FORREX Forum for Research and Extension in Natural Resources
Remote-operated jacks
A safer way to fall trees manually
by C. Kevin Lyons, University of British Columbia
M
anual tree falling is a dangerous career. In the
last two years alone, WorkSafeBC has documented 30 serious manual falling incidents. The
overwhelming hazards to a manual tree faller are
falling trees and flying debris. Finding ways to reduce hazards in this profession will greatly improve
working conditions and ensure fallers return home
at the end of the day.
In 2007, WorkSafeBC funded a research project to
analyze the mechanics of using a remote-operated
lightweight hydraulic jack to assist a manual faller.
Traditionally, a faller can only leave the work space
around the base of the tree once the tree stem has
been cut and wedged to the point where it begins
to displace from the stump. The remote-operated
jack system would allow the faller to leave sufficient holding wood to ensure the tree stem remains standing until the faller exits the hazardous
region around the base of the tree. The remote-operated jack, which would be pre-set in the backcut,
would then be engaged to force the tree stem to
displace from the stump.
When a tree commits to falling under its own
weight, the tensile stress increases on the backcut
side of the tree and fibres can be pulled out of the
tree, while the compressive stress increases on the
undercut side of the tree and cells can be crushed.
This research proposes using a design stress to
predict the point when a tree will commit to falling under its own weight. Existing data suggests
33 Mega Pascals (MPa) of pressure is a reasonable
design stress that accounts for compression in the
holding wood at the time when the tree commits
to falling under its own weight. This was supported
by calculating the maximum compression in the
holding wood from a small sample of test trees.
A “holding-wood design procedure” was then
developed to meet the objectives of this research.
. . . continued on page 19
Ionut Aron photo
Florian Noll photo
Photo, far right: Hydraulic
flange spreader.
Bottom photo: A traditional backcut with
manually driven wedge.
Preliminary research studied the mechanics of a
standing tree, and how the structure changes as it
is being cut. First, researchers needed to determine
the amount of holding wood required so a tree
stem does not displace from the stump as the faller
is cutting the tree. Second, they needed to develop
a mechanism to remotely initiate displacement
of the tree stem from the stump when the faller is
in the clear. This research assumed that trees are
to be felled in the direction of their lean. A leaning tree requires less jacking force than a vertical
tree; therefore, the research shows that lightweight
hydraulic jacks could be used as the mechanism
to initiate displacement of the tree stem from the
stump
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FORREX Forum for Research and Extension in Natural Resources
University of Victoria
Programs in natural restoration
for working professionals
The Native Species
and Natural
Processes Certificate
is a new program
specifically designed
with professional
development needs
in mind.
T
he University of Victoria is offering two certificate and diploma programs in natural restoration that will be of interest to natural resource
professionals who are working in, or would like to
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management.
The Restoration of Natural Systems Certificate and
Diploma is an award-winning program which has
been operating for 12 years to serve the needs of
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ecosystems in British Columbia, and with a conceptual framework for restoration projects, which is
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•
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Field Study in Environmental Restoration
Advanced Principles and Concepts of Ecological
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Ecorestoration Strategies: Case Studies
For more information, see www.uvcs.uvic.ca/
restore
The Native Species and Natural Processes Certificate is a new program specifically designed with
professional development needs in mind. When
fully implemented, the program will be offered in a
distance format with courses offered on an annual
rotation. The program is intended for professionals
working in restoration or related fields, and offers
advanced-level training in topics such as systems
approaches to restoration, invasive species, and the
built environment. Our next course, which will be
offered in 2009, will be Invasive Species Management (Dates TBD). For more information, see www.
uvcs.uvic.ca/nature
Both of these programs are offered in partnership
between the School of Environmental Studies and
the Division of Continuing Studies. You can take a
few courses or take the full program. For more information, contact: Janet Pivnick at jpivnick@uvcs.
uvic.ca or 250-721-8463.
Field testing is scheduled for this year
. . . continued from page 18
This design procedure had two goals. First, the
holding wood had to be sufficiently weak to
permit a 44.5 kilo Newton (kN) jack to produce the
33 MPa design stress in the holding wood. When
considering this first goal, and assuming the tree is
initially vertical, the required width of the holding
wood is significantly related to the tree diameter
at the stump height, but is not significantly related
to the total height of the tree. Second, the holding wood had to be strong enough to ensure
the tree does not move while the falling cuts are
being made, given naturally occurring destabilizing loads such as an initial lean. When considering
this second goal, the required width of the holding
wood is significantly related to the total height of
the tree, but is not significantly related to the tree
diameter at the stump height. Since the second
goal is considering a leaning tree, the horizontal
displacement of the center of mass is significant
and this is highly correlated with the total tree
height.
This research indicates that for some combinations
of diameter at breast height and total tree height,
a relatively small initial lean could compromise the
ability of the holding wood to maintain static equilibrium while the jack is being set in the backcut.
Field testing of the proposed holding wood design
procedure has been funded by WorkSafeBC, and
will be conducted in 2009. This project will determine how much holding wood is needed to resist
naturally occurring destabilizing forces before the
jack is engaged, yet still make the tree weak enough
so a small jack can remotely initiate displacement
of the tree stem from the stump.
For a link to the full journal article or an update on
the field testing contact Dr. C. Kevin Lyons at
[email protected]
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