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樹木根系には，斜面崩壊を緩和もしくは防止する働きがあることが認められている．従来，その働きの機構はくい作用と緊縛作用の二つで説明されていたが，竹下は根系にはそれらとは別の作用もあるとして，これをアーチ作用と名付けた．両側に尾根型斜面がある時，その内側にある斜面では根系が相互に接触せず，また地下深部の安定層に適していない場合でも，この作用によって斜面を安定化しているとしている．本報はこのアーチ作用の...様式を実験的に追究したものである．その結果，両側に支持土層を設け土中に根系模型を埋設した土層は，これらを有しない場合に比べ著しく安定化することが確認され，アーチ作用が存在しうる可能性が示唆された．
An additional action of tree root system called 'Arch Action' has been proposed concerning the resistance against occurrence of slope failure. In this action, longitudinal earth pressure is accepted by root block which is composed of root system with surrounding soil and the pressure is propagated sidewards to adjacent block one after another. Thus, an arch-shaped line or more lines of root blocks are formed in the slope. At the both ends of a line, stable ridges accept these pressures. And a line of root blocks supports upper unstable soil layer in the slope. This action can be compared to the action of arch dam receiving water pressure. In this paper, model experiments were made to ascertain whether this action exists or not. In some cases of experiments, columnar root blocks made of cement mortar were arranged as an arch-shaped line with openings in between and no contact to the bottom of soil layer. This arrangement of root system can be seen usually in actual forest. Moreover, side supports which are compared to stable ridges in the real slopes were set both sides of soil layer. Slope failure occured by several stages with changes of slope angle. Earth pressures measured at the lower part of side supports (end wall and side wall) and the relations between slope angle at the remaining soil weight on the slope after failures were analyzed. Consequently, as for the resistance, the followings were clarified based on the remaining soil weights. The case having a line of root blocks without side supports was more effective than that of having no root block. The case having side supports without root blocks was more effective than that of having no side support. The case having a line of root blocks with side supports was the most effective one. These tendencies are reasoned by the earth pressures that sideward pushing forces caused by shearing forces generating in longitudinal sections of soil layer are accepted by side supports eventually and that root blocks strengthen soil layer much more against shearing forces.続きを見る
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