Differential Slab-On-Grade Movement Due to Poor Drainage/Water Control

Thousands of homes and businesses every year experience problems stemming from the movement of the floors that support their structures, walls, equipment, and exterior flatwork – differential vertical slab-on-grade movement.  This is a very common insurance claim and most owners do not understand the causes.  Since the soils inArizonaare commonly expansive, but remain relatively dry for the majority of the year, the slightest bit of water infiltrating the soils results in soil expansion with vertical and horizontal forces.  Expansive soils can be defined as any soil or rock material that has a potential for swelling due to a change in water content.  In some areas of the country, homeowners are advised to “water the foundation” to keep a constant moisture amount in the soil.  InArizona, the soils are dry, and the discussion below indicates the need to keep the soil dry.

Structures inArizonaare commonly constructed as concrete slab-on-grade, which is considered a “shallow” foundation system when compared to deep drilled pier systems.  The majority of the surface water introduced in close proximity to a shallow foundation infiltrates the bearing soils below the slab-on-grade via the backfill zone.  This introduction of water into the bearing soils will result in the expansion of the soils, as well as creating excessive pressures on the underside of the slab-on-grade and foundation system.  This expansion can lift shallow foundation systems, leaving portions no longer bearing on the soil, but rather cantilevering in the air.  The movement of the expansive soils will crack the slab-on-grade and foundation systems.  In some instances, this damage can lead to the structural instability of the foundation system.

It is PIE’s opinion that slabs-on-grade should not be used for finished spaces where expansive soils exist, as there is a potential for movement, distress, and damage to the finish materials, equipment, or foundation systems.  It has, unfortunately, been very common inArizonathat builders and geotechnical engineers recommend and specify such systems on expansive soils.  The only solution to preventing distress and cracking is a deep foundation system supported by non-active zones of soil, as well as separation of the slab-on-grade from the soils.  This would be termed a structural floor supported on the deep foundation, spanning over the expansive soil without contact.  This system is standard in areas such as the Front Range ofColorado.

Placing a slab-on-grade system on unstable soil results in the manifestation of damages from the movement of the soil.  These damages involve serviceability issues where doors, windows, and walls are damaged or inoperable, or when the deflections and differential movements of the slab-on-grade exceed typical tolerances for floor slope. Depending on the use of the slab, such as a warehouse that uses forklifts or other level-sensitive equipment, the issues with movement of the slab can result in secondary damages.  These damages can be severe from equipment failure to business interruption claims.  Therefore the structural engineer, architect, soils engineer, and builder must understand the risks associated with the selection of the slab-on-grade.

Cracks in the floor finishes such as the tile grout, or sometimes cracking through the tile itself, are indications of slab-on-grade movement.


Distress to the interior, non-bearing walls begins with small hairline cracks at the top corners of doors or windows, or where the walls and the ceilings meet.  When the door or window frames become out-of-plumb, they can be difficult to open or close.  When these manifested damages are misunderstood, and the expansive soils conditions are not addressed, damage will continue and, in some events, the egress and the structural integrity of the structure will be affected.



The following items very often contribute to water infiltration below slabs-on-grade, contributing to the resultant damage:

  • Improper use of slab-on-grade systems, or shallow foundation systems on unstable soils.
  • Inadequate surface drainage/slope adjacent to foundation walls.
  • Improperly maintained/installed gutters and downspouts.
  • Over-watering of landscaping/lawns.
  • Improperly maintained/adjusted sprinkler systems.
  • Vegetation, shrubs, trees, etc., installed too close to the foundation walls.
  • Landscaping that dams or ponds water adjacent to the foundation.
  • Improperly maintained flatwork (negative slope, joints/cracks not sealed, etc.).

A very common original design and/or construction defect is improperly sloped surface grading.  According to IRC Section R401.3 Drainage: “Lots shall be graded so as to drain surface water away from foundation walls.  The grade away from foundations walls shall fall a minimum of 6-inches (152-mm) within the first 10-feet (3048-mm).”  This helps to direct water away from the foundation and expansive soils.

Another common mistake is to assume the grade sloping around the structure will not decrease with backfill settlement.  The code requires this consideration.  When the fill is not compacted properly, grade that initially had a positive slope away from the structure becomes negatively sloped and directs water back towards the structure.  Backfill soils typically have a higher permeability than the natural soil, thus, water infiltration will permeate down to the foundation soils more easily in the backfill zone.  Perimeter foundation drains are commonly used at the bottom of backfill zones to control and remove water that enters the zone.  In some systems, vertical barriers and horizontal barriers are used to capture backfill water, preventing the underlying soils’ moisture content from changing.  These barriers can be used in combination with drain systems to aid in minimizing the impact to the soils below the shallow foundation.

Misdirection of roof water run-off is also a common defect. Roof gutters and downspouts should be regularly maintained and cleaned of all debris.  All drainage pipes/downspouts should be directed away from the building and directed to grade sloping positively away from the structure, beyond any landscape edging.  In many of PIE’s observations, the roof water run-off is either directed toward the building or directed to an area of grade sloped negatively toward the building, creating ponding water that saturates the soils directly under the slab.  Above-ground downspout extensions should be used and discharged at least 5-feet away from the foundation, and be placed over the landscape edge to minimize the potential for ponding water.

Landscaping and vegetation is also of concern around foundations.  Typically, vegetation should be kept at least 5-feet away from foundations.  PIE has observed many homes that have slab-on-grade heaving/cracking; it is very common to see plants or shrubs that are planted and watered against the structure.

Sprinkler systems are also a common source of water on a property.  It is important that the sprinkler systems are checked and maintained to prevent leakage from the systems.  Sprinklers should also be placed outside and directed to spray away from the backfill zone.  Rock landscaping with perforated edgings should be used for a minimum of 5-feet around structures.  The perforations should be placed so water ponding behind the edging is directed above the yard’s landscape material.

The risk of slab-on-grade movement cannot be eliminated from these sites, but if you have a shallow foundation and slab-on-grade system, reducing water around the expansive soils is necessary in order to minimize problems that will arise.  It is also necessary to perform observations and maintenance of your property in order to ensure that water is not being introduced around the system.  When this is done, the water introduction to the expansive soils and the associated movement to the systems will be minimized.

With the monsoon season upon us, precautions should be taken to avoid issues of improper water management adjacent to foundations susceptible to damage from water introduction into the expansive soils.  The risks associated with heaving of shallow foundations and slabs-on-grade cannot be eliminated, but the intent of water control is to minimize the introduction of water into the soils.  Building owners should review their geotechnical/soils report for specific recommendations regarding water control at their property, and simply keep the foundation system dry.

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