more quickly resulting in shrinkage and collapse.

The perimeter footing would settle while the wetter soils of the crawl space would keep the interior floor elevated. The floor level pattern would thus reflect the simultaneous effects of shrinkage and swelling of the foundation soils. Conversely, in a dry climate with a dry crawl space, continuous irrigation around the building perimeter would cause heave of expansive soils and lifting of the perimeter footing while interior floors may remain unaffected.

The floor level pattern would then be the reverse of the previous example- high on the perimeter and low in the center. Expansive soils can also have pronounced effects on site improvements such as patios, walkways, and swimming pools. Severe cracking and dislocation of these materials can be the result.

Expansive soils can be particularly brutal to swimming pools and associated improvements. I have seen pools heave, rotate, and crack as a result of expansive soil.

What Causes Soils To Expand?

Once the cracking begins, the leaking water just feeds the problem. Anyone planning a new pool or planning to repair a pool should consult a knowledgeable pool engineer who will evaluate the soil and design accordingly. Pavements resting on expansive soils which are also abutted to a building foundation or a retaining wall can move laterally away from the abutting structure while also lifting up, a reflection of the principal that expansive soils exert pressure in all directions. This lateral movement of improvements can be particularly pronounced when there is a nearby slope.

Recalling that there is always a horizontal component of expansive soil movement on sloping ground, the periodic swelling and shrinkage of expansive soils on a slope, together with the forces of gravity, will result in an ongoing conveyance or creep of soil down the face of the slope.

Slope creep can be responsible for distress to on-slope and near slope improvements which can be observed and measured. Walls and fences in particular will rotate in the down-slope direction under the influence of expansive soil. Hillside improvements on creeping soils must be heavily reinforced and firmly anchored to the soil in order to prevent damage and eventual destruction.

expansive soil

Design oversight by a qualified foundation engineer is highly recommended. In new construction where expansive soil is a concern, the engineer may require controlled pre-wetting of the soil prior to placement of the foundation. This will cause pre-expansion of the soil with the idea that further expansion pressure on the new foundation will be minimized. Alternatively the soils engineer may recommend that the upper several feet of expansive soil be removed and new non-expansive material be imported and compacted to create a stable layer of soil at the building footprint.

How Expanding Soil Affects Foundation Walls in or around South Carolina.

Depending upon the severity of expansion potential, non-expansive soils may be mixed with expansive soil to lower the expansion potential to an acceptable level. Where expansive soil conditions have been causing foundation movement on existing structures, repair designs may include deepened footings, thicker slabs, and extra reinforcing in all concrete foundation elements. There are a variety of underpinning methods which include the use of grade beams, concrete piers, pipe piles, screw anchors and a variety of other systems.


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Underpinning is a separate topic on this website and the visitor is encouraged to go there for a focused discussion of the topic. A structural slab has extra thickness and reinforcing to resist movement and distress caused by the expansion pressure of the underlying expansive soil.

Backfilled Soils And Virgin Foundation Soils

Exactly what constitutes a structural slab will depend upon the engineer. In my experience, a structural slab is usually about five or six inches thick and is reinforced with half-inch reinforcing bar 4 bars at eighteen to twenty-four inch centers in both directions. If the existing load-bearing footings are to remain, a concrete saw must be used to cut through the slab, leaving those narrow sections of slab which are directly on top of load-bearing footings.

Rebar dowels are pieces of rebar which are epoxy-glued into pre-drilled horizontal holes in the vertical face of the remaining concrete. The epoxy manufacturer will usually specify the hole size and depth required for a particular size rebar.

Expansive Soils Problems

Depending upon the severity of the expansive soil problem, the engineer may require that soil beneath the proposed structural slab be removed down to a particular depth and replaced with non-expansive material. If such is the recommendation, I will usually ask the engineer to give me an alternative design to see if it may be more cost effective to increase the thickness and reinforcing in the structural slab rather than remove the soil.

There are also expansive soil chemical treatments available which are designed to alter the clay mineralogy and reduce the expansion potential. Most commonly used for treating the subgrade on highway construction projects, lime is introduced into the soil in the presence of water. Other effects of lime treatment may include the formation of cementing agents within the expansive soil. The net effect of lime treatment is mitigation of foundation problems by reducing in the shrink-swell potential and by increasing the strength of the treated soil.

High-sulfate soils do not respond well to lime or other calcium-based soil treatment methods. As a result, highway departments and soils engineering researchers are looking for new and better options. Some of the alternatives which are being tested and tried by highway departments include silica fume, amorphous silica, fly ash, cation exchange products, enzymes, acids, emulsions, and polymers.

Presently there are a number of non-traditional proprietary liquid soil stabilizer products which are being offered for the treatment of expansive soil foundation problems affecting existing buildings and structures. Research on the no-traditional treatment systems is on-going. Due to the magnitude of the foundation problems caused by expansive soil, new and better products are needed. I would caution property owners to get the facts and insist on verifiable results when considering a liquid soil stabilizer.

Toward the beginning of my discussion of expansive soils, I made the following statement: The goal in moisture control is to take actions which will keep the expansive soil at a relatively constant level of moisture content. The object is to stabilize the expansive soil by injecting moisture into the subgrade. A program of subgrade irrigation for expansive soil should be designed by an engineer and based upon an investigation of the site and testing of the potentially expansive soil. Subgrade irrigation involves the installation of pipes to conduct water into the foundation soils at various injection points.

The amount of water required depends upon the season and the humidity. When your home was being built, contractors had to dig a big hole in the ground. This was created to make space for your basement. They dug up mounds of the hard-packed earth that was there — some of which may have laid there untouched for hundreds of years or more.

As foundation walls and house framing were completed, the empty space around the foundation needed to be filled. Contractors typically backfill foundation walls using some of the excavated soil that was removed to make room for the basement.


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The excavation process breaks up and loosens the soil. Because of this, backfill soils will always be more permeable, or water-absorbent than the hard-packed earth beyond. When it rains, the water will collect in the backfilled soils, exerting hydrostatic pressure against foundation walls. Wet foundation soils have a number of possible ill effects on the soils. By adding water to soils, you can cause them to expand, add weight to the soil, and ultimately lead to enormous pressure on the foundation walls.

While sandy soils remain stable as water passes through them, soils rich in clay undergo significant changes based on their moisture content. Here, chemicals are added to the soil which change the clay chemistry and mineralogy to make it less expansive. However, the treatment may not penetrate the soil very deeply, and this method is less commonly used in residential construction.

Expansive Soil Causes Basement & Foundation Problems

As noted above, excess moisture in the soils serves as the primary cause of damage due to expanding. Consequently, the proper drainage of rainfall, snowmelt, and irrigation is a crucial aspect of reducing the risk of these damages. This includes managing the drainage of surface moisture to prevent infiltration into the soil, as well as the drainage of subsurface moisture that has already penetrated the surface. An effective surface drainage plan consists of managing roof and slope drainage, as well as the proper use of ditches and swales. Roof drainage can be managed by the proper use of gutters, downspouts, and splash blocks to direct rainfall and snowmelt away from building foundations.

Water which infiltrates the ground near the foundation can not only leak into basements, but can also cause the soil around the foundation to swell, potentially damaging the slabs and foundation walls. Generally, geotechnical reports call for a proper surface drainage system to carry surface water 10 feet away from buildings at a slope of 10 percent. Another element of proper surface drainage is the use of ditches shallow trenches and swales depressions built into the landscaping, to ensure that water is channeled away from the buildings but not directed toward neighboring structures.

In addition to effectively managing surface drainage, it is also necessary to properly manage drainage of the water which does infiltrate the soil. Good subsurface drainage will ensure that the soils do not retain an excess of this moisture. Drainage of subsurface moisture can be managed by the use of sumps, perimeter drains, interceptor drains, and area drains. A sump consists of an enclosed pit or low area that collects subsurface water.

A submersible pump in the bottom of the pit pumps water out and to the surface. The drain pipe collects subsurface water and is sloped to direct it away from the foundation and into a sump, area drain, or gravity outlet. Again, this is an important means of preventing damage to the building by keeping excess water away from the soils near the foundation.