The case study indicates that the long-distance cable logging system can be considered to be the most suitable option for commercial timber harvesting based on sustainable forest management in steep mountainous terrain of Bhutan.
First introduced by several Swiss and FAO supported projects, there has been a tradition of long-distance cable logging in Bhutan for about 25 years. Consequently, there is a number of cable cranes and the skill for operating and rigging such cable systems. These skills are readily available through either FDC directly or a number of contractors who have purchased the necessary equipment following FSD's and FDC's programmes of privatizing timber harvesting.
The following cable crane systems can be distinguished:
Traditionally, long-distance cable cranes are used only in their gravity version in Bhutan with an open-end mainline and the winch on top. For gravity operation a minimum gradient of about 20 percent for the skyline is required to allow downhill movement of the carriage. Since the mainline cable is spooled on the drum of the winch, the operating distance of the system is determined by the capacity of the winch drum which can hold about 1 700 m of cable in the case of the Gantner single drum sledge winch used at the study sites.
Using the same type of single drum sledge winch, the all-terrain system uses an “endless mainline” where the mainline is simply slung around a parabolic disk which is clamped to the winch drum. The mainline taken through the carriage is fixed with its ends to either side of a tender running a certain distance behind the carriage. In this way, the mainline which is guided by pulleys at the side of the corridor becomes a circumferential cable which moves the carriage and the tender into one or the other direction, depending into which direction the winch drum is turned (Roetzer, 1996).
Photo 25. Single drum sledge winch used at the Korila site for long-distance cable logging in gravity operation with an open-end mainline and the winch on top
In the all-terrain version, the winch can be set up at any place along the corridor and the logging operation becomes less dependent on terrain conditions. Harvesting can be extended to areas otherwise inaccessible for commercial timber harvest because of winch transport difficulties caused by irregular terrain and rivers.
The use of long-distance cable cranes in the all-terrain version is considered a prospective option for Bhutan contributing to the economic viability of forest management projects in FMUs with difficult terrain conditions for cable logging.
The main silvicultural technique applied in Bhutan's mountain forests is traditional strip-wise clear-felling for timber extraction by gravity cable cranes and subsequent replanting with indigenous species of commercial interest. After the cable way has been located by the forest engineer and support, anchor and end-mast trees have been marked in the field, about 30 m of forest on each side of the cable way are clear-felled. This way, the cleared forest area per crane set-up varies between 3.5 to 8.0 ha depending on corridor length (700–1 300 m).
To facilitate high productivity in timber extraction and to avoid damage to natural regeneration, trees should be felled in such a way that they point to the cable in a fishbone pattern. Felling and bucking is normally done by contracted crews using chain saws.
Since, in general, the felling crew is not involved in logging operations, the chainsaw operators are often not aware of the requirements of the subsequent timber extraction process for facilitation of smooth and efficient logging operations. Therefore, close supervision of the felling crew by local staff should ensure that:
directional felling is applied according to the recommendations of the responsible planning engineer;
bigger branches are cut if large tree crowns are expected to cause problems for the extraction crew;
bucking is in line with the regional grading rules to reduce wood waste.
Due to the heavy weight of trees when harvesting in primary, often over-mature forest, the assortment method is applied where trees are cut into length at the felling site. This also facilitates fully suspended loads during transport by the carriage.
Set-up of the gravity cable crane
After felling has been completed at the operation site, the long-distance cable crane will be set-up. As mentioned earlier, the cable winch has to be placed at the highest point of the corridor where logging operation always starts. The winch mounted on a sledge pulls itself up to the place of set-up with its mainline. In very difficult terrain the winch might be dismantled and pulled up by animals. While the winch operator is working to position and to ready the winch, the other crew members will set-up the skyline.
The skyline has to be mounted at a sufficient height to ensure that the load is always fully suspended when transported along the skyline. This avoids the load hitting the ground which causes additional tension to the skyline and creates soil disturbances perpendicular to the contour lines often associated with erosion.
Since the set-up of the cable crane could not be observed by work and time studies at the Korila nor at the Helela logging site, no hourly-based performance can be stated. However, the set-up and take-down times performed by the six members of the cable crew varied between ten days for the Korila logging site and seven days for the Helela logging site according to the information provided by the respective cable crew leader.
The extraction process by means of long-distance cable crane in gravity operation, as found at the Korila and Helela logging sites is described in detail below.
Photo 26. Due to the heavy weight of trees the assortment method was applied to ensure that the load was fully suspended during transport by the carriage
The cable crew normally consists of six members, the supervisor, the winch operator, three labourers at the felling site and one labourer at the landing. The supervisor normally is also trained to operate the winch. The whole logging operation is radio-controlled. One radio stays with the winch operator, another with the labourers at the felling site and one with the labourer at the landing site. All crew members can follow at anytime the ongoing extraction process.
When the empty carriage arrives at the felling site, the winch operator is told by radio where to stop the carriage and to release the hook. One labourer pulls the line out to the logs where a choker has been set. Depending on the size of the logs, up to four logs were transported in a single work cycle.
After the logs have been attached and all labourers have left the area of danger, the winch operator is told to pull in the line. Adjustment of chokers which might be required for extraction is carried out after the mainline has not only been stopped but also released. Obstacles such as tree crowns, stumps, etc., where logs have been tangled during lateral inhaul are cause for adjustment.
Photo 27. At the Helela site the forest road served as landing area
Working with two sets of choker cables improves logging productivity. The labourers at the felling site prepare the next load by setting the chokers while the previous load is transported to the landing.
At the Korila study site the logging operation was controlled by giving hand signs as the radio was out of order the day when the time and work studies were carried out. This was only possible since the corridor and the landing were within sight of the winch operator due to favourable terrain conditions. Otherwise the operation would have had to be stopped immediately after break down of the radio communication for safety reasons.
The following general observations were made either at the Korila or Helela logging site which are not in line with appropriate harvesting practices:
lack of safety equipment such as the use of helmets, gloves and boots for the labourers at the felling site and landing, and appropriate ear protection gear for the winch operator;
obstacles such as large tree crowns had not been cut and sometimes caused serious problems during the extraction process where the supports were at risk of failure due to increased tension;
supports had not been established so that heavier logs hit the ground due to deflection of the skyline, even though using the assortment method.
As at the felling site, the winch operator is told via radio the best position to stop the carriage. The labourer at the landing site unhooks the chokers and attaches the choker cables used in the previous work cycle to the hook. While the empty carriage moves to the felling site, the chokers are removed from the logs and log data recorded.
At the Korila logging site the landing was located on the uphill side of the mid-slope road. Since the logging site had almost been finished when the studies were carried out, only data on uphill logging with long-distances for transport along the skyline could been gathered. In contrast, at the Helela site only downhill logging occurred since the forest road which was used as landing area, crossed the skyline at the bottom of the clear-felled corridor.
The disadvantages of using a minor road as log landing area where the log pile cannot be bypassed are obvious. The road is blocked when timely log hauling cannot be assured. If the cable unit cannot be transported to the next corridor then operation on the nearby corridor is delayed. High non-operating times of the cable unit not only jeopardize the economic viability of a logging job but might also effect the loggers as they often depend on the income from logging to earn a living.
Photo 28. Log loading carried out manually becomes a time-consuming process when logs cannot be rolled horizontally onto the dump truck
The subsequent log hauling is carried out by means of dump trucks without any special equipment or adaptation for log loading or hauling purposes. The loading is carried out manually by up to six labourers. When logs cannot be rolled horizontally onto the dump truck (Photo 28), as at the Korila logging site, this becomes a time-consuming process. Even if the log pile does not obstruct vehicle passage, unnecessary storage time of logs must be avoided due to climate conditions and danger of deterioration in most areas of Bhutan.