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Highlights:
Diameter of logs and distance to the truck affect productivity of manual loading system.
10 to 30 cm diameter and 30 to 60 kg logs take 12 s to carry them.
Logs smaller than 30 cm in diameter at distances greater than 10 m need 15 s to carry them.
Pieces larger than 30 cm and 60 kg, more than 10 m from the truck require double manual loading time.
The efficiency of this system can be increased by setting maximum loading distances of 10 m.
Introduction
The best logging system depends on the balance between forest characteristics, management practices, supply operation level (intensive, intermediate or mechanized) and factors that may affect labor productivity (Duncker et al., 2012; Melemez et al., 2013).
For industrial forest harvesting, machinery and equipment are usually used to achieve greater efficiency in the process; however, it is also possible to harvest timber with a high proportion of manual labor (Stańczykiewicz et al., 2021). This depends on the scale of harvesting, tree characteristics, land conditions, environmental constraints, availability of machinery, labor and extraction costs (Cataldo et al., 2020).
The use of human force for loading logs is an option for handling short-sized logs that do not exceed 2.5 m in length and 50 kg in weight, to avoid injuries or accidents to the worker (Schettino et al., 2017; Secretaría del Trabajo y Previsión Social [STyPS], 2009); furthermore, due to high dispersion and relatively small volume of harvest per unit area, the use of machinery for harvesting is not very practical and expensive (Mihelič et al., 2018; Vanbeveren et al., 2015). This activity is justified because it reduces the amount of fuels and risks of forest fires, shortens area recovery time, creates commercial opportunities and jobs; it also limits the emission of greenhouse gases and particulate matter during harvesting (Eker, 2011; Labelle & Lemmer, 2019).
Although manual systems are less productive than mechanized systems, they also imply lower impacts to the soil and remaining vegetation but increase the risks to workers' safety (Gülci & Erdaş, 2018; Maesano et al., 2013; Melemez et al., 2014). Therefore, the efficient use of labor in manual loading can have a positive impact on the productivity of the system; to prove this, it is necessary to have information on the time it takes workers to perform this activity (Grzywiński et al., 2020).
In the forest region of El Salto, Durango, it is common to find areas where the loading of short-sized logs is carried out with human power, but the productivity level of this operation is unknown. The objective of this study was to evaluate the effect of the diameter of 4 ft (1.22 m) long logs and the distances to the truck on the efficiency of manual loading, assuming that these variables have no significant influence on this activity.
Materials and Methods
Study area
The study was carried out in the natural forests of UMAFOR 1008 ‘El Salto’, southwest of the state of Durango, Mexico, in the Sierra Madre Occidental mountain system. It is dominated by pine-oak vegetation associations composed of Pinus durangensis Martínez, P. cooperi C. E. Blanco, P. engelmannii Carr., P. douglasiana Martínez, P. strobiformis Engelmann, P. lumholtzii B. L. Rob. & Fernald, Picea chihuahuana Martínez, Cupressus sp. and Quercus sp. (PRO FLORESTA S. C., 2008).
Logging in the study area is prescribed by the Silvicultural Development Method and variants of the Selection Method. Trees are felled using a chainsaw; logs are cut from 8 ft to 34 ft in length plus bracing (2.44 m to 10.36 m) with a minimum diameter of 8 in (20.32 cm) and, for secondary products, with a minimum diameter of 4 in (10.16 cm) with lengths of 4 ft to 8 ft (1.22 m to 2.44 m).
Log handling is carried out with mechanical cranes, skidders or animal traction, and log transport is done manually or mechanized (Nájera-Luna et al., 2012).
Methods
Field data were collected from the performance of eight forest workers, divided into four brigades of two people per truck, who manually carry out short pine logs; in this case, 4 ft long logs (1.22 m) on relatively flat sites with no more than 15 % slope.
Each loading cycle was divided into two movements: the first, when the worker begins walking to the log to carry it, and the second, when he bends down to lift the log off the ground or roll it to the truck where he places it on the truck platform for another worker to place the load.
Work times were timed with an accuracy of hundredths of a second using the ‘back to zero’ method, which consists of measuring the time directly in each element of the work cycle, returning the stopwatch to zero to take the time of the next element (Peralta et al., 2014). Main and complementary activities to support the work were considered as productive time and breaks and interferences during the work cycle were considered as unproductive time.
In addition to work time, in situ, distance (m) of the location of each log carried to the nearest part of the transport truck platform was recorded and the diameter dimensions of each log were taken to calculate its volume using the Smalian formula (Cruz de León & Uranga-Valencia, 2013):
where,
V = log volume (m3)
A 1 = area of the larger log section (m2)
A 2 = area of the smaller log section (m2)
L = length of the log (m).
As an approximation to know the mass of the logs carried, the volume was multiplied by the average density of wood in wet or green condition for Pinus species in Mexico, which has been reported to be 900 kg∙m-3 (FAO et al., 2020).
For determining the sample size, a pre-sampling of 100 manual loading cycles was carried out, thereafter the number of work cycles required to reach an admissible sampling error of 5 % suggested by do Nascimento-Santos et al. (2018) was determined:
where,
n = manual log loading cycles required
t = Student's t-value at 95 % probability
s = variance
E = admissible sampling error (%)
Consequently, the sample size was estimated at 571 manual log loading cycles; however, 738 work cycles distributed in three diameter classes and four loading distance intervals were measured (Table 1).
Table 1 Log distribution by diameter class and manual loading distance of 4 ft (1.22 m) logs in 738 work cycles (4.40 % sampling error).
| Diameter category (cm) | Logs per category (n) | Loading distance (m) | Logs per category (n) |
|---|---|---|---|
| 10-20 | 400 | 0-5 | 535 |
| 20.1-30 | 289 | 5.1-10 | 142 |
| >30 | 49 | 10.1-15 | 26 |
| >15 | 35 | ||
| Total | 738 | 738 |
The loading cycle efficiency (R, m3∙h-1) was estimated with the data of time (t, h) and log volume (v, m3) using the relationship R = v / t (Simões et al., 2014). The efficiency of manual loading distance in the work cycle (Edc, m3∙m-1∙h-1) was derived as a function of log volume (m3), loading distance (d, m) and total work time (TT) using the following formula (Takimoto & Yovi, 2003):
Finally, the operational efficiency (OE, %) was determined from the effective time worked regarding the total work cycle time by (Cavassin-Diniz et al., 2018):
where,
TT = total work cycle time (h)
NT = non-productive work cycle time (h).
Statistical analysis
To characterize diameter, length, volume and weight of logs, as well as the distance and duration of the work cycle stages, regarding the yield per diameter and distance category, tables with statistics of central tendency and position measures (medians with the quartiles Q1 and Q3) were elaborated. The hypothesis of normality of yield and efficiency according to distance in the work cycle was evaluated with the Kolmogorov-Smirnov test.
Significant statistical differences in yield and work cycle efficiency between diameter classes and between loading distances were determined by nonparametric analysis of variance and Kruskal-Wallis median rank comparison tests (α = 0.05) using the InfoStat software version 2018 (Di Rienzo et al., 2018).
Results
General information about the manual loading cycle
Kolmogorov-Smirnov test showed that yield and distance efficiency of the work cycle were not from a normally distributed population (P < 0.0001). According to Table 2, the yield of the manual loading cycle ranged between 3.80 and 16.42 m3∙h-1 influenced by the diameter of logs and loading distance. The yield increased as the diameter of logs increases and decreased at longer loading distance.
Load volume is mostly composed of logs of 20 to 30 cm (49 %) and 10 to 20 cm in diameter (33 %), which together represent 82 % of the log load. Each log needs 10 to 35 s to complete a work cycle, where 60 % of the productive time is spent lifting, moving and placing each log on the truck loading platform.
In terms of distance, 74 % of the load volume is found in the first 5 m from the truck, while up to a distance of 10 m, this volume reaches 91 %. For the first 10 m of the truck, the loading cycle per log takes 15 s, but at distances greater than 10 m, more than double the time is required, because the work of lifting, moving and placing a log on the truck consumes up to 70 % of the productive work time. Therefore, log collection at distances greater than 10 m negatively affects productivity, even though they only represent 9 % of the volume.
The high efficiency (100 %) detected in the work cycle also is remarkable, which indicates that interruptions due to unproductive time were irrelevant in this operation, probably because it is based on piecework and not on a base salary.
It is important to know the influence that the distance factor has on the volume that can be carried in an hour of work for each meter of distance that each piece is located with respect to the truck. In this regard, the results indicate yields in the order of 2.48 m3∙m-1∙h-1 with logs 1.2 m long and 10 to 20 cm in diameter, and 5.28 m3∙m-1∙h-1 with logs larger than 20 cm. By loading distance, this indicator corresponds to 5.28 m3∙m-1∙h-1 for logs within the first 5 m with respect to the truck and decreases considerably to 0.17 m3∙m-1∙h-1 with logs that are carried more than 15 m away.
Table 2 Characteristics of the manual loading cycle for 4 ft long logs per diameter classes and loading distance.
| Log load variables | Log diameter category (cm)* | |||
|---|---|---|---|---|
| 10.0-20.0 | 20.1-30.0 | >30 | ||
| Length of logs (m) | 1.2 | 1.2 | 1.2 | |
| Diameter of logs (m) | 0.18 (0.15-0.20) | 0.25 (0.23-0.27) | 0.35 (0.33-0.38) | |
| Volume of logs (m3) | 0.03 (0.02-0.03) | 0.06 (0.05-0.07) | 0.12 (0.10-0.14) | |
| Total loaded volume (m3) | 11.27 | 16.92 | 5.99 | |
| Approximate weight of logs (kg) | 29.40 (21.60-36.50) | 59.90 (49.10-67.30) | 117.30 (101.70-135.30) | |
| Load distance (m) | 3.00 (2.0-6.0) | 3.00 (2.0-6.0) | 2.50 (2.0-5.0) | |
| Time reaching the log (s) | 4 (2-6) | 3 (2-6) | 4 (2-5) | |
| Log lifting and loading time (s) | 6 (3-10) | 8 (5-13) | 17 (10-26) | |
| Total working cycle time (s) | 11 (7-18) | 12 (8-20) | 27 (19-43) | |
| Maximum working cycle time (s) | 375 | 219 | 329 | |
| Minimum working cycle time (s) | 1 | 4 | 6 | |
| Manual load efficiency (m3∙h-1) | 8.66 (5.46-14.18) | 16.42 (10.10-26.03) | 15.99 (9.73-23.00) | |
| Load distance efficiency (m3∙m-1∙h-1) | 2.49 (0.96-5.28) | 5.28 (2.04-9.59) | 4.25 (1.16-11.97) | |
| Operational efficiency of manual cycle (%) | 100 | 100 | 100 | |
| Work cycles (n) | 400 | 289 | 49 | |
| By type load distance (m)* | ||||
| 0.0-5.0 | 5.1-10.0 | 10.1-15.0 | >15 | |
| Length of logs(m) | 1.2 | 1.2 | 1.2 | 1.2 |
| Diameter of logs (m) | 0.20 (0.18-0.25) | 0.20 (0.17-0.23) | 0.18 (0.16-0.24) | 0.21 (0.16-0.29) |
| Volume of logs (m3) | 0.04 (0.02-0.06) | 0.37 (0.02-0.04) | 0.031 (0.02-0.05) | 0.041 (0.02-0.08) |
| Total loaded volume (m3) | 25.26 | 5.8 | 0.99 | 2.15 |
| Weight of logs (kg) | 38.30 (29.40-59.90) | 36.50 (27.7-48.8) | 31.10 (24.60-55.30) | 40.90 (24.90-80.60) |
| Load distance (m) | 3.00 (2.00-4.00) | 7.00 (6.0-9.0) | 12.00 (12.00-13.00) | 30.00 (30.00-30.00) |
| Time reaching the log (s) | 3 (2-4) | 6 (5-8) | 12 (9-13) | 7 (5-10) |
| Log lifting and loading time (s) | 6 (4-10) | 9 (6-13) | 15 (13-19) | 25 (15-36) |
| Total working cycle time (s) | 10 (7-16) | 15 (11-21) | 27 (22-33) | 35 (25-50) |
| Maximum working cycle time (s) | 375 | 200 | 203 | 119 |
| Minimum working cycle time (s) | 1 | 3 | 18 | 24 |
| Manual load efficiency (m3∙h-1) | 14.36 (8.48-21.93) | 8.66 (5.65-11.75) | 3.80 (2.89-6.58) | 4.63 (2.39-9.67) |
| Load distance efficiency (m3∙m-1∙h-1) | 5.28 (2.76-9.51) | 1.06 (0.71-1.74) | 0.32 (0.23-0.57) | 0.17 (0.11-0.32) |
| Operational efficiency of manual cycle (%) | 100 | 100 | 100 | 100 |
| Work cycles (n) | 535 | 142 | 26 | 35 |
* Medians and in parentheses values of Quartiles Q1 and Q3.
Manual loading efficiency between diameter and distance categories
Yield comparison showed significant differences (P < 0.05) between diameter classes and loading distances (Table 3). According to diameter, load yield of logs larger than 20 cm is 47 % higher than that of logs between 10 and 20 cm. In terms of load distance, from 5 to 10 m from the truck, the yield decreases 40 % compared to the first 5 m from the truck and 70 % at distances greater than 10 m.
Table 3. Yield of manual load of 4 ft (1.22 m) logs between diameter categories and load distances.
| Classes | Median (m3∙h-1) | Average of ranges | Gl | C | H | P value |
|---|---|---|---|---|---|---|
| Among diameter of logs loaded (cm) | ||||||
| 10-20 | 8.66 | 291.37 a | 2 | 1 | 17.49 | <0.0001 |
| 20.1 - 30 | 16.42 | 464.02 b | ||||
| >30 | 15.99 | 449.86 b | ||||
| Among distances of logs loaded (m) | ||||||
| 0 - 5 | 14.36 | 418.48 c | 3 | 1 | 119.92 | <0.0001 |
| 5.1 - 10 | 8.66 | 275.96 b | ||||
| 10.1 - 15 | 3.8 | 124.35 a | ||||
| >15 | 4.63 | 182.39 a | ||||
Df: degrees of freedom; C: correction factor of the statistic for tied observations; H: test statistic not corrected for ties. Mean median ranks with the same letter are not significantly different according to the Kruskal-Wallis test (P = 0.05).
Efficiency of load distance between diameter and distance categories
Table 4 shows that the efficiency of distance on loading yield had significant differences (P < 0.05) between diameter and distance classes, so the volume loaded per meter of distance and hour is 71 % higher in logs larger than 20 cm compared to logs from 10 to 20 cm in diameter. Regarding the loading distance, the volume loaded per meter of distance and hour decreases 80 % in the first 5 to 10 m of distance from the truck and 97 % over 10 m.
The efficiency of manual loading is due to the variation in the dimensions of logs located at all the distances evaluated, so it is possible to increase it by decreasing the loading distances, even if this means increasing the loading cycle time for the displacement and positioning of the truck.
Table 4. Distance efficiency in manual loading yield of 4 ft (1.22 m) logs between diameter categories and loading distances.
| Clasess | Median (m3∙m-1∙h-1) | Average of ranges | Df | C | H | P |
|---|---|---|---|---|---|---|
| Between diameter of logs loaded (cm) (cm) | ||||||
| 10-20 | 2.485 | 317.69 a | 2 | 1 | 51.76 | <0.0001 |
| 20.1 - 30 | 5.28 | 422.83 b | ||||
| >30 | 4.25 | 418.95 b | ||||
| Between load distances of logs (m) | ||||||
| 0 - 5 | 5.28 | 454.89 c | 3 | 1 | 330 | <0.0001 |
| 5.1 - 10 | 1.06 | 185.74 b | ||||
| 10.1 - 15 | 0.325 | 64.23 a | ||||
| >15 | 0.17 | 36.59 a | ||||
Gl: degrees of freedom; C: correction factor of the statistic for tied observations; H: test statistic not corrected for ties. Mean median ranks with the same letter are not significantly different according to the Kruskal-Wallis test (P = 0.05).
Discussion
Gülci and Erdaş (2018) reported a yield of 3.40 m3∙h-1 in manual loading at the foot of the gap for Pinus brutia Ten logs in Turkey with 0.06 m3 per cycle in 64 s, where the most time-consuming work stage (46 %) was stacking on the truck platform. The manual loading performance of these authors is consistent with that found in this study for a distance of 12.0 m with 3.8 m3∙h-1 (Table 3), but the volume of the loaded log (0.03 m3) and the duration of the work cycle (37 seconds) differ. The above indicates that, for each loading cycle at the foot of the gap in the forests of Turkey, it is possible to load up to 2.4 logs of half the volume of the reference study and at a distance of 12 m from the truck in El Salto, Durango. It is important to emphasize that Gülci and Erdaş (2018) mention that the logs are loaded by two people and placed in the truck by two other workers, but in this study, the loading is usually carried out by one worker and another one places the load in the truck. This situation requires adjusting the techniques and work methods for manual loading according to size, volume, weight and loading distance of the logs, as well as the maneuverability restrictions for people and equipment imposed by the selective cuts. This type of cutting is characteristic of the study area, where it has been proven that productivity is affected by longer loading distances needed to complete each work cycle.
Working conditions may be acceptable as long as a harvest volume is available to cover the costs of a fragmented collection (Spinelli et al., 2017). In another study, Silayo et al. (2010) reported a manual loading yield of 3.80 to 23.30 m3∙h-1 for logs of 3.9 to 5.1 m long (average of 4.5 m) from plantations of Pinus patula Schltdl. & Cham. of 26 years in Tanzania. This yield is also similar to that found in this study (3.80 a 16.42 m3∙h-1; Table 2), despite the 3.3 m difference in log length between the two studies, although in Tanzania it is reported that the loading was at the foot of the gap carried by a crew of six workers, due to the size and weight of the logs.
Weight also increases with the increase in length and diameter of the logs, which makes their handling more difficult, so there is a need to evaluate the alternative of partial or total mechanization of loading operation (Spinelli et al., 2017). In this study, despite one person usually loads the logs, when it comes to bulky and heavier logs, at least two workers are required. Therefore, in manual loading, the size of the logs should be small enough to be maneuvered without major effort by the workers, but at the same time, their dimensions should be acceptable for the market (Gülci & Erdaş, 2018). Larger sizes involve overexertion of workers and unhealthy body postures that can affect the productivity of manual loading and could lead to an accident or physical and emotional exhaustion (Quintana et al., 2022).
In this study, the approximate weight of the logs ranged from 29 to 117 kg (Table 2), 30 % of which exceeded 50 kg and were sometimes carried by a single person, even though logs above this weight must be carried by at least two workers as stipulated by the STyPS (2009).
Despite this and as noted in this paper, forestry workers are exposed to carrying weights above tolerable limits, even when the maximum acceptable for optimal handling is 25 kg per person (Barbosa et al., 2014; Fiedler, et al., 2015; Minette et al., 2018). This load limit is based on the US National Institute for Occupational Safety and Health (NIOSH) lifting equation, which states that at least 90 % of adult persons can lift 25 kg per person without adverse health effects (Waters et al., 2021). In this study, 75 % of the loaded logs weighed more than 25 kg and were usually handled by a single worker, which implies greater exposure to overexertion and physical wear and tear.
Finally, to reduce health risks, some authors recommend rolling the logs closer and placing the truck near a slope so that gravity facilitates handling and thus reduces the physical effort of the worker (Figueredo-Fernández et al., 2020). However, this alternative is limited to harvesting areas with adequate slopes and no obstacles to move the logs downhill with the help of gravity (Kaakkurivaara & Kaakkurivaara, 2018) and not for conditions in relatively flat terrain with obstacles as was the case in this study.
Conclusions
The yield of manual loading of logs varies depending on diameter and loading distance. Logs larger than 20 cm in diameter achieve 53 % more yield than those between 10 and 20 cm. In the first 10 m distance from the truck, 91 % of the loading volume is found and 15 seconds are required to load each log. At distances greater than 10 m, the yield decreases up to 70 %, since twice as much time is required to complete a work cycle; in addition, the worker's physical effort is greater and there is a risk of bodily injury, especially when lifting logs weighing more than 25 kg. Based on the results, it is recommended, as far as possible, to bring the trucks as close as possible to the logs to be loaded at a distance of 5 to 10 m, thus increasing efficiency.
