Bridge - Drilling

Target of investigations

Drilling can provide information about the internal decay, voids, the residual shell thickness in wood members.


Drilling is a direct technique to evaluate the presence of subsurface defects. When inspectors drill into a suspect member, they can feel the resistance against drilling if a hand crank drill is used and monitor the drilling shaving. Nowadays, large drill bits (i.e., 3/8 inch to 3/4 inch) are rarely used to drill through timber bridge elements due to their destructive nature. Drilling usually leaves a sizeable hole and sometimes extends entirely through members. It has largely been replaced with the resistance microdrilling technique, which is less subjective and minimally destructive.

Physical Principle

An inspector performs the visual inspection of drill shaving related to the presence of decay or void. Using a hand crank drill, the inspector can feel the resistance against drilling, which is proportional to the wood density.(1) The low-density regions can suggest the presence of decay or void. By feeling a sudden drop in the resistance against drilling and visually inspecting drill shaving, an inspector can confirm the presence of decay or void in wood members.

Data Acquisition

While an electric power drill can be faster, a hand crank drill allows an inspector to feel resistance as he or she drills, which can help estimate the decay depth and shell thickness. Drill bits with a diameter of 3/8 inch to 3/4 inch are commonly used. When drilling, an inspector monitors wood shavings for potential decay and notes areas where drilling is easier. The growth features of wood, such as resin pockets and knots, should not be interpreted as deteriorated regions. After drilling, shell thickness can be estimated using a bent wire or thickness gauge inserted into the drilled hole.(1) Copper naphthalene should be applied to the decayed area to retard the decay progress. Precautions should be taken to avoid spillage into the environment surrounding the bridge. The drill hole should be filled with a wood dowel soaked in preservatives.(1) The drill bits should also be cleaned to avoid the transfer of microorganisms. For drilling vertical faces, drill at a slightly upward angle for drainage to flow away from the filled hole.(2)

The common areas for drilling are the ones more susceptible to moisture accumulation and decay, such as the following:(2)

  • Deck planks at the bottom next to beams.
  • Beams in the sides near the deck and at the bottom close to the pile/bent cap.
  • Cap on the top near beams and at the bottom close to the piles/bents.
  • Pile/bent on the top near the cap and at the bottom near the waterline or ground level.

Data Processing

No data processing is required

Data Interpretation

An inspector marks areas of the wood where drilling is easier.


There are many advantages to drilling, including the following:

  • Simple
  • Low cost
  • Samples obtained from the subsurface of wood members.


  • Drilling can be subjective and is a destructive technique using drill bits ranging from 3/8 inch to 3/4 inch diameter.
  •  Drilling can be time-consuming and labor-intensive depending on the number of tests.
  • Drilling does not provide quantitative information about the physical and mechanical properties of wood members.


  1. White, R. H., and R. J. Ross, eds. 2014. Wood and Timber Condition Assessment Manual, 2nd ed. General Technical Report No. FPL-GTR-234. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory.
  2. Ryan, T. W., J. E. Mann, Z. M. Chill, and B. T. Ott. 2012. FHWA Bridge Inspector’s Reference Manual (BIRM). Publication No. FHWA NHI 12-049. Washington, DC: Federal Highway Administration., last accessed January 31, 2022.