Resistance Drill Measurements to Economize Level III Timber Pile Inspections and Pile Load Capacity

Abstract: Marine port facilities are regularly inspected to assess their structural integrity. As most of the foundational elements are underwater, these inspections are typically completed by a commercial dive team inclusive of a PE Diver/Team Leader. Clients on occasion request partially destructive testing methods in order inspect the interior of the elements (Level III inspection effort) in anticipation of major repairs. A Level III underwater inspection effort seeks to discover any internal damage not detectible from a Level I or Level II inspection effort.
Damage to the interior of the pile is typically caused by a marine borer called Teredo (Teredo navalis). Teredo infests timber by digging tunnels through untreated timber located in temperate brackish and salt water. Teredo can infest even treated piles through exposed daps (notch at the top of the pile for fitting the pile into a pile cap or pile cap extension), open bolt holes, or other vulnerabilities in the wood. When inspected through visual or tactile means, an inspector is not always able to determine detailed information about the interior of the pile.
For a Level III inspection of timber, timber coring is traditionally conducted using either an auger core or a 2-inch pneumatic core drill. Samples are collected in predetermined locations based on the visual and tactile inspections. One sample is collected on each pile from the elevation that is predicted to have the most severe section loss. Once the sample is collected, the sample is documented and sent to a laboratory for testing.
The IML PD400 resistance drill has been used by structural inspectors to inspect timber structures on dry land. Due to the versatility and success of the device for inspections on land, the resistance drill was adapted for use on underwater structures. The resistance drill retrieves data using a long drill bit approximately 3-millimeters in diameter at the tip and 1.5-millimeters in diameter at the shaft. The drill will record the feed force required to push the needle into the wood and the torsional force of the drill bit known as drilling resistance. Measurements are typically recorded on each element in the tidal zone, at midheight, and adjacent to the mudline.
Pneumatic cores can only be retrieved at the location of the worst suspected damage on the pile and at no other elevation due to the structural damage one timber core alone causes to the pile. In addition to collecting more measurements in one location, divers using the resistance drill can collect data faster than with the pneumatic coring drill.
The data is analyzed in-house by the inspection team using the custom-built PDTools Pro software. The software can be used to analyze details about the timber pile for the full diameter of the pile, whereas the coring drill and the auger core are limited by the length of the barrel of the drill bit and auger, respectively. This paper will additionally demonstrate the relationship between the interior cavities measured by the resistance drill and the resulting reduced load capacity of the pile.

How does this project / topic contribute to the advancement of the industry and profession?: Traditional coring methods have been useful for determining the inherent characteristics of timber. However, undamaged timber is required to retrieve a usable sample. The cores cannot be used to determine if voids exist within the inside of the pile - core drills and auger cores are unable to capture timber that is not present. Additionally, the core drills are too short to penetrate to the center of most timber piles, limiting the area of inspection information gathered from coring to the pile exterior. The resistance drill can collect interior data at multiple elevations on the same pile.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: The topic discusses the implementation of the IML PD400 resistance drill as an alternative and alongside traditional Level III inspection techniques. The resistance drill is a tool traditionally used by arborists to check if trees can be manufactured into sufficient lumber. The resistance drill has since been adapted for structural inspections above water. Due to its success in that capacity, the drill was recently adapted for use in underwater inspection as well. This new tool is still relatively unknown to facility owners.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: The most challenging aspect of the topic was researching analyses used for hollow timber pile sections. While these exist for steel piles, there is no method explicitly developed for the flexural-torsional buckling of timber. Prior to the resistance drill, there would be no use for analyzing a hollow timber section because there was no way to know a timber section was hollow without destructive testing. I provide analyses for bearing capacity based on a reduced diameter; however, there is much room for development of this in future papers.

Who is the target audience for this paper?: The target audience for this paper is primarily port and marine facility owners seeking to optimize repair funding. This paper is also useful for inspectors looking to learn new underwater inspection techniques and designers who work with inspection data to design timber repairs.