Cyclic loading effects and stability assessment of trees and stumps used as anchors in cable yarding operations

Swing yarders in running skyline configuration using either grapples or chokers represent the most common configuration for cable yarding in coastal British Columbia. In this context, whole-tree logging and short work cycles lead to heavy and repeated loads applied to the trees and stumps when used as anchors for the rigging cables. Moreover, increased harvesting of second-growth forest stands leads to the unavailability of large and safe trees, thus introducing new challenges to identify suitable anchors and potentially increases safety risks of cable yarding operations.

The present study aimed to collect evidence of the mechanical response provided by anchors for typical cable yarders used in second-growth harvesting in coastal British Columbia, and test the suitability of innovative techniques for the stability assessment based on the relationship between the anchor root-plate rotations and the related applied tensile forces. A conspicuous dataset could be derived from all the surveys, storing anchor rotations from a total of 1522 work cycles of which 1224 work cycles included also tensions measurements. The methodological approach was proven effective for monitoring different rigging configurations giving proof that repeated loading affects the stability of a tree/stump which can rapidly change over few hours of active yarding operations. Acquired data proved also that comparing theoretical failure limits with anchor rotations could be a valid approach however a considerable amount of species-specific data from tree pulling tests is required.