ManuscriptThe role of environmental effects in timber harvesting is deserving more importance in decision making and logging planning. Therefore more emphasise has to be put on planning system, so that environmentally sensitive sites for a certain harvesting system can be recognised and the best suitable machines selected for given terrain conditions. Some rut depth models based on WES-method have been developed for single pass rut and only a few for multipass. Forwarders usually have different wheel configuration and wheel load between axles, and the wheel load varies during a transport cycle. In order to test the suitability of some multicycle model in Nordic forestry conditions some test drives were organised and the rut depth after each wheel pass recorded. It was found out, that a multipass coefficient model using wheel numeric as the input variable and a simple multipass model gave acceptable results, but more tests are needed to test the total goodness of the method

Mulari, Jyri

Rantala, Matti

Saarilahti, Martti

Helsingin yliopiston digitaalinen arkisto

MULTICYCLE RUT DEPTH IN FORWARDINGMartti Saarilahti, Jyri Mulari & Matti RantalaECOWOOD-ProjectUniversity of Helsinki, P.O. Box 27, FIN-00014 University of Helsinkimartti.saarilahti@helsinki.fiAbstractThe role of environmental effects in timber harvesting is deserving more importance indecision making and logging planning. Therefore more emphasise has to be put on planningsystem, so that environmentally sensitive sites for a certain harvesting system can berecognised and the best suitable machines selected for given terrain conditions. Some rutdepth models based on WES-method have been developed for single pass rut and only afew for multipass. Forwarders usually have different wheel configuration and wheel loadbetween axles, and the wheel load varies during a transport cycle. In order to test thesuitability of some multicycle model in Nordic forestry conditions some test drives wereorganised and the rut depth after each wheel pass recorded. It was found out, that amultipass coefficient model using wheel numeric as the input variable and a simple multipassmodel gave acceptable results, but more tests are needed to test the total goodness of themethod..1. INTRODUCTIONThe role of the environmental effects of timber harvesting is deserving more importance inthe decision making of forest operations and logging planning. In the Finnish forestry rather arough operational terrain classification is used, based mainly on the knowledge on theground vegetation and soil type. Therefore more emphasise has to be put on planningsystems, so that environmentally sensitive sites for a certain harvesting systems can berecognised and the most suitable machines selected for given terrain conditions.There is quite a large number of papers dealing with soil compaction and some paperspresenting models for wheel sinkage or rut depth [1]. But, as Holm has noticed, “very fewauthors have published papers on multipass-behaviour of wheels” [2]. The load size of aforwarder can be reduced depending on soil conditions. This affects, however, both on theproductivity and to the environmental effects of the operation. Different loading status affectson wheel loads, and simple rut depth models, based on a single wheel behaviour models,may give misleading data for comparing different forwarder constructions.This paper deals with the modelling of the rut depth of wheeled forwarders using a multipassapproach.22. MULTIPASS AND MULTICYCLEThe development of the rut, as well as in some extent the rolling resistance and the thrust, asa function of the number of wheel passes depends on the vehicle configuration. A largenumber of field tests have been carried out and WES mobility models developed using asingle wheel tester. But how these results can be applied for multi-axle vehicles? It is alsoquestionable, how well the multipass models based on farm tractor trials are applicable onforwarder transport. Therefore it is useful to distinguish between the different multipassconcepts:- single pass, a single wheel with known parameters- multipass, passing of a single vehicle- all axles have the equal wheel parameters: d, b, W (wheel multipass)- different wheel parameters by axles  (vehicle multipass)- multicycle (pass) of a forwarder, a special case for example in forestry transportation,when a forwarder travels unloaded to the stump area and returns loaded- different wheel parameters by axles, front and rear- different load parameters during the cycle, travel and return3. MATERIAL AND METHODS3.1. Data collectionData on rut depth for a pilot study were collected on three sites situated at South-EasternFinland. A 6-wheeled Valmet 840 forwarder was driven on three selected lanes (M, N, O),first 3-5 times unloaded, and thereafter loaded on the same track. The driving velocity overthe measuring points was very slow, but steady. The soil surface, and later rut depth, wasrecorded at one point by 0.1 m distances perpendicular to the track both on left and rightside. The rut depth was recorded after each wheel pass, so that there were threemeasurements for one tractor pass on the both sides. Soil penetration resistance wasmeasured at both sides from three different points using a self recording penetrometer. Soilsamples were taken from one point at each lane, 0.0-0.10 and 0.15-0.25 m depth, and thegravimetric water content, 100·water/dry mass, was determined.ForwarderA Valmet 840 6-wheeled forwarder was used in the tests. The empty mass of the tractor is11.9 t and the used load size was estimated, based on timber volume recording, to be 5.9tons, only about 60% of its full load capacity. The tractor wheel characteristics were, asfollows, Table 1.Table 1. Wheel characteristics of the forwarderWheel Tyre characteristics Wheel load, kNDesignation b, m d, m pi, kPa Empty LoadedFront wheel 600/65 - 34 0.600 1.634 200 31.5 31.53Rear wheel 1 600/55 - 26,5 ELS 0.600 1.333 370 13.6 26.5Rear wheel 2 600/55 - 26,5 ELS 0.600 1.333 370 12.7 28.4Site and soil propertiesThe first lane, M, was situated on a small clear cut area on  bare silty soil, which had beenlightly harrowed for game feeding ground. Using the Finnish forest site type classification thesecond lane (N) was on a typical spruce swamp (korpi) and the third lane (O) on Myrtillustype (MT). The surface layer was rather humified peat, and the subsoil wet silt, the watertable being rather high. The soil properties are given in Table 2. Soil penetration resistance,cone index, is the average of the 150 mm depth, which seemed more reliable, than othervalues, for example the average 0 to 150 mm.Table 2. Soil propertiesLane Site Peat Mineral Moisture, % w/d Cone index, kPalayer, m soil 0-0.1 m 0.15-0.25 m left rightM Open (Myrtillus) 0 Siltymoraine29 29 1490 1137N Spruce swamp 0.3 Siltymoraine160 28 481 513O Myrtillus type 0.15 Siltymoraine47 35 797 894The average penetration resistance profiles are given in Figure 1. The surface layer of laneM is weak because of the light harrowing, but the subsoil has a good bearing capacity.Lanes N and O are problematic, because stones and roots increases occasionally thepenetration resistance. For the left lane of site N exceptionnally 0-0,100 m averagepenetration depth was used, because of the apparent obstacle.Figure 2. Average penetration resistance, kPa. Black, right lane, Grey, left lane0501001502002503003504004500 1000 2000 3000M0501001502002503003504004500 1000 2000 3000N  0501001502002503003504004500 1000 2000 3000OFigure 1. Average penetration resistance (kPa) by lanes43.2. MethodsGeneral equation for the settlement during repetitive loading  is given in Eq. (1)[3], and thesame applies for multipass phenomena [4].z z nn a= ×11(1)wherezn sinkage, settlement after pass n, mz1 sinkage after 1st pass, mn number of loadings, passesa multipass coefficientDifferent authors have given different values for multipass coefficient a. Scholander [3] givesvalues ranging from 2 (wet silt) to 26 (dry gravel). After Abebe [4] the coefficient is 2 to 3for loose soils and low loads, and  4 to 5 for bearing soils and heavy loads. Based on fieldtests Freitag [5] found out that the second pass sinkage follows the model, Eq(2):( )z z z= +12 2 2 0 5.  (2)wherez rut depth after 2nd pass, mz1 rut depth after 1st pass, mz2 estimated rut depth after 2nd pass without no prior soil disturbance, mThis corresponds to the multipass coefficient of 2. Larminie [6] gives the multipassmultiplicators which lead to the multipass coefficient a = 3.8. Based on Dwyer & al [7]second pass rolling resistance coefficient equation the following multipass coefficientequation can be developed, Eq(3):azNNzCICI=×-æèçöø÷æèççöø÷÷ -ln( )ln.ln( )20 8961 1a(3)This permits to conclude, that the multipass coefficient is dependent on wheel numeric.Saarilahti [1] analyzed the data of some previous forwarder studies, and developed somemodels between the multipass coefficient and the wheel numeric. The analysis of thedependence of multipass coefficient as a function of the wheel numeric is given in Figure 2.Data for models P1-P3 were collected on peatlands and ANTTILA on moraine soils.50,01,02,03,04,05,06,07,08,09,010,00 2 4 6 8 10NciCOEFFICIENT aTURNAGEDWYERANTTILAP1P2P3LARMINIEFigure 2. Different multipass coefficients as a function of the wheel numeric NCIIt can be seen, that evidently the multipass coefficient is related to the wheel numeric, andthus to wheel/soil properties, but the data does not allow to determine one single “bestmultipass coefficient”. Therefore some of the multipass coefficients were tested in multicyclerut depth calculus. The Maclaurin’s [8] sinkage model was used for the first pass sinkage,and the rut depth after each single wheel calculated using the model, Eq(4).( )z z zn n a a a= +-1 11(4)where a is the calculated coefficient for the nth wheel.4. RESULTSIt was found out, that some coefficients led from the observed rut depths to ratherremarkable deviations. The best suitable model for estimating the rut depth was based ondata developed from Anttila’s data [9], (ANTTILA  in Fig 2), Eq(5)a NCI= ×150 7. . (5)It is close to one of models developed from the forwarder tests on peatland (P2 in Figure2), Eq.(6).a N ML= ×1 7 0 57. . (6)where NCI and NML are wheel numeric.The calculated and observed rut depth as a function of number of wheel passes by lanes arepresented in Figure 3.6The models as such leave quite a lot to improve, for example, the observed first pass issystematically lower than the estimated. One reason is, that the observed rut depth containsthe lug effect, being measured from the lug bottom, not that of the carcass.-80-70-60-50-40-30-20-1000 10 20NUMBER OF PASSESRUT DEPTH, mmLANE M-300-250-200-150-100-5000 10 20NUMBER OF PASSESRUT DEPTH, mmLANE N-250-200-150-100-5000 10 20NUMBER OF PASSESRUT DEPTH, mmOBS LOBS RLRLANE OFigure 3. Estimated and calculated rut depth as a function of number of wheel passesOne reason for the discrepancy between the observed and calculated rut depth may be dueto the neglection of the dynamic load and load transfer. Some big roots and stones createdrather an uneven rut profile, and the weight transfer between sides and axles, as well asdynamic loads due to swaying, are remarkable. Their influence are not included into thewheel loads, however. It seems, that on lane O a soil failure has occurred under the leftwheels, while the right wheels are supported by a thick root.6. CONCLUSIONS AND RECOMMENDATIONSThe study, based on rather a limited data, permits to conclude, that the use of  a variablemultipass coefficient based on wheel numeric may improve the multipass and multicyclemodeling. Further studies, however, are needed for testing the reliability of the models forreal operational conditions. It may be useful to use estimated true wheel loads, which takeinto account the load distribution and a certain dynamic load.LITERATURE[1] Saarilahti, M. 2001. Evaluation of the WES-method in assessing the trafficability of forest terrain andthe mobility of forestry vehicles. Part 2. WES mobility models. ECOWOOD. University ofHelsinki, Department of Forest Resouce Management. Internal Report. 29 p.[2] Holm, I. C. 1999. Multi-pass behaviour of wheels in soft soil, also in a curve. Proceedings of  the 11thInternational Conference of ISTVS. 1:222-232.7[3] Scholander, J. 1974. Bearing capacity of some forest soils for wheeled vehicles. Some technicalaspects and consequences. Skogsmarks bärighet för hjulfordon. Några tekniska aspekter ochkonsekvenser. Specialnotiser från SFM Nr 14. 97 p.[4] Abebe, A., Tanaka, T. & Yamazaki, M. 1989. Soil compaction by multiple passes of a rigid wheelrelevant for optimization of traffic. Journal of Terramechanics 26(2):139-148.[5] Freitag, D. R. 1965. Adimensional analysis of  the performance of pneumatic tires on soft soils. U SArmy Waterways Experiment Station, Report  No. 3-688.[6] Larminie, J. C. 1988. Standards for mobility requirements of military vehicles. Journal ofTerramechanics 25(3):171-189.[7] Dwyer, M. J., McAllister, M. & Evernden, D. W. 1977. Comparison of the tractive performance of atractor driving wheel during its first and second passes in the same track. Journal ofTerramechanics 14(1):1-10.[8] Maclaurin, E. B. 1990. The use of mobility numbers to describe the in-field tractive performance ofpneumatic tyres.  Proceedings of the 10th International ISTVS Conference, Kobe, Japan,August 20-24, 1990. I: 177-186.[9] Anttila, T. 1988. Metsämaan raiteistumisen ennustaminen WES-menetelmää käyttäen. University ofHelsinki, Department of forest resource management. Publications 17. 53 p. ISBN 951-45-8025-7

Multicycle rut depth in forwarding

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