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Roller machine compacting layers of soil.

Soil
Over-Excavation
(Over-Ex)

Footings on Over-Excavated Soil

Purpose of Over-Excavations

Engineers are encouraged by builders to find a more cost-effective alternative to piers, so over-excavations have become a popular approach to the design of foundations in weak or swelling soils that typically would not support footings.  Over-excavations involve the replacement of existing soil with "structural fill," which is defined as soil of a specific consistency that is moisture-conditioned (water is added) and compacted in layers to a specific hardness (density).


Over-excavations can be effective in the right conditions.  However, there are more risks related to footings on over-excavated soil than piers that are embedded deep into the ground.

See the Foundations Basics page for descriptions of footings and piers.

Diagram of layers of an over-excavated soil mass under a residential structure.
Soil Over-Excavation Process
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Why Footing Foundations Constructed on Over-Excavated Soil Can Fail


Structures founded on footings constructed upon a depth of over-excavation instead of piers can be prone to problems.  Although over-excavated soil is replaced under the supervision of a field technician to ensure it is of the right consistency, moisture content, and density, substandard construction means and methods often occur.

1.  ENGINEER'S INEXPERIENCE OR LACK OF TECHNICAL KNOWLEDGE


The design of a foundation supported by over-excavated soil requires advanced knowledge and experience with soil mechanics.  Many factors come in when determining if over-excavation is appropriate for the site.  Design constraints such as soil types, groundwater, grading, and climate must be considered.  Through soil testing and engineering analysis, it is up to the geotechnical engineer to evaluate the site, provide dimensions for the depth of over-excavation, and provide specifications to the contractor regarding the material and the method of compaction for the replaced soil.  If there are calculation errors on the engineer's part, the foundation can fail.


In addition, like all of us, engineers are human and may make mistakes in collecting data or performing calculations.  It is possible for engineers to make inadvertent errors or omissions in designing a foundation.   

2.  MOISTURE INFILTRATION


Moisture infiltration is the most significant risk factor for footing foundations on over-excavated soil.  Due to the structural fill's consistency and density, it is susceptible to wetting from the surface or from the surrounding soil.  Roof, drainage, or groundwater sources can cause settlement (compression) or expansion (swelling) of the structural fill and result in the failure of the footing foundation system.


The most common method of moisture transfer is by gravity.  The seepage of surface water, rain, and snow melting into the soil are common examples.  Moisture migration can occur in all directions.  Shrinkage cracks that develop due to moisture loss on the surface provide easy access for water to seep into the deep soils.


Capillary force is a significant means of water transfer in fine-grained soil such as clay, silt, and sand.  In clean sands, the moisture transfer can be a few inches; in fine sands, it can be up to 10 to 12 feet; and in clay, it can theoretically be more than 1,000 feet.  This means that moisture underground can travel quite a distance in certain soil conditions.

3.  CONTRACTOR MEANS AND METHODS


Over excavation, specifically, the placement of structural fill into an excavation requires good construction practices and oversight.  The contractor is required to place the soil in layers which require skilled work with water and compaction equipment.  If an improper type of soil is used, if water is not properly added to the soil, or if the soil is not compacted to the defined density, the soil will not properly support the foundation.

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