Evaluation of decay detection drill data – a semi-quantitative approach

Decay detection drilling and pick testing are common methods for evaluating decay in structural timber. In this study 63 poles in snow fence trusses at Finse in Norway, where climate change is expected to increase decay hazard, were evaluated by both drilling and pick testing. The aim of this study was to 1: assess a novel method to translate qualitative evaluation of drilling data into quantitative data, 2: compare the generated quantitative drilling resistance data to pick test data, and 3: evaluate potential gain from drilling in two different directions as opposed to one-way drilling. Graphs from decay detection drilling were evaluated visually, classifying areas of the graph as decayed or sound. The classification was translated into ratings from 0 to 4, thus translating qualitative observations into quantitative data. Decay in the outer sections of the beam was weighted higher than internal decay. Possible signs of initial decay were not considered.
No significant differences were found between drilling profiles from different cardinal directions. The first drilling was not made systematically in a specific cardinal direction. To evaluate the gain from drilling in two directions, ratings from the first drilling and the one yielding the highest rating were compared. Fewer poles were rated 1-2 and more were rated 3 and 4 when the highest rating was used, but the difference was not significant. Looking at the rating of individual poles and how it differed between drillings, 14 (22%) were given a higher rating from the second drilling. Results from the pick test corresponded better to drilling data when maximum rating was used, than to the data from the first drilling. Slight softening of the wood surface was found by pick test but not reflected in the decay drilling profiles. Several poles given low ratings by the pick test were rated higher by decay detection drilling, and several poles given high ratings by the pick test were rated lower by decay detection drilling. This shows that substantial internal decay can go undetected in a pick test, while the whole cross section can be in better condition than indicated by a pick test.