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Undercut and Over Appreciated

excavation below subgrade
The word “undercut” has a negative connotation when used to undermine or dismiss someone.  However; the term is used in construction to define the act of removing unstable sub grade or base course.  The more technical term is Excavation Below Sub grade (EBS).  While it is beneficial to stabilize the sub grade and base course of a parking lot or roadway, it is often not a planned procedure which can make it an uncomfortable experience for all parties.  Here are some things to be aware of:

How are undercut areas determined?
Unstable areas in the sub grade or base course of a road or parking lot should be removed to aid in the longevity of the pavement.  The most common way that these areas are determined is by performing a “proof roll.”  Although some municipal projects require geological testing for sub grade to reach a certain density, this would be done in conjunction with a proof roll.

What is a proof roll?
A proof roll is a test in which the tires of a heavy vehicle (usually a fully loaded dump truck) are driven over sub grade or base course in order to determine unstable areas.  If the wheels of the truck leave ruts or deflection is seen, the area is deemed unstable and should be removed.

What is the minimum depth of rutting? What is deflection?
The minimum depth of rutting is up to the discretion of the project manager, engineer, or owner.  It is usually dependent on the project type and where the proof roll is on the earth sub grade or stone base.  Deflection is the amount of movement the sub base makes while wheels ride over it.  If the area is unstable, it will appear to “pump” while driven over.

How deep should the unstable material be removed?
Unstable material should be removed until stable base is found or deep enough that the new stone thickness will “bridge” the area.  If a stable area is not found, it is recommended to lay geotechnical fabric down and fill the area with thicker stone.

Although an undercut area is often not planned, one should be aware of the possibility of discovering an area in the construction process.  Stabilizing the area will protect the investment and aid in the longevity of the asphalt.  A smart reputable contractor should always communicate the discovery of these areas with the owner.

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