Successful Felling Along Powerline Corridors

The PF Olsen Viking Timber Management (VTM) team, based in Rotorua, has recently undertaken two significant powerline felling projects of note. These operations had different challenges and constraints, but through a collaborative planning and management approach that involved the harvesting contractor both were completed successfully without incident.

The first project involved the clearance of approximately 3km of trees adjacent to the 110kV transmission main that runs between the Aniwhenua and Matahina hydro dams in the Eastern Bay of Plenty. The site was located in Tuararangia Forest and work had previously been deferred due to safety concerns about felling the trees within the fall zone of the high tension line. PF Olsen met with Nova Energy at the beginning of the year to discuss the issues. Meeting face to face with the Nova Energy team led to the development of a plan to progressively fell the powerline corridor during a series of lines shut-downs.

Nova advised PF Olsen of its planned maintenance shuts and other periods where it could shut the line down to allow safe tree felling. PF Olsen scheduled the harvest of the affected areas based on these dates. In consultation with the harvesting contractor (Rodney Hubbard of H&R Logging Limited) operations were planned to maximise the window of opportunity to safely fell trees during the shut down periods. The work involved establishing safe zones along the working corridor where multiple fallers could work simultaneously.

The second site was a 2km strip of trees located on the boundary of Okahu Forest located at Mourea at the Northern end of Lake Rotorua (see photos below). This site had an 11kv powerline running along the majority of the forest's eastern boundary. In addition to being within the powerline hazard zone the trees were also close proximity to SH33.

The hazard: trees within falling distance of live powerlines and the State Highway.

Unfortunately discussions with the owners of these lines were not as fruitful as those with Nova. Network operating constraints meant the lines could not be shut down easily without significant modification of the network. PF Olsen then considered the risk of felling these trees under a 'single shot' reclose. We considered this less than ideal as it still entails a degree of risk. As noted the last edition of Wood Matters there is still a risk of a potentially fatal electrical shock should a line strike occur under a single shot reclose.

Taking a moment to consider the risks and plan the action.

Machine assisted felling adjacent to dual hazard of powerlines and State Highway.

In progressing the option of falling next to live lines it was critical to ensure that the required level of control could be implemented to ensure trees would be fallen away from the lines. In making this decision it was critical that the contractor be involved in the planning process and be comfortable that every tree could be safely fallen away from the lines.

In planning both jobs we introduced a defined management system starting with rating the trees of concern. Each tree was rated from 1 to 5:

  1. The tree can be fallen without concern outside the two tree length zone.
  2. The tree is within the two tree length zone and outside the falling length plus 4 meter zone.
  3. The tree is within the danger zone (falling length plus 4 meters) but considered easy to fall - faller must have a machine available to him if required to get the tree over safely.
  4. The tree is within the danger zone, but assessed as difficult (includes edge trees) - the faller must use a machine to push/pull every tree over.
  5. The tree is within the danger zone and cannot be safely fallen with normal machine assisted felling - to be left standing for a specialist to take down.

In executing both projects it was agreed with the contractor that machine assisted felling using the grapple excavator afforded the greatest level of control. The consensus was that a Waratah or similar processing head does not provide the same level of control.

Controlled felling - success achieved through careful planning and skilled execution.

PF Olsen would like to acknowledge the skill and commitment of Rodney Hubbard and his team at H&R Harvesting in helping ensure success of these operations. We would also like to acknowledge the support of Craig Rowe and the team at Nova Energy in organising the power shut downs.