Tree roots with an underground lateral network play a very important role in slope stability. Since failures occur in three dimensions, lateral roots fully demonstrate their resistance against failure. This paper describes the resistance force that ƒąC lateral roots provide. Intensive field investigations and a model description were made. Since the field data show that the root strength ƒąC varies considerably among the surveyed plots, predicting the ƒąC from the plot information such as tree species, tree ages, slope and soil thickness can be difficult. To assist with prediction, this paper proposes a ƒąC model to calculate the resistance force shown by trees of any age when it is given information about the species, forest operation history, and initial tree density. The ƒąC model constitutes four components: a forest yield calculation table; Karizumi's root volume regression curve; a root distribution model proposed by Tsukamoto; and a summation process of ƒąC. The model highlights the importance of a particular parameter, which is the weight ratio of lateral roots to full roots. The model shows that the ratio of lateral roots directly affects the size of ƒąC. By inspecting the field investigation data and the output of the ƒąC model, it has been proved that the change of ƒąC over time is well represented by a simple logarithmic curve which has only one coefficient multiplied by the logarithmic term itself. In comparing logarithmic curves to the output of the ƒąC model, the logarithmic coefficient corresponds well to the weight ratio of lateral roots. The variance of ƒąC shown by the field data is lumped into the coefficient. It has the potential to describe lateral root strength in a simple manner and could be a good index of ƒąC. Also, when considering a forest thinning operation, the change in lateral root strength might easily be represented by the coefficient.
Key wordsFslope failure, tree root, lateral root, weight ratio of lateral roots, logarithmic coefficient

Key wordsFslope failure, tree root, lateral root, weight ratio of lateral roots, logarithmic coefficient