Study on evaluating method using root pull – out resistance for tree roots effect to prevent shallow landslide

Ryota KAKETANI, Hideki TAKIZAWA, Izumi KOSAKA, Waka SONOHARA, Iturou ISHIGAKI and Kazutoki ABE

Abstract

The effect of the root system on preventing shallow landslides is exerted when roots are pulled out by means of surface soil displacement. Accordingly, resistance may be exerted immediately against the displacement of surface soil or after any significant displacement occurs due to differences in root shape or the hardness of soil in which the roots grow. In addition, the radial growth of roots with the rootstock as a nucleus will result in the roots being pulled out in various directions when a shallow landslide occurs, and that may influence the manner of developing root resistance. That being the case, this study conducted a pull‐out test of Cryptomeria japonica roots with the objective of clarifying the effect of differences in the pull‐out displacement and pull‐out direction (pull‐out angle) of roots on the manner of development regarding pull‐out resistance and pull‐out displacement, and studied a method of evaluating the effect of the root system on preventing shallow landslides in consideration of the pull‐out displacement and pull‐out angle. In this study, both pull‐out displacement and pull‐out resistance were measured, with the pull‐out angle being varied from0degrees to90degrees. The study revealed that (1) the maximum pull‐out displacement value differs among the respective roots, (2) the pull‐out resistance of vertical roots with the same diameter increases in line with a higher pull‐out angle, and (3) the maximum pull‐out displacement tends to increase in line with a higher pull‐out angle. Based on the findings above, we proposed a method of calculating the resultant force of pull‐out resistance per the displacement of several roots by calculating the resistance according to the pull‐out displacement exerted by one root, at intervals of1mm. The use of this method to evaluate the effect of roots on preventing shallow landslides revealed that it is approximately 40% of the effect evaluated by using the conventional method of summing up the maximum pull‐out resistance.

Key words

tree root, pull‐out test, pull‐out displacement, shallow landslide