Repairing a Foundation Settlement

Friday, May 7th, 2021 by Jessica LaFlesch

Challenge

The Owners of this 1980’s Prescott, AZ single family basement home reached out to us after noticing signs of a potential foundation problem.

These foundation problem symptoms included:

Interior wall cracks
Exterior wall cracks
Doors that are out of square
Cracks in the brickwork

After completing an initial, level A visual inspection, it was determined that a Forensic Engineered Report was needed.

After examining the home and performing the manometer survey, Arizona Foundation Solutions believes the home could be experiencing foundation settlement at the northern, southern and western portions of the Basement as shown by the Signs of Stress and lower readings on the Topographical Map. The drop off in floor elevations on the topographical map is consistent with a foundation settlement pattern. Foundation settlement can be caused by one or any combination of many factors including sub-grade saturation of moisture due to poor drainage, years of storm runoff, plumbing leaks, improper compaction, the lack of a proper foundation system, and/or (in most cases) natural earth movement.

A minor foundation heave pattern is observed in the southwestern portion of the basement as indicated by the higher elevation readings in the 3D Topographical Map. This phenomenon usually occurs in areas where structures are built on expansive clays. Moisture from one or any combination of the following: storm runoff, poor drainage around the foundation, plumbing leaks and/or underground moisture sources will allow the moisture/vapor to accumulate underneath the foundation. The moisture then interacts with the clayey soils, causing them to swell. The clay soils take the path of least resistance and expand upwards and lift the foundation.

The Foundation Performance Association (FPA) "Guidelines for the Evaluation of Foundation Movement for Residential And Other Low-Rise Buildings" were adopted to correlate acceptable and unacceptable distress phenomena with actual survey elevations. Deflection and Tilt calculations were performed and compared to allowable values. For this engineered analysis, the deflection of the slab (L/99) was more than the allowable deflection limit of L/360. In addition, the tilt of the slab (0.60%) was less than the allowable tilt of 1.00%.

Solution

We believe the proper way to permanently stop the perimeter foundation settlement is to underpin the areas that are experiencing movement. Underpinning is the process of installing deep foundation elements called piles. Piles are engineered foundation supports that are driven down past the unstable soils and are then locked up into load bearing strata, which can support the loads that are transferred to them. Once the piles have been installed, they can be used to lift the perimeter foundation up to its Highest Practical Maximum. The piles should be spaced approximately six to eight feet on center and should start and stop near the hinge points of movement (exact spacing to be determined after load bearing calculations). In this case, the piles would be located at the northern, southern and western portions of the Basement.

Arizona Foundation Solutions believes that the proper way to deal with floor settlement, in this situation, is to compaction grout or poly level the low areas in distress. Either method can be acceptable depending on homeowner goals and are summarized as follows:

PolyLevel: this is done by drilling small 3/8” holes in the slab after which polyurethane grout is injected directly under the slab to raise it up to it highest practical maximum.

Compaction Grouting: this is done by coring 2-3” diameter holes in the floor. The holes would be spaced approximately 6' O.C. Steel casing is driven to refusal after which low very thick cement grout is pumped under high pressure. Poly level is very economical but does not stabilize or improve any soils that need densifying. Compaction Grouting is several orders of magnitude more expensive however it does improve and stabilize any loose soils that are in need of improvement. Prior to installing compaction grout, AZFS recommends a soil sample be performed to determine what type of grout mix would be necessary.

AZFS does not suggest a heave repair plan at this time as there is no significant damage to correspond with the high elevation readings. The home should be monitored and should damage arise in the future, AZFS should be contacted to perform a comparative monometer survey at a discounted price.