Potential for using through-boring to improve groundline treatment of Australian wood species: A preliminary study J.J. MorrellS Jack Norton Abstract An aging electricity distribution system and reduced availability of naturally durable tropical hardwoods in Australia will combine in the next decade to produce a major shortage of poles. One approach to mitigating this shortage is to utilize lower durability species and improve the penetration of preservatives into the refractory heartwood by introducing additional pre- treatment processes. A potential method for improving preservative penetration in the critical ground-line zone is through- boring. This process, in which holes are drilled through the pole perpendicular to the grain in the ground-line zone, is widely used in the western United States for treatment of Douglas-fir and may be suitable for many Australian wood species. The potential for improving heartwood penetration in eucalypts with alkaline-copper-quaternary (ACQ) compound was assessed on heartwood specimens from four species (Eucalyptus cloeziana F.Muell., E. grandis W.Hill ex Maiden, E. obliqua L’Her. and E. pellita F.Muell.) and Lophostemon confertus (R.Br.) Peter G.Wilson & J.T.Waterh). Longitudinal ACQ penetration was extremely shallow in L. confertus and only slightly better in E. cloeziana. Longitudinal penetration was good in both E. obliqua and E. pellita, although there was some variation in treatment results with length of pressure period. The results suggest that through-boring might be a reasonable approach for achieving heartwood penetration in some Eucalyptus species, although further studies are required to assess additional treatment schedules and to determine the effects of the process on flexural properties of the poles. Australian utilities have long employed naturally dura- Over the past decade, a variety of hardwood plantations ble hardwoods to support their overhead electrical distribu- have been established across Australia. While many of these tion lines, and over 5 million poles currently serve this role stands have been established for the production of pulp for across the country (Francis and Norton 2006). Many of these paper production, some species planted also have potential poles were installed in the post World War II expansion for use as poles. One drawback to the use of some plantation period and are potentially reaching the end of their useful hardwood species that could be used for poles is the relatively service lives. At the same time, changes in public land policy low natural durability of the heartwood. Australian utilities have placed many formerly accessible hardwood resources had a large resource of highly durable hardwoods. This re- that might serve as replacements off limits to continued har- source allowed utilities to pay little attention to protection vest. The same report estimated that demand for poles by of the heartwood in the preinstallation preservative treatment Australian utilities will increase from the current rate of process because protection of sapwood was adequate for 60,000 to 65,000 poles per year to well over 100,000 in obtaining acceptable performance of these species (Crews 2014. Presently, there is an insufficient supply of Durability Class 1 and 2 (Australian Standard AS5604, 2005) poles to meet this burgeoning demand. Francis and Norton (2006) The authors are, respectively, Professor, Dept. of Wood Sci. and summarized this problem and identified a number of possible Engineering, Oregon State Univ., Corvallis, Oregon (jeff.morrell@ strategies for bridging the supply gap including improving oregonstate.edu); and Research Scientist, Queensland Dept. of Pri- maintenance of in-service poles to reduce the need for re- mary Industries and Fisheries, Indooroopilly, Queensland, Australia placements, the utilization of plantation softwoods, and ([email protected]). This paper was received for pub- lication in April 2009. Article No. 10609. the use of less durable hardwoods, treated in a manner to S Forest Products Society Member. confer effective protection to the refractory, low durability ÓForest Products Society 2009. heartwood. Forest Prod. J. 59(9):61–66. FOREST PRODUCTS JOURNAL VOL. 59, NO.9 61 et al. 2004). It is likely that those days have passed, and utilities will need to identify methods for improving the per- formance of less durable heartwood. The obvious approach to improving eucalypt durability is preservative treatment; however, many of the plantation spe- cies produce relatively thin bands of permeable sapwood surrounding a thick core of low durability heartwood. Conventional pressure treatments are designed to treat the sapwood and are largely unable to produce acceptable heart- wood penetration. In the absence of improved heartwood treatment, users of less durable plantation hardwoods will be faced with continuing internal decay or insect issues at ground line that will increase maintenance costs and reduce service life. Current maintenance and remedial practices are specifically focused on improving heartwood durability, and Figure 1. — Example of a through-boring pattern where the many of the utilities now employ holes drilled abaxially into distances between individual holes (A, B, C, and D) can be the poles around the critical ground-line zone (Greaves and varied to produce the desired degree of treatment. McCarthy 1986, 1993). These holes are used to introduce diffusing preservatives with the intent of creating a treated and protected inner zone. An important aspect of the ability of a species to be through- A related development, in which holes are drilled through bored would be acceptable longitudinal preservative penetra- the poles, has been adopted in the United States to protect thin tion into the heartwood. In general, longitudinal penetration sapwood, low durability poles. The U.S. Pacific Northwest should be between 50 and 75 mm so that preservative move- has an extensive supply of Douglas-fir. This species grows ment from holes can create a sufficient degree of overlap to tall and straight, but also has a thin sapwood shell surrounding produce protection. Previous studies of Douglas-fir suggest a ‘moderately’ durable heartwood core (Graham 1980). In the that poles will be free of internal decay in the through-bored 1960s, utilities using this species were experiencing exten- zone, even when as little as 60 to 70 percent of the heartwood sive internal decay that reduced service life to as little as 12 is treated. This relationship may differ in hardwoods, given years. In their search for solutions to this problem, they de- the more variable treatment patterns produced by differential termined that drilling numerous holes through the pole, per- treatment of the vessels and fibers, but it provides a starting pendicular to the grain, in a regularly spaced pattern around point for assessment. the ground line could be used to improve heartwood treat- In this paper, tests designed to assess longitudinal penetra- ment in this critical zone and thereby the life of a pole with- tion of preservative into selected Australian plantation hard- out adversely affecting flexural properties (Merz 1959, woods subjected to various treatment cycles are described. Brown and Davidson 1961, Graham et al. 1969, Grassel Results are presented and discussed and the direction of fur- 1969). Termed ‘‘through-boring’’, the process involves dril- ther work outlined. ling regularly spaced holes at a slight downward angle through the pole beginning 900 mm below ground line and Materials and methods extending 600 mm above that line (Figs. 1 and 2). The holes expose additional end-grain to preservative flow and the re- The timber species tested were Eucalyptus cloeziana sult is a ground-line zone that has nearly complete preserva- F.Muell., E. grandis W.Hill ex Maiden, E. obliqua L’Her., tive protection. As a result of their efforts, through-boring is E. pellita F.Muell., and Lophostemon confertus (R.Br.) Peter widely used for treatment of Douglas-fir poles (Mankowski G.Wilson & J.T.Waterh. Heartwood of air-dried lumber that et al. 2002). Subsequent tests have shown that the through- was primarily quartersawn was obtained and cut into test bored zone remains free of decay decades after installation specimens (Table 1). One end of each test specimen was and Douglas-fir poles now have average service lives that are coated with a marine sealant to retard longitudinal fluid pen- estimated to exceed 60 years (Morrell and Schneider 1994). etration. The test specimens were then weighed (nearest g). A recent, extensive pole test in which 140 Douglas-fir poles Two test specimens of each species were allocated to each were loaded to failure has shown that holes as large as 13 mm treatment trial. Trials 1 to 3 were performed using test spec- in diameter can be used on poles with no significant loss in imens that were 400 mm long, while Trials 4 to 7 used test flexural properties (Elkins 2005, Elkins et al. 2007). These specimens 200 mm long. results, if transferable to the eucalypt species from Austra- Test specimens were vacuum-pressure-impregnation (VPI) lian plantations, could provide an alternative method for treated using vacuum periods ranging from 5 to 45 minutes, markedly enhancing the durability of these species. followed by pressure periods varying from 1 to 16 hours. All Dale (1977) designed a machine for boring out the center but one trial used a 1.5 percent solution of alkaline-copper- of eucalypt poles providing a means of treating the refractory quaternary (ACQ) compound formulated using monoethanol heartwood while taking out the low strength, low durability amine, while the last treatment used the same solution wood close to the pith of the tree; an added bonus was re- amended with 3.0 percent ammonia. ducing the risk of pole split development. The machine was For each trial, the samples were placed into the treatment used briefly on hardwood poles in Papua New Guinea but, as vessel, a vacuum of 295 KPa was drawn over the wood for the yet has not been used in Australia as a means of treating required time, then the preservative was slowly drawn into the eucalypt pole heartwoods.