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Once a site preparation method has been chosen (e.g., spot mixing), the appropriate implement and prime mover must be selected. Generally, prime movers such as wheeled skidders, forwarders, crawler tractors, or excavators are used, however, farm type tractors and flex track equipment (FMCs) have also been used. The type of prime mover selected will usually depend upon limiting/restricting site factors, special requirements of the site preparation implement chosen and cost. Restricting site factors include slope, ground roughness (including frequency and size of obstacles, and frequency and size of stumps), soil bearing capacity, slash loading, and evenness of terrain. Special requirements of the site preparation implement include factors such as drawbar pull requirements, hydraulic requirements, weight and balance considerations, travel speed limitations, and special attachments to raise and lower implements for turning or travel between treatment areas.
In the past, skidders or crawler tractors were simply taken from harvesting operations and used for prime movers in site preparation. This was most often the case for contractors who logged in the winter and did site preparation in the summer. Unfortunately, due to differences in the nature of the duty cycle, this was often a costly and unsuccessful decision. The site preparation duty cycle differs from harvesting in the following respects: necessity to cover 100% of the ground, continuous operation under load, heavy loads with high peak forces, frequent end-of-pass turning, on-road travel between sites, constant start and stop operation, push/pull requirements, slow steady working speeds, and necessity to carry a load. Common problems included overheating and not having sufficient power at low speeds. Contractors dedicated to site preparation have dealt with many of these problems by purchasing equipment with specifications suited for site preparation work. Duty cycle considerations for prime movers are outlined in the following sections.
Wheeled skidders are the most common prime mover for site preparation. They are generally used to pull light implements but can also be used in heavier applications such as powered disc trenching. Skidders are limited to relatively easy terrain and light slash conditions. Some contractors have modified their skidder blades or have replaced them with light rakes for clearing and aligning slash. It is recommended that skidders be limited to slopes of 15% or less for contouring and 25% for downslope operation. Skidders equipped with wide tires have greater stability on side slopes, however, they are more prone to slipping when slash is wet. Wide tires also cause increased wear on the drive train and are expensive to purchase. Good chains are essential and they must be checked regularly to ensure that they are tight.
Skidders used for site preparation often have extra oil coolers for their hydraulic system and have transmissions designed for operation at slow travel speeds. Specialty skidders, such as the Franklin and SK250, have been designed specifically for site preparation.
Forwarders have also been used as prime movers for site preparation. Forwarders are more powerful than skidders and are better designed for carrying a load (such as an auxiliary engine). Six- or eight-wheel drive forwarders having one or two sets of bogies generally have higher floatation than skidders equipped with standard tires. Forwarders also offer greater stability on slopes, and with their bogies, will ride over obstacles more easily than skidders, thereby reducing the jarring to the operator. Forwarders are more expensive to operate than skidders and have therefore seen limited application.
Crawler tractors are better-suited for "pushing" than skidders and in the past have been used primarily for piling heavy accumulations of slash or for blading treatments. Recently, they have been used for disc trenching operations where the need to part heavy slash or to traverse rough terrain has called for extra power. Crawlers can contour on slopes up to 35% and can work downhill up to 45%. However, crawlers are slower and cost more to operate than skidders.
Excavators are the most versatile of all prime movers. Unfortunately they are the slowest and most expensive to operate. Excavators are used for spot treatments on broken terrain or slopes, and for mounding or ditching on wet sites. Excavators can work on slopes up to 50%, and can treat small steep pitches given access such as skid trails. Excavators can be equipped with a variety of attachments and offer complete operator control over microsite creation and spacing. When choosing an excavator for site preparation, ensure that it has been properly guarded for bush applications and that it is equipped with the proper track pads and grousers for the job.
For additional information on excavators and their use in silviculture, see the video Excavators: Silvicultural Attachments and Treatments .
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