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Tree Protection During Construction at the Florida Supreme Court

An Air Spade was used to locate roots to establish a tree protection zone around each tree.
If smaller roots are located crossing beyond the tree protection zone trench, these can be cut. If larger roots are located, then the tree protection zone is expanded whenever possible to accommodate these larger roots.

Project: Water Intrusion Project for the Florida Supreme Court
Task: Find roots so a real tree protection zone (not an estimated one) can be designed to protect four large live oaks on the grounds of the Florida Supreme Court in Tallahassee

Trees: four live oaks (Quercus virginiana) approximately 34 to 36 inches DBH
When: January through March, 2008 and December, 2008
Location: Tallahassee, Florida. 

A project was begun to solve water intrusion problem at the Florida Supreme Court Building in Tallahassee, Florida. Excavation was required to the foundation some 35 feet below the soil surface. A hole that deep around part of the building perimeter required a wide hole – in some cases so wide that it extended into the drip line of the protected specimen tree.  

Generally we try to protect a tree root zone at the dripline of the tree. In this case the contractor asked to be able to expand the excavation closer to the tree than the dripline. The only way we could do that would be to use an Air Spade and determine where the roots were actually located. First a trench was cut around the dripline of the tree. If no roots were found, we moved toward the tree and cut another trench. We continued making concentric rings until roots were encountered.

The end result was a tree protection zone that reflected the actual location of the roots. In some cases roots were encountered at the dripline and were hand pruned at that point. In other cases no roots were encountered until I was halfway between the dripline and the tree. The end result was a tree protection zone that was slightly irregular in shape.

Although roots generally extend well beyond the dripline of a tree, the majority of the roots are located within the dripline. Furthermore most roots can be found in the upper 18 inches of the soil, at times roots can be found at 3 or 4 feet below the soil surface depending upon soil and site conditions. Once the outer limit of the roots down to a depth of 12 to 18 inches was established, which is the working depth of the Air Spade, a small backhoe removed soil to that level. 

The Air Spade was then used to excavate down another 12 to 18 inches. If no roots were found, the backhoe would remove more soil.

The Air Spade and backhoe continued to be used until we reached a depth of 36 to 48 inches where we were certain no more roots would be found. After all the work, it turned out that only located one large root, about 4 inches in diameter, that was located below a depth of 18 inches. That root was located at about 3 feet below the soil surface.