3.1 Pole supply, failures, and the cost of replacements in Uganda
In total, 1,007,290 treated wooden poles were supplied between 2017 and 2021, averaging 201,458 poles annually. A total of 83,605 prematurely failed poles were replaced over the same period costing Ugshs. 30,913,673,440 (USD 8,832,478) in replacements averaging over Ugshs. 6 billion (USD 1,700,000) annually. Trend analyses for poles replaced and their cost of replacements showed a general downward trend (Fig. 1 and Fig. 2).
UMEME (2019) indicates that the downward trend in wooden pole failure is attributed to the adoption of new technologies such as backfilling of poles in concrete and the use of polesaver sleeves which they perceive to improve the service life of wooden utility poles to the expected 40 years. The use of concrete backfilling is a technology commonly used in construction to protect the poles from getting in contact with the soil which harbours the biodegrading organisms (Peterson, 2006). Polesaver sleeves, on the other hand are said to stick on the pole keeping it dry and airtight, creating a barrier between the pole and the soil (Van Acker et al. 2023). Globally, the United States replaces 3.6 million wooden poles per year and yet adds 2.4 million poles on the distribution line annually (Kamweru 2022). Australia has an estimated 5 million wooden electricity poles in service with more than 75,000 new poles installed per annum and $40–50 million spent on maintenance and replacements annually (Baraneedaran 2009). This indicates that, despite the failures in wooden poles, large economies have still maintained their use in electricity transmission, however, the efficiency of these poles is reliant on efficient and effective management and maintenance strategies.
3.2 Cause of pole failure
The leading causes of premature pole failure were fungal decay (53%) and termite damage (21%) while bird nesting (2%) and mechanical forces (2%) were the least (Fig. 3). This is attributed to the poles being erected in contact with the soil thereby exposing them to microorganisms which reside in the soil, consequently, explaining the majority (74%) of poles that failed at the bottom – within one meter below or above the ground line. Hodgson et al. (2009) indicated that rapid deterioration of wood by termites occurs when the wood is in contact with the soil which offers favourable conditions for their activity. Furthermore fungus-infected wood is highly susceptible to termite attack compared to sound wood, thus indicating that the two biodegrading agents work together in deteriorating wood (Matsuo and Nishimoto 1973). This is because fungi feeds on lignin and cellulose (Arantes et al. 2011; Baldrian and Valášková 2008), undermining the structural integrity of the wood, thereby, making it easier for the termite to break down the wood (Nofal and Kumaran 2011).
3.3 Relationship between causes of pole failure and preservative chemical used
The overall chi-square results (X2 = 16.158, P \(<0.05\)) indicated a significant association between the cause of pole failure and the preservative chemical used. Creosote-treated poles had more (59%) failures compared to CCA-treated poles (41%) which is attributable to variations in the chemical retention levels. However, the leading causes of pole failures identified in this study, that is, fungal decay and termite attack accounted for 85.3% of CCA-treated poles and 67% of creosote-treated poles (Table 1). This disagrees with Antwi-Boasiako and Atweri-Obeng (2012) findings who reported that chemical retention was higher in CCA treated wood compared to creosote. That notwithstanding, premature failure among treated wooden poles can be attributed to on-site climatic conditions, the duration of seasoning, handling and the treatment methods used. Furthermore, failure of wood is at times caused by bacteria which is capable of decomposing and predisposing both CCA and creosote treated wood to fungal attack (Clausen, 1996). Zabel et al. (1980) indicate that trees can be colonized by fungi prior to harvest and if not well treated can persist and manifest after the pole installation. For example, some of the poles pecked by birds for nests revealed a shallow depth of penetration by the chemicals used. This inadequate impregnation of the chemical makes it easier for the poles to be attacked or degraded by termites or fungi (Gezer et al. 2015).
Table 1
Cause of failure by the preservative chemical used
| Number and percentage of pole failure by Preservative chemical used | |
Causes of failure | CCA No. (%) | Creosote No. (%) |
Decay | 59 | 62.1 | 64 | 47.1 |
Termites | 22 | 23.2 | 27 | 19.9 |
Split | 6 | 6.3 | 16 | 11.8 |
Wind breakings | 0 | 0.0 | 8 | 5.9 |
Vandalism | 0 | 0.0 | 7 | 5.1 |
Fire | 2 | 2.1 | 4 | 2.9 |
Birds | 1 | 1.1 | 4 | 2.9 |
Mechanical forces | 2 | 2.1 | 3 | 2.2 |
Bees and other insects | 3 | 3.2 | 3 | 2.2 |
| 95 | 100 | 136 | 100 |
3.4 Relationship between causes of pole failure and geographic location of the poles
The overall chi-square results (X2 = 98.558, p \(<0.05\) ) indicated a significant association between the cause of pole failure and the geographic location of the pole in service. Western Uganda had the highest (45%) pole failure while northern Uganda had the least (10%) (Fig. 6). This was attributed to their differences in weather conditions, with western Uganda generally receiving two seasons of rainfall a year which keeps the soil moist enough to support termite and fungal activity while the Northern region is generally dry because it receives one major rainy season a year and as such the soil is dry and not moist enough to support microbial activity. Furthermore, bird nests were only observed in western Uganda (Fig. 4 and Fig. 6) and these holes caused by the woodpeckers created openings for water to enter the heartwood of the poles consequently causing the poles to split and thus creating access for termites and fungi into the poles. According to Obua et al. (2010), the western region is the most bird diverse region in Uganda comprising more than three quarters of Uganda’s bird species, this could have been the reason for more woodpeckers in this region. It is important to note that fire was mainly observed in Northern Uganda (Fig. 5) because of the largely dry and vacant land in this region that is characterized by frequent bush burning thus exposing the poles to fire. This affects the structure of the pole thus weakening it and making it susceptible to breaking by wind and other mechanical forces.
In, eastern Uganda, most of the observed poles were located in wetland areas thus accounting for the high fungal activity observed in this region. Consequently, this explains the high (27%) failure recorded in this region (Fig. 8), that notwithstanding, the region also had sugar cane plantations which recorded low pole failure attributed to the farm management activities practiced in these areas. The central region harbours the capital of Uganda with the highest power connectivity in the country. Therefore, owing to this fact, the observed rate of pole failure was generally low (18%) because of the high demand for electricity in this region thereby causing regular replacement of failed poles and timely maintenance in terms of remedial treatments thereby killing the biodegrading agents.
The observed association between the geographic location of failed poles and the causes of pole failure can be attributed to variations in altitude which makes the western region with a higher altitude generally colder and wetter than the northern region with a lower altitude, thus experiencing differences in temperature and soil moisture, moreover, these are some of the key factors that influence termite and fungal activity (Brischke and Rapp 2008; Teodorescu et al. 2017). According to Siles et al. (2009), differences in altitudes result in differences and uniqueness in soils which leads to variation in the abundance and composition of fungi. Fungal community composition and diversity have also been reported along altitudinal gradients with fungal gene abundance being positively associated with altitudinal gradients (Shigyo et al. 2019; Tanaka et al. 2019). Wang and Wang (2012) developed a timber decay model based on data obtained over 35 years from 77 timber species and this showed that the rate of decay of timber is a factor of climate changes, these changes are determined by annual average temperature and rainfall at any location, thus these significantly affect the rate of timber deterioration.
3.5 Relationship between causes of pole failure and point of the pole failure
The overall chi-square results (X2 = 317.951, p \(<0.05\)) indicated there was a significant association between the cause of failure and the point of pole failure i.e. Bottom, Middle point, Top and Through.
The observed association between the cause of pole failure and the point of pole failure can be attributed to the fact that wooden poles are erected outdoors in contact with the soil which exposes them to various causes of pole failure and these are specific to the point on the pole, for example, termites damage and fungal decay are likely to happen at the bottom as these reside in the soil, and split will happen through the pole while wind and mechanical forces are likely to break the pole at the middle point. The highest failure at the bottom (within the ground line of the pole) (Fig. 8) can be attributed to the fact that the main causes of pole failure identified i.e. termite damage and fungal decay have their agents within the soil where the wooden poles are erected. This corresponds with the findings of Palfreyman and Bruce (1994) and Kües et al. (2007) who indicated that the heaviest damage by brown rot fungi is observed at ground level. The soil provides a conducive environment that drives the activity of fungi such as moisture content in the first layers of the soil which keeps the wood above 30% moisture content which is optimal for brown rot decay (Palfreyman and Bruce 1994; Ribera 2017).
3.6 Relationship between causes of pole failure and hazard class of the poles.
The overall chi-square results (X2 = 31.895, p \(<0.05\)) indicated there was a significant association between the cause of pole failure and the hazard class. The highest (88%) pole failure was observed in poles located in seemingly dry areas while the least (3%) pole failure occurred on private farms (Fig. 8). However, Fig. 10 depicts the seemingly dry areas had underlying water in the ground which could have provided a conducive environment for moisture and temperature-dependent wood degrading agents. All poles in wetland areas were damaged by fungi (Fig. 9) because these wet areas contained water which provided favourable conditions in terms of moisture and temperature that support fungal activity.
The observed association between causes of pole failure and hazard class of failed poles can be attributed to variations in temperature and soil moisture content in wetlands, dryland areas and private farmlands. Nicholas (1973) and Shrivastava (1997) indicated that climatic conditions are the main factors favouring or disfavoring termites and fungi activity in all tropical and sub-tropical regions in the world. The high-water table for the seemingly dry areas indicated that there was significant moisture in the ground which could have provided a conducive environment for biodegrading agents’ activity. The fact that all the failed poles damaged by termites were found in dryland areas while all the failed poles located in wetland areas were due to fungal decay agrees with the findings of Basu et al. (1996) and, Kemabonta and Balogun (2014) who reported the relative abundance of termites to be significantly higher in dry areas compared to wet areas. Therefore, Uganda being a tropical country is generally dry and consists of a wide variety of termites and fungi. Shrivastava (1997) reports termites as the most important wood-degrading pests in all tropical and subtropical regions. Tropical regions generally experience two seasons of rainfall a year and this keeps temperatures moderate and thus provides the optimum moisture for fungal activity. On the other hand, private farmlands i.e. maize and sugar cane plantations had the least (3%) failure which can be attributed to the farm management regimes and practices such as weeding, and application of pesticides and herbicides which hindered the activity of biodegrading agents hence only vandalized poles were observed in these areas as seen in Fig. 11.
3.7 Service life of prematurely failed poles
Only 49 (21%) of the 231 failed poles that were sampled had installation tags on them showing the preservative chemical used, the date of pole installation and the manufacturer details. These were used to compute the average service life of the failed poles (Table 2). The average life span of poles installed in Uganda was found to be 10 years, however, this was limited by the lack of installation tags on some samples which could have jeopardized the analysis. Notwithstanding, this average service life is lower than the economically recommended service life of 40 years for a well-treated wooden electric pole (Muthike and Ali 2021). The districts of Kamuli and Luuka had the least life span (7 years) of failed poles compared to other districts and this is attributed to the difference in the installation years compared with other districts.
Table 2
Ages of prematurely failed poles
District | Year of installation | Number of failed poles | Age |
Kiruhura | 2012 | 5 | 10 |
2011 | 3 | 11 |
Ssembabule | 2012 | 7 | 10 |
2011 | 5 | 11 |
Iganga | 2011 | 7 | 11 |
Mbale | 2012 | 6 | 10 |
Kamuli | 2015 | 13 | 7 |
Luuka | 2015 | 3 | 7 |
Average age | | | 10 |
3.8 Important factors in premature pole failure
Two components with Eigen values greater than one (1) were extracted from the four initial factors i.e. the cause of pole failure, point of pole failure, geographic location of the failed pole and hazard class of the failed pole using the principal components extraction method.
The extracted sums of squared loadings indicated that the first component extracted explains 39.1% while the second component explains 66.2% of the total variance in the observations respectively (Table 3).
Table 3
Principal Components Analysis showing the total variance explained by each component
| | Initial Eigen Values | Extraction Sums of Squared Loadings | Rotation Sums of Squared Loadings |
Component | Total | % of Variance | Cumulative % | Total | % of Variance | Cumulative % | Total | % of Variance | Cumulative % |
| 1 | 1.563 | 39.072 | 39.072 | 1.563 | 39.072 | 39.072 | 1.515 | 37.872 | 37.872 |
2 | 1.083 | 27.078 | 66.151 | 1.083 | 27.078 | 66.151 | 1.131 | 28.278 | 66.151 |
3 | .925 | 23.126 | 89.277 | | | | | | |
4 | .429 | 10.723 | 100.000 | | | | | | |
Varimax rotation method using the 0.5 minimum factor loading, extracted two factors i.e. the cause of pole failure (0.888) and point of pole failure (0.797) into component 1, whereas, only one factor i.e. geographical location of the pole (0.923) was extracted into component 2 (Table 4). Thus, the identified factors to explain pole failure in Uganda were the cause of pole failure, point of pole failure and geographical location of the pole.
Table 4
A rotated component matrix obtained from Varimax with Kaiser normalization
| Component |
1 | 2 |
Causes of pole failure | .888 | .156 |
Point of pole failure | .797 | − .325 |
The geographical location of the failed pole | .095 | .923 |
Hazard class of the failed pole | − .289 | .385 |
The three important factors identified to explain premature pole failure in Uganda i.e. the cause of pole failure, point of pole failure and geographical location of the pole need to be dealt with during the handling of wooden utility poles to minimise their premature failure. Ryan et al. (2015) reported the most significant factor affecting the service life of wood utility poles as being decay caused by inadequate impregnation, deep cracks, wood insects, splits, and fungi. Wright (1992) studied the point of failure from a mechanical properties context and noted that loading of the pole increases its bending stress until the yielding point strain is reached causing the pole to break. Furthermore, Bhat and Meliopoulos (2016) developed a model that predicts failure among wooden poles on a distribution line by establishing the link between stress on the pole and weather parameters to predict the breakage point of the pole, it showed that geographic locations which are more prone to harsh weather were more likely to have distortions in their distribution systems caused by pole failure. Rahman et al. (2015) suggested the implementation of inspection and maintenance strategies which focus on factors leading to the decay of poles at or below ground; these are normally a result of the reduction in dimension and strength of the pole at this point thus exacerbating fungal and termite attack.