Geogrid for unsealed forest roads: installation considerations and bearing capacity testing in New Zealand

ABSTRACT

This study established 10 field trials on corporate forest roads in New Zealand to demonstrate geogrid installation procedures and test for differences in bearing capacity, hereafter referred to as road strength, for road segments with and without geogrid reinforcement. The primary objective of this research was to determine if thinner aggregate surface layers could be used in conjunction with geogrid reinforcement without significant reductions in road strength. Each trial consisted of three 25-m long road segments randomly configured with the following pavement designs: (1) Control section that consisted of a single aggregate layer overlying a compacted subgrade soil; (2) Geogrid reinforcement that used the same aggregate thickness as the Control; and (3) Geogrid + Reduced Aggregate treatment that used a thinner aggregate layer. Road strength was measured with a Clegg Hammer for the prepared subgrade, finished road surface (i.e. before traffic), and after one winter of log truck traffic. Overall, there were no clear differences in road strength among treatments before or after trafficking. Several factors related to studying operational forest roads are thought to have contributed to this finding, including relatively low traffic volumes (285–1220 loaded trucks) and variability in aggregate thickness within and among sites. Geogrid-reinforced roads may perform better over time and with more traffic. In terms of cost, this finding supports the common practice of simply using thicker aggregate layers to achieve a desired strength as long as aggregate is cheap, local, and readily available.

KEYWORDS:

• Forestry
• unpaved road design
• geosynthetics
• road-bearing capacity
• cost-benefit analysis

Acknowledgements

Simon Fairbrother provided technical guidance for the field work. We would like to thank Maccaferri Ltd (www.maccaferri.co.nz) for providing both technical knowledge as well as supplying Geogrid for the trial. We also thank the forest management companies and professionals that helped with the installation. Finally, Papa Masseck Thiam of FPInnovations and two anonymous reviewers provided feedback that significantly improved the quality of this manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.