Croatian Journal of Forest Engineering (Apr 2011)
Ecoefficient Timber Forwarding Based on Nominal Ground Pressure Analysis
Abstract
This paper presents the research of possibilities of forwarding timber in an efficient and environmentally friendly way, under conditions of limited bearing capacity of gley soils of Croatian lowland forests due to their increased moisture all the year round. Reduced soil bearing capacity causes restricted mobility and decreases the forwarder productivity, and also increases the level of damage caused to forest soil, reflected in its compaction and rutting (Fig. 1).In lowland even-aged forests of Croatia, main felling is mostly carried out based on wood cutting and processing by chain saws and forwarding of timber, processed by cut-to-length method, by six-wheel forwarders, usually not equipped with semitracks on rear wheels of bogie axle (Fig. 1).The analysis was carried out on the example of a medium-weight six-wheel forwarder Valmet 840.2 (nominal load capacity of 12 t, whose dimensions and load distribution of empty vehicle are shown in Fig. 4), and this analysis involved as follows: 1) the effect of load reduction on forwarder efficiency, 2) forwarder environmental soundness under conditions of limited soil strength based on nominal ground pressure (Mellgren 1980) of front and rear wheels of the vehicle depending on the mass of loaded timber, vehicle with narrower (600 mm) and wider (710 mm) tires, or with tracks on wheels of the rear (bogie) axle. The limit value of environmental soundness would be the allowed ground pressure of limited bearing capacity <60 kPa (Owende et al. 2002).The effect of load reduction (4 t, 8 t with respect to 12 t of the vehicle nominal load capacity) on forwarder efficiency with respect to the distance of timber forwarding, is expressed according to the multicriteria calculation model of productivity of these vehicles (Stankić 2010), which takes into account: 1) forwarder class 2) soil strength, 3) use of tracks, 4) felling density, 5) volume of the average felling tree and 6) distance of timber forwarding. The unit cost of timber forwarding was calculated according to machine rate made by the company »Hrvatske šume» d.o.o Zagreb for the forwarder Valmet 840.2 (433.19 kn/h).For calculating the nominal ground pressure (Fig. 3), a theoretical model of axle load distribution was used, the case of vehicle at standstill on level ground, depending on mass and length of loaded logs in the bunk area of the forwarder (Poršinsky and Horvat 2005). The analysis of axle load distribution was based on an average length of logs (4 m) made by cut-to-length method in the area of the Croatian lowland forests, and the mass of 1800 kg of a pair of semitracks. The wheel load assumed even load distribution of axle load by pertaining wheels. The contact area between wheels (tracks) and soil was calculated according to Mellgren (1980), for narrower (front – 600/65-34, rear – 600/55-26.5) and wider (front – 710/55-34, rear – 710/45-26.5) tires recommended by the manufacturer of forwarder.The analysis of the forwarder environmental soundness was based on: 1) values of nominal ground pressure of front and rear wheels of the vehicle depending on the mass of loaded timber, and use of narrower and wider tires, i.e. or use of tracks, and 2) the upper limit value of the allowed ground pressure (<60 kPa) of limited bearing capacity (class 3 – Fig. 2), which prevails at the time of main felling in the Croatian lowland forests.The results of efficiency analysis of timber forwarding (Fig. 5) showed that the reduction of load under conditions of limited bearing capacity of gley soils, as a measure for providing vehicle mobility as well as for diminishing the level of damage caused to forest soil, is highly unacceptable due to the decrease of productivity and increase of unit costs.The analysis of nominal ground pressure (Fig. 7) under wheels of the front axle and wheels (tracks) of the rear axle, showed that under conditions of limited soil bearing capacity: 1) the forwarder nominal load capacity may be used and that efficient timber forwarding may be performed, 2) the environmental soundness can be provided by the application of four-axle (eight-wheel) forwarders equipped with wider tires and half-tracks on wheels of front and rear bogie axle of the vehicle.These results should be used as guidelines for purchasing forwarders, and such approach would provide in future ecoefficient timber forwarding under conditions of limited bearing capacity of gley soils in main felling sites of lowland forests.
