Wood Pole Testing
Thomas Hay Ph.D., P.E.
Introduction to Wood Pole Testing
Wood pole inspection is a surprisingly large inspection market which includes public and private utilities, municipalities, school districts, and many other private/public entities that own and maintain wood utility poles. Wood pole testing is primarily carried out using two nondestructive testing (NDT) methods — relative density measurements across the pole diameter and visual inspection. Density measurements are made using a resistograph, a device that measures and digitizes the drill torque, through the pole cross-section. The wood pole inspection method is used to detect decay and rot below the ground line.
Figure 1: Wood pole visual inspection looks for obvious issues like wood pole leaning but also searches for localized pole defects that may affect wood pole strength.
Visual testing of wood poles is more widely deployed and involves inspecting wood poles for for excessive leaning, loose or damaged guy wires, dry rot, visible voids, flaking of shell and any other defects the pole might have. Visual defects, such as flaking of the shell, will not automatically fail a pole on its own but is often an indication that its overall structural integrity is in poor condition and should be further analyzed. Voids observed from the outside of the pole are measured in depth, width, and height. Such observations are later recorded. These are a good indication that the pole’s structural integrity has been diminished. ANSI O5.1-2017 Wood Poles - Specifications And Dimensions is the American National Standards Institute standard for wood poles.
A visual inspection of wood poles is done from the groundline to the top, before excavating for groundline treatment. Visual observation shall include the species of wood and original treatment, circumference, shell rot, woodpecker hole, evidence of ants or termites, split tops, or physical damage to appurtenances that can be spotted from the ground. Equipment attached to the pole should be observed, and irregularities (e.g., broken ground wire, loose hardware, broken or split cross arms, and broken guy wire) should be reported as remarks.
Wood Pole Testing - Strength Evaluation using Resistograph Measurements
Resistance drilling of wood poles is used for assessing decay, rot and remaining intact shell-wall thickness. The interpretation of drilling resistance data requires an experienced and knowledgeable profession in wood pole testing, remaining strength and knowledge of decay mechanisms. Drilling resistance measurements are a semi-destructive method and requires a 3 mm drilling needle by driven through the wood pole. The drilling torque is digitized for on-device analysis or off-device analysis software. Changes in relative wood resistance are documented in graph as changes in amplitude, or drill torque, over drilling depth in cm / inches. Sound wood a has higher relative wood resistance (higher amplitude) than decayed wood. Example wood pole testing data is show with the onset of internal decay is shown in Figure 2.
Figure 2: Wood pole test results showing onset of internal decay.
The following cursors are used for data assessment:
Ø Orange lines: Drill start and end point.
Ø Red lines: Start of critical shell decay region.
Ø Yellow lines: End of critical shell decay region.
Ø Light red decay line: Decay line that is generated during the automatic diameter measurement dips below this indicate decay.
Drone Inspection of Wood Poles
While the condition of the wood pole is more likely to decay and deteriorate faster at and just below ground level, it is critical to assess the entire elevation of the pole. This can increase the cost of utility pole testing significantly due to the access requirement. In many cases a bucket truck or man lift would be required to inspect the entire elevation. The add additional time, safety, logistical and cost implications to wood pole testing projects. Safety concerns include working at heights and fall protection. Many wood pole testing locations are inaccessible by bucket trucks or pose additional risk to worker safety. Traffic control in some cases case exceed the cost of wood pole inspections. These financial and safety implications can result in a lower frequency of testing for this market of critical infrastructure.
Drone-based NDT is an attractive alternative to traditional wood pole testing, especially from a financial perspective. Financial advantages of drone-based wood pole testing NDT include both the tangible and intangible expenses incurred with traditional pole access methods. For wood pole inspections, the technician may be required to work at a certain height above the ground, and a drone-based woold pole inspection is better than a human eye in this case, both in terms of safety and overall accuracy.
A manlift is a viable alternative, but it is imperative to note that they require 2-3 inspectors with a qualified operator and NDT technician in the platform and ground personnel for traffic control. Additionally, fall protection is required at all times during the inspection. Therefore, drone-based wood pole inspection will be used more and more as the technology progresses.
Wood Pole Damage Assessment using software
To evaluate wood pole strength quickly and accurately, the remining strength software us used. The software is a useful tool for engineers to evaluate the effect of damage or decay on a pole’s strength. Whereas traditional methods determine pole adequacy based on average remaining shell thickness or reduced circumference measurements, our software package allows the field inspector to enter the wood pole properties, dimensions, and locations of the observed damage. Example input geometries include heart damage, pocket damage, woodpecker decay, bolt holes, etc. The remaining life model then uses these inputs to compute the percent remaining strength of the pole. The major benefits are a quantifiable approach, and thus eliminating the element of subjectivity from maintenance recommendations. A major feature of the modelling software is the ability to analyze irregularly shaped and multiple concurrent damage. The software has built-in ANSI O5.1Reference Tables, and thus compliance with the relevant standards for wood pole inspection is guaranteed.
Figure 3: Wood pole remaining strength model inputs parameters.
Conclusion
This article discusses the two major methods for wood pole inspections — the resistograph instrument and visual inspection. It then highlights an alternative to the traditional methods, that being drone-based wood pole inspections. Drone-based wood pole inspection offers major and clear benefits over traditional NDT methods of wood pole inspection. Finally, the use of remaining strength modelling is discussed as a quantitative way to assess damage in a wood pole due to damage or decay.
References
1. ANSI O5.1-2017 Wood Poles - Specifications And Dimensions is the American National Standards Institute standard for wood poles.
2. STRENGTH AND RELATED PROPERTIES OF WOOD POLES, Lyman W. Wood, E. C. 0. Erickson, A. W. Dohr, ASTM WOOD POLE RESEARCH PROGRAM, FINAL REPORT 1960.
3. J. J. Morrell, Wood Pole Maintenance Manual, Research Contribution 15, Forest Research Laboratory, Oregon State University, Corvallis, Ore, USA, 1996.
4. G. L. Daugherty, “In-place wood pole inspection program,” in Proceedings of the International Conference on Utility Line Structures, Fort Collins, Colo, USA, March 1998.
5. J. Taylor, “Pole maintenance-its need and effectiveness,” in Proceedings of the American Wood-Preservers’ Association (AWPA ’78), Granbury, Tex, USA, 1978.
6. R. Birtz, Reliability of the Various Groundline Pole Inspection Methods and Proper Application of Fumigants, 7th Wood Pole Institute, Colorado State University, 1981
7. Bodig, “Practical definition of wood pole strength: relation to construction standards,” in Proceedings of the Wood Pole Conference, J. J. Morrell, Ed., 1986.