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Skogsavverkning och markskador

Nordfjell, Tomas (2020). Skogsavverkning och markskador. UNSPECIFIED.

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Forskning har visat att kombinationen av jordart och markfuktighet är starkt avgörande för om man får
stora markskador eller ej vid skogsavverkning. Grovkorniga jordarter har bra bärighet även vid hög
markfuktighet men för finkorniga jordarter försämras bärigheten kraftigt vid ökad markfukt. Vid
drivningsplanering är det viktigt att bestämma jordart för att veta hur hög markfuktighet som kan tillåtas
vid drivningstillfället. För steniga marker har det visat sig att en kombination av blockkvot och
maximalt stickdjup med jordsond ger en god skattning av markens aktuella bärighet. En praktiskt
tillämpbar klassificering av skogsmarks bärighet utifrån enkelt mätbara variabler presenteras.

Authors/Creators:Nordfjell, Tomas
Title:Skogsavverkning och markskador
Alternative abstract:

<p>Ground based forest harvesting means that heavy transportation has to be done on forest ground. The<br>total mass of logs and machines that passes on main logging trails roads from the harvesting site to<br>roadside is several thousand tons during an ordinary harvesting operation. When forest soil is burdened<br>beyond its bearing capacity, it will become compacted and rutted.<br><br>The strength of the soil is largely determined by the soil type (particle size of the mineral soil) and its<br>water content. Soils with a high proportion of gravel or sand and / or very rocky soils have good bearing<br>capacities even at high water contents. Fine-grained soils with a high proportion of fine particles are<br>strongly dependent on the water content for their bearing capacity. A volumetric water content (VWC)<br>of about 40% seems to be an approximate threshold value when it comes to soil bearing capacity.<br><br>The stress that a forest machine exerts on the ground depends on the machine’s mass and how it is<br>transmitted to the ground. The mass that loads a single wheel affects the depth of the ground<br>compaction. High wheel loads compact the ground deeper than lower wheel loads. The trend over the<br>past many years has been that forwarders’ wheel loads have increased.<br><br>Ground pressure is strongly linked to rutting. The size of the contact surface depends on the following:<br>diameter, width and tire pressure of the wheel; the magnitude of the wheel load; the deflection of the<br>tire caused by the wheel load; and the plasticity of the ground. The larger the diameter and width of a<br>tire, the lower the ground pressure. The lower the tire pressure, the lower the ground pressure.<br><br>Any development of tires that are larger than today’s tires would encounter practical limitations. One<br>such limitation would be that the machine become so large that it cannot be transported on public roads.<br>Another limitation would be that very large tires on a forwarder reduces the space available for cargo.<br>Hence, the only remaining practical solution for reducing ground pressure is to reduce the wheel load.<br><br>The use of bogie tracks increases the contact area between the machine and the ground, thereby reducing<br>ground pressure. Measurements of ground pressure shows that the bogie tracks’ effect is more about<br>the wheels becoming wider than about the contact surface between the bogie wheels becoming longer.<br>Indeed, it is at the wheel’s centerline that the pressure on the ground becomes greatest. This is because<br>the tracks have no support between the bogie wheels. Rut depth is largely affected by the same variables<br>as compaction. A high soil water content causes greater rut depth than if the water content is low. A<br>fine-grained soil type leads to deeper ruts than a coarse-grained soil type. The rut depth also increases<br>with increasing number of crossings.<br><br>To reduce soil damage as much as possible, the following basic recommendations should be adhered to:<br>• Determine the soil type of the harvesting site and main logging trails (and if possible also the<br>variation in soil type within the area).<br>• Once the soil type has been determined, assess also the soil moisture content. Classifying the<br>bearing capacity of forest soils for forwarders is done on the basis of these two variables.<br>• Plan the forwarding so that the most frequently used trails are located on where the bearing<br>capacity is the highest. Beyond this factor, try to find as short total extraction distances as<br>possible.<br>• Use bogie tracks, especially on the forwarder's rear bogie or if the ground has poor bearing<br>capacity. Many studies have shown that rut depth decreases if tracks are used.<br>• Reinforce the ground, especially at sensitive areas or where many crossings will take place.<br>• If sufficient amounts of logging residues are not available or if a terrain section has very poor<br>bearing capacity, logs (generally low-value/pulpwwod logs) can also be used to drive on.<br>• There are also specially made "ground protectors" that can be laid out when frequently passing<br>over shorter sections with very poor bearing capacity. Such ground protectors can also be used<br>to build temporary crossings over ditches or very small streams.<br></p>

Year of publishing :2020
Number of Pages:13
Publisher:Institutionen för skogens biomaterial och teknologi, Sveriges lantbruksuniversitet
Publication Type:Other
Article category:Popular
Version:Published version
Full Text Status:Public
Subjects:(A) Swedish standard research categories 2011 > 4 Agricultural Sciences > 401 Agricultural, Forestry and Fisheries > Forest Science
Permanent URL:
ID Code:18759
Faculty:S - Faculty of Forest Sciences
Department:(S) > Department of Forest Biomaterials and Technology
Deposited By: SLUpub Connector
Deposited On:26 Nov 2020 15:03
Metadata Last Modified:15 Jan 2021 19:44

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