Construction on a building site usually requires relocating some of the soil.  The soil is removed from one location (below the original ground surface). This is called excavation, or cut.  The soil is then either replaced or added to other areas (above the original ground surface). This is called embankment, fill, or backfill.  Moving soil makes the site more suitable for its intended purpose.

It is usually the embankment soil we are concerned about here.  The type of soil, its moisture content, and the loads on it will dictate the correct placement and compaction procedure.  Written, industry-standard soil placement and compaction procedures are readily available to those in the industry.  The failure to follow the recommended procedures may result in settlement later on.  It is common for the settlement to become apparent several years later.

Note that this type of settlement is not applicable to deep foundations, which have their own standards.  Deep foundations can only be designed and evaluated by a competent Geologist or Geotechnical Engineer.

Try This:

See if you can obtain a copy of the construction drawings and compaction specifications for the project which placed the fill that is now settling. There may be a set of drawings called “as-builts” which depict the site as it was constructed. Sometimes you can get this from the agency which issued the construction permits.  Then, contact a local soils test lab which is qualified to perform a soils test called ASTM D1557, which determines the relative percentage of soil compaction (as compared to a standardized amount of compaction).

Now, you have a basic path to a possible solution, as follows:

  1. Using the construction drawings, verify that the location of the settlement is in an area where fill soil was placed by the project.
  2. Using the specifications for the project, determine what the required level of compaction was at the time of construction. If those numbers cannot be found, then use 90% outside of structures and 95% for soils beneath structures (including foundations and pavement). Those are percentages of the ASTM D1557 compaction and they are good for many types of projects.
  3. Excavate down and have the soils test lab perform in-place density testing at various depths. The technician will need to take a sample of the soil back to the lab.  It might take a week or so to get the results back. If the soils do not meet the 90% or 95% standard for compaction, then it may tell the story of what went wrong.

Resources for Further Research

Everything we need to know to reduce subsidence or settlement to an acceptable amount is already known in most cases.  You will probably need the services of a Geotechnical professional to persue this further.  Until then, check out the following examples of standards and guidelines if they are relevant to your situation:

  • American Society for Testing and Materials (ASTM) Standard D3276 Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort.
  • State of Florida Department of Transportation Soils and Foundations Handbook.
  • Edward J Monahan, PE:  Construction of and on Compacted Fills.