Foundations and Soils

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Building foundations are perhaps one of the most important aspects of building construction.  They are responsible for supporting the building structure in its entirety, so when failure of the foundation occurs, it can have a significant impact on the construction materials that constitute the rest of the structure.

However, building foundations are notorious for having limited accessibility and visibility, and when failure of a foundation occurs, determination of the cause of failure as well as optimum methods of repair can be very difficult.  For this reason, a qualified structural engineer with expertise in foundation failures should be consulted in these situations.

One of the most commonly used procedures to determine the means of failure in a problematic foundation is observation of cracking patterns in the walls, ceilings, and floors.  Movement and/or failure of a foundation can put stress on the structural components above as well as the finish materials.  The areas in which wall/ceiling/floor cracking is occurring indicates the path of least resistance to this stress, and by mapping where these cracks are located in relation to one another, a qualified engineer can determine the general means in which the foundation is moving and/or failing.

More often than not, failure of a foundation is associated with differential movement of the soil beneath the foundation.  Differential soil movement can be associated with either settlement (compressive soil effects) or heave (expansive soil effects) depending on the conditions and composition of the soil.  According to section 1803.5.3 of the 2012 International Building Code (IBC), soils must meet the following provisions in order for the soil to be considered ‘Expansive’:

  1. Plasticity index (PI) of 15 or greater, determined in accordance with ASTM D 4318.
  2. More than 10 percent of the soil particles pass a No. 200 sieve (75 micrometers), determined in accordance with ASTM D 4318.
  3. More than 10 percent of the soil particles are less than 5 micrometers in size, determined in accordance with ASTM D 422.
  4. Expansion index greater than 20, determined in accordance with ASTM D 4829.

If item 4 is conducted, however, items 1, 2, and 3 are not required.  The above information can be acquired in an approved existing or new geotechnical report based on a subsurface soil investigation of the area.

Floor slope measurements can give a preliminary determination as to whether the soil beneath the foundation of a building is settling or heaving.  For example, a downward slope towards a concrete slab foundation crack can indicate settlement of the soil in that area.  On the other hand, an upward slope towards a concrete slab foundation crack can indicate heave of the soil in that area.

There are several potential causes of differential soil movement beneath a building foundation.  One of the most common causes is poor building pad preparation during construction of the building.  This can include failure to properly compact the aggregate base course (ABC) and/or subgrade (native soil or imported fill soil), and failure to clear building pad of debris prior to compaction of the soil and placement of the building foundation.

Other potential causes of differential soil movement include excessive moisture accumulation in the soil beneath the foundation due to under-slab leakage, surface water erosion, improper drainage provisions around the perimeter of the building, and ground water induced instability.

A qualified structural engineer will be able to determine which of these factors (or combination of factors) are contributing to failure of the foundation based on observation of damages, analytical reasoning, and acquiring a timeline of events for the building via research and interviews with the building owners/occupants.