The anchorage capacity of Pinus radiata guyline stump anchors used in cable logging operations

Abstract

Testing was carried out to assess the anchorage capacity of Pinus radiata guyline stumps and to evaluate the effect of two contrasting soil types on stump anchorage capacity. Testing was performed on predominantly clay soils at Whitford Forest, Auckland, and sandy loam soils at Mangatu Forest, Gisborne.

Diameter at breast height (D.B.H.) was found to be the best indicator of stump anchorage capacity. Diameter at stump height, inside bark (D.S.H.) was also found to be a good indicator given a standard stump height.

Stump displacement was recorded under failure loads. An arbitrarily chosen 20 mm was selected to lie near the elastic limit for stumps at both sites. Stump anchorage capacity at 20 mm of displacement (S.A.C. (20 mm displacement)) was approximately two thirds of peak load at both sites studied. An approach for increasing the safety of cable logging operations based on the amount of displacement associated with the elastic limit, was discussed.

Two approaches for increasing the safety of the guyline and stump anchor system, based on the D.B.H. versus peak load relationships, were discussed. These were a probability based approach and a factor of safety (F.O.S.) approach.

The merits of each of these approaches were assessed on the grounds of safety and workability, with reference to the Whitford Forest and Mangatu Forest soil types. Reasons for selecting the probability based approach as the preferred guyline-stump anchor design approach are given.