Effects of brush mats and machine characteristics on dynamic peak loads and surface contact pressures exerted by forest machinery

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University of New Brunswick


Most forest operations are now fully mechanized and require machines weighing between 15 and 40 metric tons to harvest, process, and transport trees within the forest stand. The off-road traffic of such heavy machines can cause severe soil disturbances that can negatively affect tree growth and quality. This research examined two methods of reducing dynamic peak loads exerted by forwarders, while traversing two types of surfaces (rigid steel plates and flexible sand layer). First, the ability of steel flexible tracks installed on a forwarder's rear bogie axle to reduce peak loads on rigid and flexible surfaces was compared to peak loads of tires. Second, we quantified the ability of brush mats (branches, tops, and foliage) placed on the two test surfaces to distribute applied loads of a moving forwarder below the mats. To assess load distribution patterns, all tests were performed at full-scale in a laboratory on a prototype steel (rigid) load test platform or on the sand (flexible) covered platform. Following tests with the steel flexible tracks, the platform was covered with varying amounts of softwood brush (5, 10, 15, 20, and 30 kg m⁻²) and subjected to repetitive loadings of an eight-wheel forwarder. In addition to the full-scale tests, load distribution capabilities of small-scale hardwood and softwood brush mats were assessed in a load test structure placed inside a load frame. Lastly, based on different stand types and percent tree removal, we quantified the effect of varying machine operating trail dimensions on the amount of brush required for soil protection and the amount remaining for other use. Important findings from this dissertation are as follows: 1. Tracks operated on the flexible surface lowered mean peak loads by 30% compared to tires operated on the flexible surface as indirectly derived from the load distribution of front wheels on sand; 2. A minimum brush layer of 10 kg m⁻² (green mass) was required to statistically lower mean peak loads recorded below the forwarder; 3. Best results with respect to reduction of maximum peak loads exerted to the ground were achieved by brush mats of 20 to 30 kg m⁻² (green mass); 4. Small-scale tests revealed that softwood brush mats had a slightly better capacity than hardwood mats at laterally distributing applied loadings, and 5. From the three factors assessed (brush amount, trail width, and trail spacing), trail width had the highest effect on the required brush amount for trail protection. By enhancing the understanding of machine and operating environment interactions, results from this project will provide guidelines for soil protection during mechanized forest operations.