Behavior of vegetation protection slopes during freezing and thawing

冬季の凍上と春季の融解沈下を繰り返す地域の法面では，融解期に表層すべりによる被害が多く発生する。凍上の発生方向は熱流方向に一致し，融解沈下の方向は鉛直下方に作用する重力の影響を受ける。この時，法面では凍上が発生する方向と重力の方向が異なるため，凍上した地盤の表面は元の位置に戻らず凍結前の位置より斜面下方に移動する。この移動の毎年の累積が，法面の崩壊を誘発すると考えられる。しかし，凍結・融解過程における実斜面の挙動をとらえた報告はほとんど見られず，合理的根拠に基づいた融解時表面すべり崩壊の対策工法は未だ確立していないのが現状と言える。本研究では，北見工業大学構内の盛土法面で，植生方法の異なる4つの試験区間を設け，5シーズンに渡って凍結・融解過程における法面の挙動を実測した。結果として，1）凍結・融解過程における有限長の法表面の凍上変位，融解沈下及びそれらの成分として現れる移動量が実測値として定量的にとらえられた。2）法面のすべり崩壊を誘発すると考えられる法面下方への移動は融解沈下時に発生し，その大きさは発生凍上量に加えて，植生の成育状況，さらには植生補強の有無によることなどが明らかになった。 Slopes are often affected by slipping of the surface layer during the spring thaw through the continual cycle of frost heaving in winter and subsidence due to thawing in spring. When frost heave occurs, the ground is lifted to the same direction of heat flow, while during thawing, the force of gravity acts downward. This means that the direction of lifting due to frost heave and the direction of gravitational force acting during subsidence are different on a slope. Consequently, the ground moved by frost heave does not return to its original position, instead it is moved to a position further down the slope. This movement due to the repeated freeze-thaw process is believed to trigger the collapse of the slope.<br>In the present study, four experimental plots planted with different types of vegetation were set up on the slope of an embankment within the campus of the Kitami Institute of Technology and the behavior of these slopes during the freeze-thaw process was observed over five seasons. The following results were obtained; (1) the behavior characteristics of the surface of the slopes in the freeze-thaw process of seasonally frozen ground were measured; (2) it was shown that movement down the slope, which is believed to trigger collapse by slipping, occurred during the thaw period and that the amount of movement depended on the extent of the frost heave, the vegetation cultivation conditions and the presence of reinforcing vegetation