Construction Knowledge (Earthwork) Series: What is Over-Excavation?

The immediate mental picture that comes to mind when you hear the term over-excavation is an overzealous operator on an excavator who dug deeper than they were supposed to.  However, Over-Excavation is a technical term.

 When building on native soil, it’s difficult to determine how compacted a soil is.  Native soils are usually deemed acceptable when they are “firm and unyielding”.  Soils that do not meet these criteria are classified as “unsuitable”.  The removal of unsuitable soils below plan excavation level is called Over-Excavation. 

 In the field, an onsite engineer might prescribe a 2 foot over-excavation and backfill with suitable soils.  Adding geotechnical textiles and quarry spalls is also a common backfill for over-excavation.  Typically, the soils do not improve at the bottom of over-excavation but this added layer of suitable soils helps bridge the unsuitable soils enough that a footing or pipe run can be installed.

 On projects with expected over-excavation, a cubic yard unit price is often agreed upon prior to starting the work and the earthwork contractor would get paid for the over-excavation that was required to complete the project.