Resistance Drilling Tree Inspection, Kent, WA

Kaibara Park

DRG provided a resistance drilling inspection to determine the overall health of an at-risk tree in Kent, WA.

Project Narrative:

The City of Kent, WA requested DRG to perform a Level 3 risk assessment using a resistance drill on a historic Deodar cedar (Cedrus deodara) tree in Kaibara Park. The tree had an observable lean to the west. The road and parked cars were the most likely targets in the event of trunk failure. Decay pockets on the east side of the tree were visible and suspected when sounded with a mallet. The client had established from visual tree assessment (Level 2 Risk Assessment) that the tree was a high risk rating for trunk failure.  The tree is historic and prominent in the city park so the city wanted a more detailed analysis.

Resistance drilling was conducted at 13 locations around the tree in an effort to describe the extent of the decay, and to identify locations where the tree was at risk of mechanical failure based on declining wood strength.  Measurements from the drill revealed acceptable levels of wood strength at critical locations.  The results from the Level 3 risk assessment indicate that the likelihood of trunk failure is improbable in the next three years, and that the overall risk rating for this tree is considered “Low.”

DRG advised that the tree can be retained, but should be visually inspected annually by an ISA Certified Arborist with Tree Risk Assessment Qualification (TRAQ). In 2024, the tree should be re-inspected with a resistance drill to monitor changes in wood strength. DRG also recommended that the city consider expanding the park by removing parking stalls and pavement within striking distance of the tree. This would have an added benefit of improving the growing conditions for this tree and other trees in the park.

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