the impact of brush mats on forwarder ......the impact of brush mats on forwarder surface contact...

34th Council on Forest Engineering, June 12-15, 2011, Quebec City (Quebec) 1

THE IMPACT OF BRUSH MATS ON FORWARDER SURFACE

CONTACT PRESSURE

Eric R. Labelle1, Dirk Jaeger

2, and Benjamin J. Poltorak

3

1PhD Candidate

Phone (506) 447-3132, fax (506) 453-3538

Email [email protected] 2Associate Professor 3MScFE Candidate

Faculty of Forestry and Environmental Management, University of New Brunswick, PO Box

4400, Fredericton, NB, E3B 5A3, Canada.

ABSTRACT

During mechanized cut-to-length forest operations, forest biomass or brush (tree limbs, tops, and

foliage) is placed as a covering layer (brush mat) on the surface of machine operating trails to

improve trafficability. More recently brush has also been used as a source of renewable energy to

offset carbon emissions from fossil fuels. However, these two uses are mutually exclusive; once

brush is used on operating trails and mixed with mineral soil its calorific value is significantly

reduced and can no longer be used as a bio fuel, while using brush solely for bio fuel will leave

operating trails uncovered and result in severe soil disturbance. To manage the two competing

uses of brush, the objective of this study was to quantify the impact of different brush mat

amounts on machine surface contact pressure by placing these mats over a testing device and

driving a forwarder on top of it. The testing device (load test platform) recorded the loading

below the mats using high capacity load cells. In total, 20 test scenarios were performed with an

8-wheel forwarder to analyze differences in peak pressures recorded underneath brush mats of 5,

10, 15, 20, 25, and 30 kg m-2

each subjected to two, six, and 12 forwarding cycles. Results

indicated a 24% lower average peak surface contact pressure underneath the 30 kg m-2

brush mat

compared to when the machine was driven in direct contact with the load test platform.

Keywords: Biomass, brush, forest machinery, surface contact pressure, soil protection

INTRODUCTION

For the majority of the first half of the 20th

century, anthropogenic disturbances on forest soils

were quite low, both in frequency and magnitude, and were most often limited to the damages

caused by horse traffic or sporadic uses of skidders. Nowadays, to be productive, efficient and

safe, forest operations depend on heavy equipment to process and transport trees. Soil

disturbances are predominantly associated with in-stand timber extraction processes when

machines expose, compact and/or displace mineral soil while transporting timber from the felling

site to a landing adjacent to a hauling road. The Canadian forest industry applies two main

mechanized harvesting methods (cut-to-length, CTL; and full tree) to harvest and transport wood

efficiently and safely from felling site to road side. The gross mass of machinery ranges from 10


to 40 metric tons and exerts nominal ground pressures of 60-180 kPa. This machinery operates

directly on the forest floor, thus having the potential to cause severe soil disturbance (Nugent et

al. 2003). The most frequent and depleting disturbance is soil compaction, which is defined as an

increase in soil density (Craig 2004). By increasing a soil’s mechanical resistance, the

densification process can have a direct impact on plant growth through a reduction of air

exchange and infiltration rate (Forristall and Gessel 1955, Froehlich and McNabb 1984, Corns

1988). Mechanized CTL operations usually require a harvester to fell and process trees and a

forwarder to transport the logs from the machine operating trails to a landing accessible by

trucks. When applying the CTL harvesting method, which dominates in Atlantic Canada,

harvesting equipment travels on trails usually covered by harvest residues (limbs, tops and

foliage of trees) resulting from the processing of harvested trees. This debris acts as a so called

brush mat which helps to disperse machine loads over a greater area, thereby lowering peak loads

exerted on forest soils and, as such, mitigates soil disturbances and related negative impacts on

plant growth (Bettinger and Kellogg 1993, Richardson and Makkonen 1994). However, the high

and volatile price of fossil fuels (oil and natural gas) combined with the need to reduce carbon

emissions because of an apparent climate change, has focused interest of forest stakeholders in

using harvest residues, such as limbs and tree tops, as a source of bioenergy.

A pre-requisite for any viable bioenergy operation is that brush be free of contaminants such as

mineral soil, thus maintaining its full calorific value. To avoid such contamination, operators

delimb trees on the side of machine operating trails to avoid any contact with the machine

running gear and the forest floor, thereby eliminating the possibility of creating a brush mat to

distribute the load (Eliasson 2005). With the absence of brush, a machine’s surface contact

pressure is directly and fully exerted to the ground, leading to potential increases in soil density

and other disturbances. In short, brush used on machine operating trails for soil protection cannot

be re-used for bioenergy generation and using all brush as bio fuel may cause severe soil

disturbances along unprotected machine operating trails. In order to optimize the two competing

uses of brush, knowledge of minimum quantities and qualities of brush for effective soil

protection on machine operating trails is needed. With this knowledge, brush amounts necessary

for soil protection could be allocated and the remaining brush utilized as bio fuel without

compromising forest soil integrity along machine operating trails. The study attempts to provide

necessary information in this respect by addressing the following objective.

Research objectives 1- Quantify the impact of brush mats on forest machinery surface contact pressure.

METHODOLOGY

Testing device To measure and record dynamic loads exerted by forest machines, a load test platform composed

of three separate sections, ramps, in- and out-feed, and load test platform itself was designed and

constructed (Figure 1). The principal part of the structure was the load test platform measuring

4.09 m by 2.54 m for a total area of 10.4 m2

and equipped with 24 high capacity (450 kN) load


cells, each able to measure independent loads on a 30.5 x 30.5 cm resolution (size of a loading

plate). In- and out-feed sections were built at the same height (19.4 cm) as the platform to permit

testing at a zero percent gradient, thus avoiding potential wheel slip and a change in machine

centre of gravity. Both in- and out-feed sections were of sufficient length to allow the full wheel

base of the forwarder to be stopped without having any axle on the load test platform. Following

a pass-over of both forwarder bogie axles over the load test platform, a period of no load

(forwarder resting on in- or out-feed section) was necessary to allow load cells to decrease to a

zero load, thus making it easier to differentiate between the various loading events. Depending on

the required load resolution, load cells could be placed in different arrangements, in so called

layouts, within the platform. Two load cell layouts (clustered and transect) were used during

testing. To specifically quantify the impact of the forwarder, load cells were first positioned in a

clustered pattern (4 clusters of 6 load cells each arranged in 2 adjacent rows of 3 load cells wide),

directly located in forwarder tracks (Figure 1). This load cell layout offered the highest resolution

to capture machine footprints. It was also of interest to understand how the brush could distribute

applied loadings laterally. Therefore, load cells were also installed in a transect layout on two

adjacent rows throughout the full width of the load test platform (12 load cells wide).


Figure 1: Schematic of load test platform. Green circles represent load cells placed in a clustered

layout and blue circles show load cells positioned in the transect layout. Photograph illustrates

the three fully assembled sections (in-feed, load test platform, and out-feed).


Forwarder specifications A 2000 Timbco TF820-D forwarder with a tare mass of 23,500 kg and a load capacity of 20,000

kg was used for all tests (Table 1). This 8-wheel forwarder had two independent bogie axles.

Olofsfors steel flexible tracks with widening plates, weighing 1,100 kg per unit, were installed on

the rear bogie axle during all test scenarios. For better manoeuvrability over the load test platform

and associated in-/out-feed sections, steel flexible tracks were not installed on the front bogie axle

during testing. Based on the Pascal software (FPInnovations' ground pressure calculator),

nominal surface contact pressure underneath the front rubber tired bogie axle was 67.7 kPa

loaded and 64.5 kPa on the rear tracked loaded bogie axle. These surface contact pressures are

based on a full load of 20 metric tons.

Table 1: Timbco TF-820D forwarder specifications.

Nominal surface contact pressure

Tire size

Tare

mass

Load

capacity

Loaded

mass

Front

unloaded

Rear

unloaded

Front

loaded

Rear

loaded

Front

axle

Rear

axle _________________kg

_________________

___________________kPa

____________________

23,500 20,000 43,500 63.2 25.5 67.7 64.5

28L-26†

† rear bogie axle equipped with Olofsfors Eco-track

Sampling procedure

Control parameters

To establish control parameters, the Timbco forwarder was first driven unloaded over and

afterwards stopped on the bare load test platform (without brush cover). The resulting dynamic

and static loads for each axle were recorded by the load cells and stored in a 25 channel data

acquisition system. For data analysis, the loads (kN) recorded by each load cell were converted to

surface contact pressure (kPa) by relating the recorded load to the area of the so called virtual

active zone (930.3 cm2, the size of a loading plate; Figure 1). The control test was replicated with

the forwarder driven over the platform at the same position to verify the accuracy and precision

of the load recording system. The same procedure was then repeated with the forwarder loaded

with 6,680 kg of dry logs. Due to the extended time required to perform all tests scenarios, logs

with relatively stable moisture content were chosen to limit mass fluctuations associated with

varying log water content. As a result, we were not able to fill the log bunk to its full capacity of

20 metric tons before reaching its volume capacity.

Brush mat construction and forwarder traffic

After control parameters were assessed, actual testing with brush of varying quantity and quality

was performed. Fresh softwood biomass (balsam fir and black spruce) imported from on-going

CTL forest operations was stored inside the storage hall to reduce air drying and avoid further

increase of moisture content due to precipitation. Prior to any brush amount test, branches used to

create a brush mat were characterized individually by specie, diameter, and length. Aside from

specie identification, branches were assigned to one out of four diameter classes (x ≤ 10 mm, 10

< x < 30 mm, 30 ≤ x ≤ 60 mm, and x > 60 mm) and to one out of five length classes (y ≤ 1 m, 1 <

y < 2 m, 2 ≤ y ≤ 3 m, 3 < y < 4 m, and y ≥ 4 m). Following classification, branches were weighed


with a digital scale and placed perpendicular to the direction of travel on the platform to simulate

branch positioning of in-wood delimbing by a processor until the target brush amount (Table 2)

was reached.

Table 2: Load test platform testing variables.

Testing surface type Load cell

layout

Target brush amount

(kg m-2

)†

Traffic frequency

per test (cycles)

Replic.

‡

Brush in contact with

loading plates

Clustered 5, 10, 15, 20, 25, 30 2, 6, 12 2

Brush in contact with

loading plates

Clustered 10, 20, 30 2, 6, 12 0

Brush in contact with soil Transect 10, 20, 30 2, 6, 12 2

† green mass

‡ replications

Once a brush mat was completed, the forwarder was driven over the brush covered platform

unloaded and loaded at varying traffic frequencies per test (Table 2). Due to space limitations at

the testing site, the empty forwarder was driven backwards (at a speed of 1.5 km h-1

) into the hall

over ramps, in-feed section onto the platform and further on to the out-feed section until the front

bogie axle was passed the platform. From there, the forwarder was driven at the same speed in a

forward movement, again, onto the platform and in-feed section with ramps outside the hall.

During this traffic, all 24 load cells had the potential to record dynamic loads. Afterwards, the

forwarder was loaded with the same load as in control tests (6,680 kg) and driven over the

platform in the same pattern. These two unloaded and two loaded passes over the brush mat

constituted two forwarding cycles. Therefore, each cycle represented eight individual loadings

(two loadings from each of the four forwarder wheels). For this project, 2, 6, and 12 forwarding

cycles were studied to determine the capacity of the brush mat to attenuate surface contact

pressure over repetitive loadings.

When all traffic frequencies were completed on a specific brush mat, the platform was cleared of

the compressed brush and new brush, undamaged by machine running gear, was used for the next

test. Replacing brush between tests was essential since the properties (strength, compressibility,

yield point, etc.) of branches could have been altered by machine loadings. Due to branch size

variation and potential moisture content differences, the six brush amounts (5, 10, 15, 20, 25, and

30 kg m-2

) tested over the steel covered platform with load cells placed in a clustered layout were

replicated twice (Figure 2). Following theses tests, load cells were re-positioned into a transect

layout over the full width of the platform to quantify the ability of a brush mat to distribute

loadings laterally (Figure 2). With this transect layout, three brush amounts (10, 20, and 30 kg m-

2) were tested directly over the steel covered platform without replication to verify accuracy of

the load cells in their new locations.


Figure 2: Schematic of clustered load cell layout identified by green virtual active zones and

transect layout shown in blue virtual active zones.

After successful testing with the new load cell layout, brush mats of 10, 20, and 30 kg m-2

(each

amount replicated twice) were tested on top of a 20 cm thick layer of mineral soil placed on the

platform to obtain the response of the load cells under a flexible surface (Table 2). Prior to any

tests, the soil layer located on the platform was compacted using a plate compactor for three

minutes. The creation of brush mats on top of the soil layer and the forwarder traffic were

performed the same way as for tests without soil layer described before. After all forwarder

traffic cycles had been completed for a respective scenario, brush was removed from the platform

and discarded and the soil was loosened with a shovel and re-compacted with the plate compactor

before testing the next brush amount.

Statistical analyses Statistical analyses were performed with SPSS and Minitab statistical software. Dependant

variables were load or surface contact pressure readings obtained directly from the load cells e.g.

peak surface contact pressure, sum of peak and second highest surface contact pressure, etc. To

determine the impact of an independent variable (brush amount, log bunk load status (unloaded

and loaded), forwarder traffic frequency, etc.) on the chosen dependant variable, a series of one

way ANOVA's were performed and a probability level of 0.05 was chosen during all statistical

tests.

RESULTS

Impact of brush on machine surface contact pressure Due to the limited length permitted for this article, only results from the rear loaded axle will be

presented. Furthermore, because the area of contact underneath a tire of the forwarder was greater

than the surface area of one loading plate, total wheel load could not be completely captured by a

single loading plate. Therefore, in order to adequately compare machine impacts, the sum of peak

and the second highest surface contact pressures from an adjacent load cell will be presented.

When combining all replicas, loaded rear axle mean surface contact pressures recorded from the

clustered load cell layout tests decreased from 311 kPa during the no brush (0 kg m-2

) scenario to

OUT-FEED

IN-FEED


238 kPa for the 30 kg m-2

brush mat, equalling a 23.5% reduction in peak pressure (Figure 3A).

Modifying load cell position from a clustered to a transect layout did not seem to have an impact

as mean surface contact pressures decreased from 313 kPa with the no brush scenario to 236 kPa

for the 30 kg m-2

brush mat, which translated to a 24.6% reduction (Figure 3B). As a reference

point to brush amount in kg m-2

, pre impact average brush mat thickness was 20, 40, and 60 cm

for the 10, 20, and 30 kg m-2

brush amounts, respectively. A statistical difference of mean surface

contact pressure existed between 0 and 10 kg m-2

brush mats indicating a beneficial effect of

having a minimum of 10 kg m-2

of brush to statistically lower machine surface contact pressure

(Figure 3A-B). A further increase of brush also statistically lowered mean surface contact

pressures up to the maximum brush amount studied of 30 kg m-2

.

Adding a soil layer on top of the steel platform lowered on average mean surface contact

pressures by 25% in comparison to tests done directly over the steel covered platform (Figure

3C). The rear axle exerted lower mean sum of peak and 2nd

highest surface contact pressures

when it was in direct contact with the soil then when the platform was covered with 10 and 20 kg

m-2

brush mats placed on top of the soil. This was a surprising result since we were expecting

brush placed on top of the soil layer to further decrease mean surface contact pressures. However,

upon further investigation we determined that the percentage of the 3rd

highest average surface

contact pressure to the sum of the four load cells wide (half cluster) was much higher when the

machine was in direct contact with the soil than when brush was added. Therefore, combining all

three highest surface contact pressures per loading (indicated with dashed lines in Figure 3C),

showed a decrease of pressure from 270 kPa for no brush to 233 kPa for 30 kg m-2

brush amount.


Figure 3: Mean sum of peak and 2nd

highest surface contact pressures per brush amount (left

ordinate). A different letter indicates a statistical difference at the 0.05 probability level. (Rear

loaded axle only). Percent of mean 3rd

highest surface contact pressure to the half cluster sum

(right ordinate).

Impact of traffic frequency on the ability of brush mats to lower surface

contact pressure Results presented in Figure 3 combined loadings recorded from all traffic frequencies per test. To

determine the ability of a brush mat to distribute loads over repetitive loadings, we averaged

surface contact pressures readings recorded during 1-2, 3-6, and 7-12 loaded forwarder passes

3020100

350

300

250

200

150

Brush density

Me

an

of

Su

m o

f p

ea

k a

nd

2n

d h

igh

est

GP

Chart of Mean( Sum of peak and 2nd highest GP )

3020100

350

300

250

200

150

Brush density

Me

an

of

Su

m o

f p

ea

k a

nd

2n

d h

igh

est

GP


3020100

350

300

250

200

150

Brush density

Su

m o

f p

ea

k a

nd

2n

d h

igh

est

GP

Interval Plot of Sum of peak and 2nd highest GP

3020100

350

300

250

200

150

Brush density

Su

m o

f p

ea

k a

nd

2n

d h

igh

est

GP


Mea

n s

um

of

pea

k a

nd 2

nd

hig

hes

t su

rfac

e co

nta

ct p

ress

ure

s (k

Pa)

Clustered layout over steel

Transect layout over soil

Brush (kg m-2)

0 10 20 30

0 10 20 30

302520151050

350

300

250

200

150

Brush density

Me

an

of

Su

m o

f p

ea

k a

nd

2n

d h

igh

est

GP


302520151050

350

300

250

200

150

Brush density

Su

m o

f p

ea

k a

nd

2n

d h

igh

est

GP


150

200

250

300

350

150

200

250

300

350

200

250

300

350

1500 10 20 3025155


Transect layout over steel

N = 32N = 288

N = 48N = 96

N = 32N = 576

A

B

C

a

ab

cd

ef

a

b

c

d

ab ab

c

20

15

10

5

0

20

15

10

5

0

Mea

n 3

rdhig

hes

t su

rfac

e co

nta

ct p

ress

ure

to h

alf

clust

er s

um

(%

)

20

15

10

5

00

5

10

15

20

0

5

10

15

20

0

5

10

15

20


and identified them as 2, 6, and 12 passes in Figure 4. In the majority of cases, mean surface

contact pressures slightly increased with an increase of traffic frequency and were more apparent

as brush amount increased (Figure 4). There also seemed to be a larger difference of mean

surface contact pressures between two and 12 loaded passes as brush amount increased from 15

to 30 kg m-2

.

Figure 4: Mean sum of peak and 2nd

highest surface contact pressures per brush amount and

loaded forwarder passes. A different letter indicates a statistical difference at the 0.05 probability

level per forwarder passes and brush amount. (Rear loaded axle only)

Brush density

Forwarder cycles

3020100

12621126211262112621

350

300

250

200

150

Me

an

of

Su

m o

f p

ea

k a

nd

2n

d h

igh

est

GP


Brush density

Forwarder cycles

3020100

12621126211262112621

350

300

250

200

150

Su

m o

f p

ea

k a

nd

2n

d h

igh

est

GP


Brush density

Forwarder cycles

3020100

12621126211262112621

350

300

250

200

150

Me

an

of

Su

m o

f p

ea

k a

nd

2n

d h

igh

est

GP


Brush density

Forwarder cycles

3020100

12621126211262112621

350

300

250

200

150

Su

m o

f p

ea

k a

nd

2n

d h

igh

est

GP


Brush density

Forwarder cycles

302520151050

12621126211262112621126211262112621

350

300

250

200

150

Me

an

of

Su

m o

f p

ea

k a

nd

2n

d h

igh

est

GP


Brush density

Forwarder cycles

302520151050

12621126211262112621126211262112621

350

300

250

200

150

Su

m o

f p

ea

k a

nd

2n

d h

igh

est

GP


N = 32N = 96

Transect layout over steel

Transect layout over soil

Mea

n s

um

of

pea

k a

nd

2n

dh

igh

est

surf

ace

con

tact

pre

ssu

res

(kP

a)

Forwarder passesBrush (kg m-2)

150

200

250

300

350


150

200

250

300

350

150

200

250

300

350



0 5 10 15 20 25 30

0 10 20 30

0 10 20 30

N = 192N = 288

N = 48N = 16N = 32N = 48

N = 32N = 48N = 96N = 144

a aaa aa

abb

a a a

aab b

ab bc

aa a

a a a

a aa

2 6 12 2 6 12 2 6 12

2 6 12 2 6 12 2 6 12

ba a

a

bbc

ba

c

2 6 12 2 6 12 2 6 12 2 6 12 2 6 12 2 6 12

A

B

C

2

5

4


DISCUSSION

Impact of brush on machine surface contact pressure Previous studies had focused on determining the impact of machine traffic over brush on soil

physical conditions mainly through the assessment of soil mechanical resistance and density

changes between pre- and post-impact measurements. Even though this study concentrated on

surface contact pressure recorded underneath different brush amounts, our results offered similar

trends to what was reported by Han et al, (2008) where a 15 kg m-2

brush mat statistically

lowered penetration resistance on a soil of medium moisture condition at a 10 cm depth. Poltorak

(2011) also reported benefits of using a 20 kg m-2

brush mat to statistically lower soil density

increase (compaction) caused by mechanized forest operations compared to machine traffic

directly over bare soil.

In both scenarios where brush was in direct contact with loading plates, an increase in brush

amount lowered mean sum of peak and 2nd

highest surface contact pressures. However, the same

could not be concluded for the soil covered scenario since lower mean surface contact pressures

were observed when the forwarder was driven directly over the soil compared to when the soil

was covered with 10 and 20 kg m-2

of brush. We assume a reason for this to be the increased

surface contact area between both track and tire to the soil in combination with the ability of a

soil to distribute loads diagonally within the 19 cm thick soil horizon. However, expanding the

zone of analyses from two to three load cells wide gave similar results (i.e. lower pressures as

brush increased from 0 to 30 kg m-2

) as to when the forwarder was driven over steel rather than

soil covered platform. The amount of brush required to protect forest soils is largely dependent

on site characteristics (soil moisture, soil texture, organic content, stand type, etc.) and is

therefore difficult to predict. However, based on the results obtained from the load test platform,

we would recommend leaving a minimum brush layer of 10 to 15 kg m-2

on sensitive sites to

lower machine surface contact pressure. Operating heavy equipment on highly susceptible soils

(silty clay, clay at high water contents) could require the maximum brush amount tested of 30 kg

m-2

or more depending on the number of passes required to extract the timber.

Impact of traffic frequency on the ability of brush mats to lower surface

contact pressure In forest operations, the frequency of off-road machine traffic is a function of harvested wood

volume and its location throughout a cut block, and can vary between a single cycle to 12 cycles

or more near a main landing where wood is being accumulated. For this reason it was of interest

to quantify the response of a brush mat to lower machine surface contact pressure over repetitive

loadings. The ability of a brush mat to lower machine surface contact pressure was reduced when

traffic frequency increased from two to 12 loaded forwarder passes. As a brush mat was being

compacted by repetitive loadings of the forwarder, branches were broken by the machine running

gear which decreased the overall strength of the mattress. The difference between mean surface

contact pressures recorded after two and 12 passes increased with increasing brush amount. This

would mean that brush of higher amounts (20-30 kg m-2

) were more beneficial in reducing mean

peak surface contact pressures at lower traffic frequencies. Nevertheless, these high brush


amounts were still more suitable at distributing machine loads following 12 loaded passes than

were the thinner 5 and 10 kg m-2

brush mats after just two passes.

CONCLUSIONS

This study attempted to quantify the impact of different brush amounts as a covering layer on

machine surface contact pressure with the use of a load test platform. Brush mats >10 kg m-2

were proven to be beneficial in statistically reducing surface contact pressures of an 8-wheel

forwarder compared to a no brush scenario. Furthermore, increasing traffic frequency from two to

12 passes caused brush mats to be slightly less efficient at distributing applied loads but remained

beneficial at the highest traffic frequency tested. The competing uses of brush between acting as a

mattress on trails to lower machine impacts and as a potential source of bio fuel for a clean

source of energy will only get more severe with time. However, leaving brush on machine

operating trails remains an essential and proactive method of mitigating soil disturbances during

mechanized forest operations and needs to be an integral part of best management practices.

ACKNOWLEDGEMENTS

This work was financially supported by the Natural Sciences and Engineering Research Council

of Canada (NSERC), FPInnovations, the New Brunswick Innovation Foundation (NBIF), the

University of New Brunswick Research Fund, and the New Brunswick Department of

Transportation. Forwarder and operator were generously provided by Debly Forest Services Ltd.

Assistance with statistical analyses and programming tasks was obtained from Dr. William

Knight from the Applied Mathematics Department at the University of New Brunswick.


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(11/12): 59-64.

Corns, I.G. 1988. Compaction by forestry equipment and effects on coniferous seedling growth

on four soils in the Alberta foothills. Canadian Journal of Forest Research. 18: 75-84.

Craig, R.F. 2004. Soil Mechanics. Seventh edition. Spon press. British Library Cataloguing in

Publication Data. 447 p.

Eliasson, E. 2005. Effects of forwarder tyre pressure on rut formation and soil compaction. Silva

Fennica. 39 (4): 549-557.

Forristall, F.F., and S.P. Gessel. 1955. Soil properties related to forest cover type and productivity

on the Lee Forest, Snohomish County, Washington. Soil Science Society of America

Proceedings. 19: 384-389.

Froehlich, H.A., and D.S. McNabb. 1984. Minimizing soil compaction in Pacific Northwest

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North American Forest Soils Conference. Knoxville, TN.

Han, H.S., D. Page-Dumroese, S-K Han, and J. Tirocke. 2006. Effects of slash, machine passes,

and soil moisture on penetration resistance in a cut-to-length harvesting. International Journal of

Forest Engineering. 17 (2): 11-24.

Nugent, C., C. Kanali, P.M.O. Owende, M. Nieiwenhuis, and S. Ward. 2003. Characteristic site

disturbance due to harvesting and extraction machinery traffic on sensitive forest sites with peat

soils. Forest Ecology and Management. 180: 85-98.

Poltorak, B.J. 2011 (in revision). Mitigating soil disturbance in forest operations. MScFE thesis.

University of New Brunswick. Canada. 107 pages.

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