Landslides are common in the Oregon Coast Range because of the combination of steep slopes, shallow soils, and prolonged and sometimes intense winter rainfall. Historically, road-related landslides have been the dominant source of management-caused erosion. As road construction and maintenance practices have improved, however, road-related landslides have diminished in importance, and the emphasis has shifted to landslides elsewhere in harvest units.
In-unit landslides result, in theory, when the roots of harvested trees decay, reducing soil reinforcement and thus soil strength. The goal of this research is to develop a model to predict the increase in soil strength attributable to root reinforcement. The objectives of this project are 1) to develop a process-based, mechanistic model of root reinforcement in shallow forest soils, and 2) to carry out a parameter study with the model and compare its results with data available in the literature.
The conceptual development of the analytical model was reported in COPE Report 2(2):4-7. The concept for the model is based on reinforced-earth theory, with roots treated as reinforcing elements in the soil and modeled mathematically as axially and laterally loaded piles. The differential equations governing the behavior of the reinforcing roots are solved by means of a finite-difference approximation.
The analytical model is fully programmed and functioning. The model is now undergoing a sensitivity analysis or parameter study to determine how it will perform over a range of soil and root properties. The model results will subsequently be compared with values in the technical literature.
This project will help increase our understanding of how roots reinforce shallow forest soils, and will therefore be of value in managing landslide-prone terrain. The model should also be valuable for those interested in investigating the effect of vegetation type and density on the stability of high-risk sites and assessing the effects of alternative silvicultural practices on soil strength. Present slope-stability models that require information on root reinforcement could be improved by incorporating information from this model.
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