The traction effect of lateral roots of Pinus yunnanensis on soil reinforcement: A direct in situ test

A traction effect by lateral roots is one way in which roots can contribute to lateral in-plane reinforcement of a shallow soil mass. In contrast to the effect of vertically-extending roots, whereby soil is reinforced by an increase in its shear strength, the traction effect reinforces the soil by enhancing the tensile strength of the rooted soil zone. To verify whether or not a traction effect exists in the root system of a pine forest (Pinus yunnanensis French) in the Hutiaoxia Gorge, SW China, and to investigate the magnitude of this effect, a direct in situ test was conducted at two sites in the gorge. The results from the two sites showed that, in the topmost soil (0 similar to 20cm), the lateral roots are able to provide a tractive force of up to 421.19 similar to 561.05 N over a vertical cross-section area of 20x50 cm(2), or an increase in the pulling resistance of the rooted soil by 33.2 similar to 37.58%. This is equivalent to a tensile strength increase of the shallow rooted soil by 4.2 similar to 5.6 KPa. The test results imply that, together with the pine's vertical roots, which anchor the shallow rooted soil zone to the deep and more stable soil mass, the lateral roots of the pine trees, through their traction effect, are able to mitigate against shallow instability in the forested slopes, such as shallow slide and creep, to a certain degree.