ADDRESSING UNSTABLE SOILS CONDITIONS

By: Darrien Shuquem

If you have observed damage in or around your home or building such as cracks in drywall or stucco (especially from the corners of doors and windows), doors and windows that are difficult to open and close, uneven floors, or cracks in slabs, patios, sidewalks, and block walls, you should investigate whether your home or building is being impacted by “soils movement.” 

When designing and constructing a home or other building, the design professionals and general contractor must consider the underlying soils conditions.  A typical first step in this process is a “soils report” by a geotechnical engineer, which involves taking soil samples at the site, from various locations at various depths, and doing tests and evaluations to determine the nature and characteristics of the soil.   Based on these findings, the geotechnical engineer will offer recommendations to prepare and condition the building pad, for the design and construction of the slab/foundation, and for grading and drainage around the structure. 

Unstable soils are generally either “expansive” or “collapsible,” but in both cases soils movement is triggered by moisture infiltration.  Expansive soils are “clayey” and are made up of microscopic particles which absorb moisture. Much like a sponge, expansive soils expand when wetted, and contract when dried.   When there is an increase in the moisture content of expansive soils beneath a structure, the upwards “swell pressure” can cause concrete slabs, sidewalks, driveways, and walls to crack, heave or move. 

Collapsible (or compressible) soils are generally “sandy” and loosely packed, and usually found in arid or semi-arid conditions.  When these soils are wetted, they will collapse or compress to become more densely packed. As with expansive soils, the soils movement that is triggered by water infiltration can cause damage to concrete slabs, sidewalks, driveways, and walls.

Unstable soils usually do not pose an insurmountable hurdle to construction, so long as certain precautions are taken.  First, the soils must be properly conditioned and the building pad prepared at appropriate moisture content and compaction levels (as determined by the soils engineer).   Soils beneath a building that are too dry are more susceptible to expansion or collapse.  

Additionally, it is critical that there is good drainage around the structure.  At the most basic level, this involves grading the lot so that water does not pond near the foundation, but instead moves away from the structure and off the lot or to an on-lot retention basin.   Building codes typically require that the grading slope at least 5% (a 6 inch drop over 10 feet) away from the foundation. Drainage can be further improved by the use of gutters and downspouts, to collect roof runoff and disperse it away from the foundation. 

In recent years, builders have increasingly made use of “post-tension” slab systems to address unstable soils conditions.  Before the concrete is poured, steel tendons are placed from each end of the forms. As the concrete cures, these tendons (which are anchored at one end) are pulled and tightened to create tension and strengthen the slab.  (In a home with a post-tension slab, there will be a stamp in the garage floor which states “Post Tension Slab — Do Not Drill Cut or Core.”)

Post-tension slabs are generally effective at resisting differential movement.  In other words, if a post-tension slab should crack, one side of the crack should not lift higher than the other.  However, post-tension slabs are not immune to movement if the soils beneath the slab become wet.  

A common problem found with post-tension slabs over expansive soils is “edge curl,” where the edges of the slab raise or curl up higher than the interior portions of the slab.  When this happens, the framing and trusses along the exterior walls are also pushed upwards, causing cracks in walls and ceilings, floor tiles, and exterior stucco. It is also possible for a post-tension slab to “tilt” as a result of soils movement, whereby one end or one side of the slab is higher than the other; this causes the same sort of damage to the home. 

Following construction, there are tactics homeowners and associations can employ to avoid or lessen the impact of soils movement:

1. Install gutters and downspouts in areas where roof runoff falls at or near the foundation; make sure the downspouts discharge the water at least several feet from the foundation.
2. Keep plants, shrubs, trees, etc., at least three feet from foundations, and avoid irrigation near foundations.
3. Avoid installation of planters, pavers, sidewalks, etc., that may hinder drainage or trap water near foundations. 
4. Routinely inspect landscaped areas around buildings to make sure water is not ponding near the foundation. 
5. Pay attention to damage that may be indicative of soils movement, such as cracked drywall or stucco, especially if running diagonally from the corners of archways and window and door openings.   Cracked floor tiles, baseboards separating from walls, and doors and windows that are out of square or difficult to operate, may also be indications of soils movement.

If a homeowner or association is concerned about soils movement causing damage to their property, a geotechnical or structural engineer should be consulted.  The engineer will review the soils report, the grading and drainage plans, the foundation plans, and other construction documents, and may also take soils samples and do a manometer survey of the slab.  The manometer determines the elevation of the slab across multiple locations, and is a useful tool to determine if the slab has moved or deflected since construction. A slab may have an elevation differential of up to 1 inch (or slightly more) as a result of typical construction practices, but elevation differences of 2 inches or greater usually indicate post-construction soils movement. 

Should it be determined that a building is suffering damage as a result of soils movement, repair protocols typically include the precautionary measures discussed above, and improving the grading and drainage.  If the lot configuration does not allow dirt to be moved around or added to improve the slope away from the structure, subsurface drainage systems are often utilized.  

In cases of substantial damage from soils movement, “cut-off walls” are often employed; a “cut-off” wall is a plastic membrane or concrete installed several feet deep around the perimeter of the foundation, to provide a physical barrier to stop moisture from migrating beneath the slab.  Sometimes, chemicals are injected into the soils beneath the slab which make the soils resistive to moisture. And where the slab is significantly out-of-level, the slab or portions of it may need to be removed and replaced, or raised or lowered.

If you believe your home or building is suffering damage from soils movement, you should not delay in investigating and addressing the issue, to protect the value of your property. If the underlying soils movement is not remedied and the damage not repaired, you will likely be legally obligated to disclose this problem to any subsequent buyer.  And should you determine that you need to assert a legal claim against the builder and/or seller, it is important to note that every state has statutes of limitations and repose which place a time limit on your right to do so. If you delay and do not assert your claim before it is time-barred, you will lose the opportunity to have the responsible parties address the soils movement and the resulting damage to your home or building.