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8. STUDY METHODOLOGY AND ANALYSIS OF DATA

The efficiency of road construction either by hydraulic excavator or bulldozer as applied in the road projects under review as well as of long-distance cable logging as applied at the two study sites under review was investigated with work and time studies analysing the work performance of the operations.

Work and time studies were done in accordance with the forest work-study nomenclature (IUFRO, 1995). The study methodology used for all time studies conducted during road construction and cable-logging operations was cumulative timing exclusively, with the time for each work element subsequently obtained by subtraction.

In road construction either by excavator or bulldozer the equipment operator was observed by work and time studies. The long-distance cable crane activities conducted by the winch operator were the subject of observation. Information on workers assisting on construction and logging activities was collected to support the analysis of time consumption and the interpretation of time distribution of work elements.

8.1. Work and time studies on forest road construction

Road construction by excavator or bulldozer comprises a set of activities undertaken to provide the designed road. (See Tables 14 and 15.) Depending on terrain conditions, type of equipment used and the operators' way of arranging the distinct phases of road construction, these activities vary with respect to their frequency of occurrence.

Nevertheless, at each construction site a sequence of regular work elements can be found for the construction operation either by excavator or bulldozer that constitutes the work cycle. Work cycle is defined as a sequence of work repeatedly applied to every work object (IUFRO, 1995). A work element is considered a sub-division of a given work task and is limited by break points. Depending on the occurrence in every work cycle, a work element can be considered as a repetitive or an occasional element.

Only workplace time, which is defined as the portion of the total time that a production system or part of a production system is engaged in a specific work task (IUFRO, 1995), has been considered in estimating production rates and costs at the study sites.

Figure 6 shows the structure of work time elements that occurred during time studies on road construction either by excavator or by bulldozer.

Figure 6. Structure of workplace time (WP) concepts

Figure 6

The classification and the percentage of workplace time consumption observed for each work element in construction operations by excavator at the study site can be found in Appendix 2, for the bulldozer operations in Appendix 3. General information on construction sites where time studies were carried out on operations by excavator is stated below. The experienced excavator operator trained two new operators.

Subject of observationSection 1
Trainer)
Section 2
(Trainee 1)
Section 3
(Trainee 2)
Kharungla road
Work time12 h 45 min12 h 43 min7 h 43 min33 h 11 min
Non-work time3 h 12 min2 h 15 min1 h 12 min6 h 39 min
Workplace time15 h 57 min14 h 58 min8 h 55 min39 h 50 min
 
Side slope75–80%70–75%65–70%70–75%
Subgrade width5.0 m5.0 m5.3 m5.1 m
Length of section79m82m62 m223 m

On average the workplace time per metre of road constructed by excavator was 10.72 min for the Kharungla road section. The corresponding figure for the trainer was 12.11 min, 10.95 min for trainee 1 and 8.63 min for trainee 2. The time required per metre of road serves as the basis for estimating production rates treated in Chapter 8.2.

At the study site of the Kharungla road project, the excavator operator who trained the Bhutanese was found to accomplish construction work by following the stated order of the five distinct phases (see Chapter 5.2), whereas the trainees chopped and changed between activities of distinct phases. The way single activities had been arranged by the trainees was mainly determined by lack of experience and routine rather than by following their personal optimum sequence of construction activities and resulted in high time shares for fill slope construction. However, the careful performance of work resulted in a lower productivity for the trainer.

However, in order to compare and analyse the operators' construction work, the average productive work time required per metre in length of road constructed by excavator and time distribution of work elements assigned to the productive work time are stated in Table 14 for each excavator operator.

Table 14. Distribution of productive work time elements in excavator construction

Work elements(classification)TrainerTrainee 1Trainee 2Kharungla
Log removal(MW)2.23%3.55%1.17%2.48%
Topsoil removal(MW)28.52%28.99%26.14%28.14%
Excavating base(MW)------------
Fill slope construction(MW)29.57%42.58%53.83%40.23%
Subgrade/cut shaping(MW)39.21%24.48%18.86%28.83%
Complementary work(CW)0.46%0.40%---0.33%
Productive work time(PW)100.00%100.00%100.00%100.00%
(PW)9.45 min8.95 min7.39 min8.69 min

On average the productive work time required per metre of road constructed by excavator was 9.45 min for the trainer, the respective time for the trainees was 8.95 and 7.39 min. On average the corresponding figure amounted to 8.69 min for the Kharungla road section. The complementary work covers the time for movement from and to the construction area within the sections under review.

The figures on time distribution stated in Table 14 clearly indicate that a distinct work element “excavating base for fill foundation” did not occur at the study site under review. This was not a result of poor construction technique but was caused by the fact that at the toe of the anticipated fill slope a preliminary road became necessary due to soil conditions characterized by a lack of cohesive materials and boulders required to build up a solid foundation for the fill.

Turning to the Korila extension project, general information on construction sites where time studies were carried out on operations by bulldozer are stated below for the Korila extension section.

Subject of observationSection 1Section 2Section 3Section 4Korila extension
(same operator in all sections) 
Work time9 h 43 min6 h 17 min6 h 21 min2 h 35 min24 h 57 min
Non-work time59 min51 min52 min11 min2 h 53 min
Workplace time10 h 42 min7 h 08 min7 h 13 min2 h 46 min27 h 50 min
 
Side slope50–55%60–65%50–55%40–45%50–55%
Subgrade width4.6 m3.9 m4.6 m4.8 m4.5 m
Length of section150 m92 m75 m62 m379 m

On average the workplace time per metre in length of road constructed by bulldozer was 4.41 min for the Korila extension section. The corresponding figure for the distinct sections of the study site reflecting different terrain conditions varies considerably, ranging from 2.68 min for section four with extreme favourable conditions to 5.77 min for section three. The time required per metre of road either of workplace time or of work time serves as the basis for estimating production rates, treated in Chapter 8.2.

In order to compare and analyse the operator's construction work in the four sections characterized by different conditions for road construction, the average productive work time required per metre of road constructed by bulldozer and time distribution of work elements assigned to the productive work time are stated in Table 15 for each section.

Table 15. Distribution of productive work time elements in bulldozer construction

Work elements(classification)Section 1Section 2Section 3Section 4Korila
Log removal(MW)---------------
Stump removal(MW)5.89%3.80%3.39%1.04%4.20%
Cut slope(MW)49.03%69.71%62.82%87.18%61.88%
Subgrade construction(MW)44.10%22.34%28.61%11.78%31.17%
Uprooting trees(MW)---1.68%------0.42%
Complementary work(CW)0.97%2.47%5.18%---2.33%
Productive work time(PW)100.00%100.00%100.00%100.00%100.00%
(PW)3.36 min3.56 min4.57 min2.29 min3.48 min

On average the productive work time required per metre in length of road constructed by bulldozer was 3.48 min for the Korila extension section, ranging from 2.29 min for section four to 4.57 min for section three. The figures on time distribution stated in Table 15 indicate that in road section four with most favourable conditions for road construction the subgrade can almost be established just by cutting the slope. The absence of major stumps and boulders to be removed resulted in less time needed for finishing the subgrade.

The complementary work covers the time for movement from and to the construction area within the sections under review as well as the time required for changing the blade angle as the bulldozer turns for finishing the coarse subgrade constructed in previous work cycles. It also includes a negligible time-share for checking the road gradient of constructed sections by the operator.

The figures on time distribution stated in Table 15 also indicate that a distinct work element “log removal” did not occur at the study site of the Korila extension project. This was because all logs had already been removed manually from the construction area before construction work by bulldozer commenced.

In contrast to the Kharungla road section where the productive time required per metre of road constructed increased with steeper side slopes, this pronounced relationship was overlaid with heterogeneous soil conditions in the Korila extension section.

8.2. Estimating production rates in road construction

Since productivity is defined as the rate of product output per time unit for a given production system, the production rates of a studied system can easily be estimated if time studies combined with measurements of the output of production have been completed.

The estimated production rates for excavator and bulldozer operations stated in Tables 16, 17 and 18 are based either on the total workplace time or total work time used to perform a certain length of road by the equipment operator at the road construction study sites.

All other construction activities where an excavator or bulldozer might be employed (culvert, shaping after surfacing, retaining structures, etc.) are not included in the estimates of production rates stated in Tables 16, 17 or 18.

Table 16. Estimated production rates in road construction by excavator

Kharungla roadSide slopeSubgrade widthProduction rateProduction rate
(%)(m)(m/h WP)(m/h WT)
Section 175–805.04.956.19
Section 270–755.05.486.45
Section 365–705.36.958.03
Study site70–755.15.606.72

Note: WP=workplace time
WT=work time

Based on the hourly productivity found in the studies and on an assumed workplace time of nine hours per day including a one hour lunch break, the productivity rates for excavator construction range from 45 to 63 m of road per day. The first figure can be considered indicative of steeper side slopes (75–80 percent) and the latter of easier conditions (side slopes 65–70 percent) in mountainous terrain. It is noteworthy that the actual workplace time observed during the studies often exceeded the regular workplace time for employees of nine hours per day as the operators tended to make use of favourable weather conditions for construction work.

Table 17. Estimated production rates in road construction by bulldozer

Korila extensionSide slopeSubgrade widthProduction rateProduction rate
(%)(m)(m/h WP)(m/h WT)
Section 150–554.614.0115.43
Section 260–653.912.8914.64
Section 350–554.610.3711.80
Section 440–454.822.3723.95
Study site50–554.513.6115.19

Note: WP = workplace time
WT = work time

Based on the hourly productivity found in the studies and on an assumed workplace time of nine hours per day including a one-hour lunch break, the productivity rates for bulldozer construction range from 93 to 201 m of road per day. The first figure is indicative of less favourable site conditions (boulders to be removed, etc.) but also steeper side slopes and the latter of most favourable conditions. It is noteworthy that, as in excavator construction, the actual workplace time observed often exceeded the regular workplace time for employees of nine hours per day for the same reason.

Considering the unexpected high production rate in steeper terrain of section two, one has to keep in mind that this rate was achieved by reducing the subgrade width to 3.9 m on average in order to avoid rock work. To construct the required subgrade width of 5 m would have made the removal of rock boulders necessary and slowed the work progress dramatically.

Photo 29

Photo 29. At the Korila section, a major rock outcrop was demolished manually ahead of construction work as it could not have been accomplished by bulldozer

Furthermore, it has to be noted that a major rock outcrop was demolished manually ahead of construction work as it could not have been accomplished by bulldozer. In contrast, the rippable outcrop could have been removed by excavator and the material incorporated in the road structure. Six labourers equipped with iron bars were engaged 30 hours to break up the rock outcrop of about 30 m3 into pieces. This effort can only be afforded as long as labour costs are low.

Table 18. Comparison of estimated production rates in road construction

 Side slopeSubgrade widthProduction rateProduction rate
Study site(%)(m)(m/h WP)(m/h WT)
Kharungla road70–755.15.606.72
Korila extension50–554.513.6115.19

Note: WP = workplace time
WT = work time

The figures stated in Table 18 on average production rates found in the respective road sections for both road projects may easily mislead one to give preference to bulldozer construction where on average production rates were more than twice as high as in excavator construction. Considering the following facts will help to evaluate the different construction techniques more clearly.

First, one has to keep in mind that the average side slope gradients of the two construction sites differ considerably and under similar soil conditions the production rates decrease as side slopes increase. The steeper the slopes the more rock fragmentation excavation will be required since rock outcrops cannot be bypassed or rock surfaces avoided.

Second, and probably more important, is the fact that the desired road standard was not achieved in the Korila extension project. To construct the required subgrade width of 5.0 m following the marked grade line would have increased the earthwork, increased the need to uproot stumps and to remove boulders. These are well-known as time-consuming activities.

In addition to the technical aspects, the environmental damage caused by bulldozer construction which is sometimes on a substantial scale have not yet been taken into consideration (see Chapter 8.4).

The figures found in the literature on production rates for hydraulic excavators and for bulldozers as in Gorton (1985) and FAO (1989) are difficult to compare with those stated in Table 18. One main reason is that the activities performed either by excavator or bulldozer are not defined in these papers.

If the figures do not cover the first phase of excavator construction, namely, log removal from construction area, this would explain to a high degree the difference between the higher figures found in both papers and the lower rates derived from the data collected in the time studies at the Kharungla site. Another reason is the lack of information on the time basis of the production rates stated in the literature mentioned above. However, the performance rates found for the excavator in this study fit quite well with results of a similar study carried out in mountainous terrain of the Alps in Austria (FAO, 1998).

Although in principle the same problem mentioned above exists in comparing the figures on bulldozer production rates, the performance found in this study at the Korila site is within the range of production rates given in the above-mentioned papers. For side slopes between 40 and 60 percent, Gorton (1985) gives a production rate of 12.9 m/h for a subgrade width to be established at 5.0 m, and in FAO (1989) rates between 8.0 to 14 m are stated for subgrade width from 6.0 to 7.0 m.

8.3. Estimating costs of road construction

The estimation of production costs is based on the production rates stated in Chapter 8.2 for construction operations and the hourly costs for workforce and equipment involved in the construction operation at the study sites. For the Kharungla road the hourly costs for workforce and equipment are based upon information obtained from FDC, for the Korila extension information was provided by the road contractor.

Table 19 shows the costs for equipment and workforce as well as costs for each activity carried out and observed by the work and time studies. As conditions for construction work may vary considerably along the road course, the construction costs stated in Table 19 refer to the particular road section under review for each road project only.

Table 19. Estimated costs of road construction

  Excavator - study siteBulldozer - study site
Production costs factorUnitCost/unit
(US$)
Cost
(US$/m)
Cost/unit
(US$)
Cost
(US$/m)
Road construction
Construction equipment
(h)51.669.2336.082.65
2 labourers
(h)  0.280.05  0.450.03
Cut slope improvement
22 labourers
(h)   n.a.      n.a.17.550.06
Subgrade shaping
6 labourers (30 hours)
(h)  n.a.      n.a.36.080.36
Rock disintegration
6 labourers (30 hours)
(h)  n.a.     n.a.40.382.97
Total construction cost  9.28 6.07

Note: n.a. = not applicable
Cost of construction equipment includes the cost for the operator and one handyman

The cost estimates stated in Table 19 refer to the road in the stage of finished subgrade as described in Chapter 5.2 and do not include any further construction activity which might be performed by excavator or bulldozer at a later stage such as install culverts, build retaining structures or final shape the road after surfacing.

Furthermore, the cost calculation for the excavator is based on the purchase price of this particular excavator used at the Kharungla construction site. This excavator is the most recent model manufactured, whereas the bulldozer is an older one. This has to be kept in mind when comparing the figures on cost per metre of road established by the distinct construction techniques. The short-term economic advantage (about 35 percent) of bulldozer construction might mislead one to favour the bulldozer.

8.4. Qualitative assessments of environmental impacts

Only enhanced forest road construction practices, both environmentally friendly and economically feasible, will meet the challenges and requirements of road construction in difficult terrain. The features and advantages of the hydraulic excavator itself and its use in road construction can be described as follows (FAO, 1998):

Although different types of buckets and attachments are not yet available in Bhutan, the skilled and experienced operator is provided with an effective tool to minimize environmental impacts of road construction activities when making use of the advantages of the hydraulic excavator.

Focusing on environmental impacts of road construction practices, the advantages of the environmentally friendly construction technique by means of hydraulic excavator are obvious.

Comparison of four distinct phases of environmentally sound road construction by excavator and by bulldozer

Excavator Bulldozer
i) Area dedicated to forest roads  
Subgrade width is kept to the absolute minimum as additional road width for manoeuvring is not needed. Depending on type of bulldozer used in a particular road project, additional width for manoeuvring might be required.
Overall construction width is reduced as the establishment of a proper fill foundation to build upon reduces the length of fill slope significantly. Dength of fill slope is determined by the angle of repose of side cast material and will increase significantly in sleep terrain; on side slopes over 75 percent a fill cannot be established at all.
Overall construction width is reduced by balanced road construction as full benched road sections can be restricted to sleep slopes where rock surfaces are to be crossed. Full bench construction with side cast material is the most common bulldozer technique with the potential of making large areas unproductive and inflicting scars on the landscape.
ii) Disturbance of landscape  
Clearing width is reduced as a result of minimized overall construction width and consequently, canopy disturbance and related impacts are reduced. Depending on the stands alongside road the canopy disturbance is often increased by trees uprooted by excess material side cast downhill (Photo 17).
Cut heights are reduced in balanced road sections compared to full bench construction sites. Full bench construction results in higher cuts with large surfaces exposed to erosion which render blending them into the landscape difficult.
Less rock fragmentation is needed and less material is to be excavated where roads are designed to balance cuts and fills. Full bench construction results in 25–35 percent more excavated material compared to balanced road design (FAO, 1989).
Rock broken out by means of the excavator's bucket provides material suitable in size to be incorporated in the road structure and reduces the need for borrow pits alongside the road corridor. If rock fragmentation can be carried out manually ahead of the construction operation, blasting can be avoided; otherwise blast-related damages can be expected along with side cast material.
iii) Water drainage and erosion control  
Drainage facilities, in particular the hillside ditch, can easily be established during the ongoing road construction process at any time required. Culverts and retaining structures will be established after completion of bulldozer work and therefore might not be in place when needed.
Most of the excavated material is very likely unavailable for erosion as it is incorporated in the road structure and properly compacted. Side cast and wasting practice has been responsible for the highest erosion rates in road construction (FAO, 1989) with the potential of making areas alongside road unproductive (Photo 5).
Cut and fill slopes are final shaped and compacted by means of the excavator's bucket to prevent erosion. Steep cuts unfavourable for revegetation and uncompacted fills are highly exposed to erosion.
Soil exposed to erosion is minimized as branches removed from the construction area are spread on the fill slopes. No measures which would prevent erosion are inherent features of road construction by bulldozer.
Retaining walls for slope stabilization can be established during road construction at any time required. Retaining structures have to be constructed manually after road construction.
iv) Damage to stands alongside road  
Suitable material is incorporated into the road structure, other excavation is deposited with accuracy and care between the construction limit and the established filter windrow. Loose, unconsolidated side cast material is exposed to erosion since incorporation of excavation in the road structure is limited to road sections where draws are to be crossed.
The placement of materials can excellently be controlled by the skilled excavator operator and loose material which might escape during construction activities will be trapped by the filter windrow. Unsatisfactory control in placement of the excavation results in side cast material causing damage to the forest stand downhill of the road.
The high ripping capability and breakout force of the excavator results in less need for blasting where hurled stones or escaping rocks are likely to cause damage. Where rock breakage can no longer be carried out manually and blasting is specified, blast-related damages on the forest stand and also on the rock formation can be expected.

Photo 30

Photo 30. Steep cuts unfavourable for revegetation and uncompacted fills highly exposed to erosion are often features of bulldozer construction

Focusing on environmental impacts of road construction in both road projects, the superiority of excavator technique in difficult and/or steep terrain over bulldozer construction techniques becomes obvious although the short-term economic benefits of bulldozer might favour their use.

Apart from the above-mentioned environmental advantages, economic components are important aspects of consideration in the environmentally friendly road construction practice as side slope increases and rock fragmentation is involved. As indicated in the literature by FAO (1989) and Gorton (1985), the economic advantage of road construction by bulldozer is most likely to disappear on slopes of about 50 to 60 percent and above.

The obvious advantage of balanced road design is that loose, unconsolidated wasted material is minimized compared to traditional road construction techniques by bulldozer. To build up the fill foundation and the fill itself, requires highly skilled and experienced operators, since fill failures, due to improper construction techniques, may not only destroy parts of the roads but may also trigger landslides in steep terrain making large areas unproductive and inflicting scars on the landscape.

Furthermore, the establishment of a fill foundation reduces the length of fill slopes considerably and makes road construction in steep terrain feasible. This feature is of great importance when side slopes exceed 40 percent as balanced road sections can be achieved with considerably less excavation (FAO, 1989). In contrast, applying traditional side cast techniques by bulldozer the length of the fill slope would be about 3.5 m, 12 m and 22 m for slope gradients of 45 percent, 60 percent and 70 percent assuming an angle of repose for side cast material of about 37° and on slopes of over 75 percent a fill cannot be established at all (Gorton, 1985).

8.5. Work and time studies on logging operations

Timber extraction by long-distance cable crane comprises a set of activities (see Table 20) undertaken to transport timber from the felling site to the landing. In contrast to other harvesting operations, these activities vary little with respect to their frequency of occurrence.

The sequence of regular work elements for long-distance cable operations at each logging site constitutes the work cycle. Definitions of work cycle, work element and workplace time can be found in Chapter 8.1, general information on the study methodology in Chapter 8.

Only workplace time has been considered in estimating production rates and costs at the study sites. Figure 7 shows the structure of work time elements that occurred during time studies on long-distance cable logging.

Figure 7. Structure of Workplace Time (WP) concepts

Figure 7

On average the workplace time per cubic metre of timber extracted by long-distance cable crane was 15.48 min at Korila study site, where only heavy logs of broad-leaved species were logged. The corresponding figure for Helela study site was 11.98 min where only coniferous logs were extracted. The time required per cubic metre of timber extracted either of workplace time or of work time serves as the basis for estimating production rates, treated in Chapter 8.6.

Subject of observationKorila logging siteHelela logging site
Work time9 h 15 min9 h 08 min
Non-work time0 h 02 min0 h 00 min
Workplace time9 h 17 min9 h 08 min
 
Average distance333.8 m418.2 m
Volume logged35.98 m345.76 m3

In order to compare extraction works at the two logging sites, the average productive work time required per cubic metre and time distribution of work elements are stated in Table 20.

Table 20. Distribution of productive work time elements

Work elements(classification)Korila logging siteHelela logging site
(min)(%)(min)(%)
Carriage move - empty(MW)1.278.251.6515.70
Attaching load(MW)7.9551.854.9046.56
Carriage move - loaded(MW)3.0519.911.2611.95
Unloading at landing site(MW)3.0719.992.7125.79
Productive work time(PW)15.34100.010.52100.0

The average productive work time required per cubic metre of timber extracted by long-distance cable crane was 15.34 min at the Korila site. The corresponding figure for the Helela site was only 10.52 min although the average yarding distance was about 80 m longer than at the Korila site.

The figures on time distribution stated in Table 20 reflect the different conditions in cable logging at the two study sites. In downhill logging, as carried out at the Helela site, the empty carriage is pulled uphill and the loaded carriage moves downhill to the landing by gravity braked by the winch's air wing brake. In uphill logging it is the opposite, which results in the low time share for the work element “carriage move-empty” and a high time share for “carriage move-loaded” as one can see from the figures stated in Table 20.

8.6. Estimating production rates in cable logging

The production rates estimated by time studies combined with measurements of the output (for details see Chapter 8.2) are stated in Table 21. At the Korila site the non-work time was negligible (see Appendix 4) and at the Helela site a non-work time did not occur so that the performance stated in Table 21 would not have changed if based on the work time.

Table 21. Estimated production rates in long-distance cable logging

 Side slopeLogging distanceProduction rate
Study site(%)(m)(m3/h WP)
Korila site42333.83.88
Helela site50418.25.01

Note: WP = workplace time

The higher productivity at the Helela site can easily be explained by the very short lateral extraction distances along the cableway between the clear-felled patches where lateral pulling distances reach their maximum of 30 m as in the traditional technique at the Korila site.

Based on the hourly work time performance found in the studies and on an assumed workplace time of nine hours per day including a one-hour lunch break. The production rates range from 31 m3 to 40 m3 of timber volume per day, where the first figure can be considered indicative of uphill logging keeping in mind the difference in logging distances. It is noteworthy that the figures given for the daily production rate of logging by cable crane includes an hour lunch break which actually did not occur at both logging sites when carrying out the studies.

8.7. Estimating costs of long-distance cable logging

The estimation of production costs is based on the production rates stated in Chapter 8.6 and the hourly costs for workforce and equipment involved at the logging sites. The hourly costs for workforce and equipment are based upon information obtained from FDC.

Table 22 shows the costs for equipment and stand by workforce as well as costs for each activity carried out and observed by the work and time studies.

Table 22. Estimated costs of cable logging

  Korila logging siteHelela logging site
Production costs factorUnitCost/unit (US$)Cost (US$/m)Cost/unit (US$)Cost (US$/m)
Long-distance cable     
Cable crane unit
(h)92.4023.8192.4018.44
1 supervisor
(h)  0.47  0.12  0.47  0.09
5 labourers
(h)  n.a.  1.60  n.a.  1.60
Total logging cost  25.54 20.13

Note: n.a. = not applicable

The cost estimates stated in Table 22 refer to the operating cost per hour of the cable crane assuming a depreciation time of four years, a life time of 4 000 hours for the winch and 1 000 hours for the hauling cable, a rate of 60 percent for repair and a 15 percent interest rate per year as suggested in the literature for this type of machinery. The estimates do not include the labour costs for setting up and taking down the cable crane as they vary considerably according to the number of supports needed at a particular logging site.

Furthermore, one has to keep in mind that a case study limited to one day of observation is just a snapshot and does not necessarily represent the production rates which can be achieved on average (for the whole logging job).

8.8. Qualitative assessments of environmental impacts in cable logging

Traditionally, long-distance cable logging has been associated with clear felling in Bhutan. This practice follows the basic rule “the higher the total volume of timber to be extracted per crane set-up the lower the cost per cubic metre of timber transported to the landing”. As a rule of thumb for developed countries, one cubic metre of timber to be transported is required per metre of skyline established in each crane set-up under difficult terrain conditions for economical, viable timber extraction by long-distance cable crane (Trzesniowski, 1985), e.g. 1 000 m3 of timber should be logged in a crane set-up with 1 000 m length of skyline.

Considering 1.2 m3/m of skyline at the Korila logging site where the traditional strip-wise clear-felling technique has been applied, and much lower labour cost compared to developed countries, there is still a margin to modify this practice as done at the Helela site.

However, it would not be realistic to harvest less than about 50 percent of the volume normally harvested per crane set-up or to fell and extract “single trees” only. The first would most likely not permit an economical quantity of wood, the second would result in unacceptable damages to the remaining forest stand during the felling and extraction operation (Roetzer, 1996).

The group selection felling as applied at the Helela site is considered a first modification with the objective of reducing the adverse environmental impact of the traditional practice. This modified practice reduces the loss of biodiversity and the danger of erosion while maintaining economic viability and using the skill and equipment available for harvesting in Bhutan.

As one can see below, the silvicultural techniques applied to a particular forest stand will affect the environmental impact of long-distance cable logging operations.

Long-distance cable logging
Strip-wise clear-felling Group selection felling
Creation of forests with poor species composition by replanting of only a limited number of indigenous species.Retaining biodiversity by natural regeneration completed by indigenous species of commercial interest where missing.
Creation of even-aged, one-storey forest stands.Uneven-aged, multi-storey forest stands will be created in the long run.
Clear-felled strip-corridor is exposed to erosion wherever ground cover is missing.Cleared patches are embedded in the undisturbed forest and therefore the potential for erosion is reduced.

“Case studies” as recommended in Roetzer (1996) comparing various degrees and shapes of forest canopy opening by group selection felling along the cableway should determine minimum economic harvesting volumes and the maximum size of cleared areas to ensure the positive “edge effect” of the surrounding forest such as seeding of autochthonous species and protection of desired natural regeneration. To evaluate the silvicultural results, monitoring over the time of the trials has to be ensured.

Because of the increasing criticism of applying the traditional clear-felling practice to mixed broad-leaved forests of Eastern Bhutan, environmentally more acceptable silvicultural techniques are to be developed for the sub-tropical, the warm broad-leaved and cool temperate broad-leaved forest zones, which meet the requirements of retaining biodiversity, avoiding erosion and maintaining the natural habitat for wildlife of the region.


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