Federal Forest Research Centre Vienna, AUSTRIA
Although operational costs of the chute seem high, this method of operation should be used when the stand, the ground and the young growth need special care, and when other methods of extraction are impossible, or simply cannot be employed. Obtaining even just a little free output, or perhaps none at all, is however more worthwhile than thinning in thickly populated spruce stands and allowing uncontrolled stands to grow with associated risks. Longer racks with larger quantities of wood than those described in the above example would significantly reduce the cost per cubic metre for building, installing and dismantling the chute.
As far as personnel working on the site are concerned, this was a worthwhile operation on a site that had no dangerous exhaust fumes, machine vibrations or noise.
Timber chutes are slides constructed from synthetic materials shaped into channels that carry timber by using the cheapest form of energy: gravity. Their length resembles the slope on which they are laid, but they extend the distance over which logs can travel, and reduced friction enables quicker delivery than on an earth slide.
Slides were mentioned in history as long ago as the seventh century BC when Nebuchadnezzar II had wood slides constructed to transport cedars. In Austria a fourteenth century chronicle tells of specially constructed timber channels in the Tirolean Weistümer area, and there is also an eighteenth century account of similar constructions in the Pontirone Valley in Tessin, Switzerland. There are other reports of slides in Ennstal and Baden Württemberg, therefore, this type of timber transport can be shown to have been used in these areas since the Middle Ages until the present.
Of course their construction and maintenance required a great deal of labour and materials, and therefore over time they were replaced by other methods of forwarding, such as for example skyline systems.
With the arrival and manufacture of modern materials based on mineral oils, new possibilities opened up for the construction of slides and, as early as the 1970s, the Austrian firm, Leykam, developed a plastic slide, known as the Leykam Log Line. This was demonstrated to forestry professionals in 1975 for the first time.
Technical and operational details
The Log Line is composed of a semicircular plastic channel made of polyethylene - which is a material that retains its shape, is robust and can operate over a wide temperature range, both above and below zero - all of which guarantees a long service life. At first, the system consisted of pipes for the main run and open sections at the delivery point.
Nowadays only open channels are used. They have an internal diameter of 350 mm and walls 9 mm thick. Each channel has a length of 5 m and weighs 25 kg. The individual shells are fixed together in a line of required length, using stainless steel quick couplings, which are laid down the slope. For ergonomic reasons, and because they travel better, wood lengths of 2 to 4 m are most efficient. Depending on the radius of the curves on the slide and how straight the logs are, log lengths up to 6 m, and with a diameter of 30 cm at the wider end, can be accommodated.
Because logs can build up speed, 200 m stretches are the most practicable. The individual channels are tied with nylon rope to neighbouring trees, stumps, etc. If none are available, stakes can be used which can be quickly cut and sharpened from treetops using a chainsaw, and driven in next to the channels. To prevent the channels from sagging, branches and brush can be placed beneath them. To avoid the possibility of breakage, the chute should not be laid over loose rocks or bumpy ground. For similar reasons corners with a narrow radius should be avoided, since there is danger that the wood could slide out of the channel. If this is unavoidable, the corners should be built with higher sides.
Depending on the weather and the condition of the wood, a minimum gradient of 15 to 25 percent is required for trouble-free operation. Maximum gradient is 50 percent, which requires braking measures to be taken or, if possible, the chute should be laid across the slope. According to studies undertaken by Horst Tauer BSc in Ort, Gmunden, spruce with the bark still on has a dynamic coefficient of friction of 0.22 to 0.25 in dry weather, while wet wood in wet weather has a coefficient of friction of 0.15 to 0.18. Foreign bodies introduced into the slide during operation, such as earth or vegetation, increase these values significantly.
Speeds of over 15 m per second are to be avoided for the following reasons:
New openings for the Log Line
Despite its success in the past, the timber chute was, as mentioned above, increasingly overshadowed by new extraction methods. Only a few contractors or woodland owners have installed timber chutes in recent years, and their use continues to decline. With new harvesting technology and an increasing awareness of ecological considerations, there are however new areas of operation, the value of which should not be underestimated.
Recognizing this, the Technical Development Branch of the FBVA in Vienna has carried out work studies on the Leykam Log Line to determine whether this alternative method of extraction is worth considering on steep harvesting sites where current forwarders or cable cranes are not usable. It was difficult to find a contractor who still uses this method of operation, and who was prepared for this type of investigation, as well as able to fit his own schedules to our own.
Site conditions unsuitable for forwarders or cable cranes
The operation took place on a slope of around 35 percent in Rendsina. Chalky ground of this type with an A/C profile is loose and consequently does not provide a good grip, and in wet weather it cannot be driven over at all. Dry weather is essential for harvester operations, as was in fact the case during this study. Even though harvester operations in these conditions are acceptable for ecological and safety reasons, this is not the case with the forwarder. Repeatedly driving up and down the harvesting racks would leave deep tracks every 20 m. which would turn into erosion channels, resulting in humus being washed away and would lead to karstification. For this reason and because of difficulties in driving across the terrain (this could not be carried out economically without a full load, and also could not be performed safely), the option of forwarding never was considered.
Skylining was also not an option, since with these low volumes of wood per stand (see Table 1), the small stem diameters, with a distance of 20 m between the stands, would also have been uneconomical. Nor were there suitable anchor trees. For these reasons, if the stand was to be completely mechanically thinned at an acceptable cost, the only option was the chute.
Specific operational procedures after harvesting
As is the case when using a skyline after harvesting, it would be advantageous to lay out assortments of wood near the racks. However, a different approach would be for the line of the chute to follow the path taken by the harvester through the stand. To position the chute to best advantage for a convenient outlet, the harvester operator must be informed of the extraction plans and bear this in mind when laying out the assortments of wood. If possible, the operator should lay the wood in a herring-bone pattern, pointing down the hill in the direction of the chute, which would make it easier to position and place the logs in the chute. The best results may be anticipated where both the harvester and the chute are the responsibility of the same person, i.e. where a harvesting contractor is free to choose how to install the Log Line and when there is only one person in charge of both operations.
The same could be said for all forwarding operations following harvesting. Harvester operations generally succeed under the same conditions as those that favour the Log Line. The chute is therefore easily set up in the felling direction in the rack. There is usually enough brush in hand after harvesting to place some under the chute as support. Generally, brush can prove something of an obstacle when walking or bringing in logs. Naturally, after clear-fell harvesting there will not be any trees to tie the chute to and, therefore, stakes have to be driven in. These are, as already mentioned, quickly made with a few strokes of the chainsaw from the lop and top lying nearby. If they are driven in up against the chute, this has the advantage of giving additional stability to the chute components, which is especially important on corners.
Harvesting comprised the first thinning of an even-aged spruce stand, interspersed with a little beech, in the 20- to 40- years-age class. Because of other commitments, the harvesting operation that preceded the extraction could not be studied. This was, however, not crucial to the study since performance data on tracked harvesters - in this case a Neuson 11002HV - were already available, and there were no noteworthy differences between this site and other similar ones. Tree size, with a breast height diameter (bhd) of 18.9 cm, was ideal for the Logmax 3000 head, which has a maximum delimbing diameter of 380 mm.
Table 1. Main data
|Rack I||Rack II||Rack III||Rack IV||Rack V||Rack VI||Total|
|Volume (m3) over bark||15.07||12.16||12.26||8.31||4.58||5.02||57.40|
|Log diameter over bark (cm)||15.27||14.68||14.22||14.94||14.17||13.38||14.59|
|Breast high diameter (cm)||19.96||19.06||18.36||19.46||18.28||17.07||18.70|
|Average log length (m)||3.98||3.84||3.82||3.70||3.77||3.80||3.84|
|Average log value (m3)||0.07||0.07||0.06||0.06||0.06||0.05||0.06|
|Extraction rack length (m)||110||113||119||119||68||45||---|
|Chute length (m)||90||90||90||90||46||32||---|
|Number of chute elements||20||20||20||20||10||7||---|
|Chute gradient %||34||35||36||38||30||34||---|
|Average distance between racks (m)||22 metres||---|
It was not possible to do a time study on the initial construction or dismantling of the chute. The pieces of the chute were carried to the operating area individually. Frequently they are attached together in a long line in the valley and then pulled up the hill by means of a small Ackja winch. Consideration was given to pulling them up the hill on a rope behind the harvester on its first foray into the woods. The possibility of placing the logs in the chute directly from the harvester was also considered, an idea, which because of technical driving considerations, could hardly be realized in practice.
Stakes were prepared on site in the manner described, which were then driven into the ground with a metal hammer and attached to the chute with nylon rope. Operations were easiest with log lengths of 2 to 5 m. Seventy percent of the logs lay in the ideal 4 m range, 3 percent in the 2 m range, 19 percent in the 3 m range while only 8 percent were 5 m or more. Average length was 3.84 m (see Table 1). A chute ideal in length and gradient resulted in an acceleration pattern, which did not require a braking area to be built.
Table 2. Output
|Rack I||Rack II||Rack III||Rack IV||Rack V||Rack VI||Average m3/man|
|Number of operators||2||2||3||3||2||2||---|
|m3 with bark/h net||2.55||2.53||2.40||1.96||1.66||2.57||2.32|
|m3 with bark/h with breaks||2.04||1.73||1.99||1.57||1.42||1.92||1.81|
Piles of branches on top of the logs sometimes made it difficult to pull them out - a situation that should have been avoided by the harvester operator. As can be seen in Table 2, two or three men were used alternately to change the position of the slide, to drag logs towards it and to lay them in the chute with the hook.