Rainwater Infiltration into Slope Soil Restrained by Capillary Barrier of Soil and Plant Cover over Soil Surface

土の毛管遮断機能（キャピラリー・バリア）を利用することにより、地盤に浸透した降雨水を表層部で効果的に捕捉し、斜面末端に流下させることができる。これにより、地盤深部への雨水浸潤が低減し、土の強度の低下あるいは自重の増大を抑制することができる。この雨水浸透抑制効果により、自然地山斜面やため池堤防斜面を対象とした斜面すべり減災技術の開発が可能となる。本研究では、キャピラリー・バリアの基本性能を確認するため、圃場斜面に砂層と礫層の2層構造の地盤を造成し、キャピラリー・バリアのもつ雨水浸透に対する抑制効果を調べた。キャピラリー・バリアの表層部は砂層である。雨滴衝撃による表面浸食が起きやすく、性能が短期間に劣化する可能性が大きいため、植生被覆による法面保護が必要となる。本研究では、合わせて、植生斜面における土中水分動態の観測を行い、降雨浸潤の抑制に及ぼす植生被覆の相乗効果についても調べた。圃場で造成した砂層と礫層の層構造地盤における土中水分動態のモニターにより、雨水浸透に対するキャピラリー・バリアの抑制効果を確認することができた。また、植生被覆により降雨の斜面浸透が効果的に抑制されることから、キャピラリー・バリアとの相乗効果を定量化していくことが今後の重要な課題となることを指摘した。これにより斜面減災技術の実務性は格段に増すと期待される。Capillary barrier is a simple soil layer system which is composed of a fine soil layer underlain by a coarse soil layer. Water infiltrated into soil is halted just above a boundary surface between the fine and coarse soil layers due to a physical difference in water retention characteristics of both layer soils. When the boundary surface between soil layers tilts, water retained flows along the boundary surface, and a downward movement of infiltration water into the soil below the boundary surface is restricted. Waterproof by the capillary barrier with a tilted boundary surface decreases rainwater infiltration into soil, and keep a soil slope stable because of small deterioration of soil strength due to wetting as well of small increase of soil mass. This special and attractive feature leads to a possible adoption of the capillary barrier as effective measures for slope protection of natural soil or embankment. In the study soil moisture contents were measured in sand soil layer underlain by gravel layer constructed in a natural sand slope. After a trench about 50cm wide was excavated in the natural sand slope, gravel was compacted into the layer of 10cm thickness in the trench, and then the sand excavated was put back and compacted into about 20cm thick layer. Inclination of the boundary surface between the soil layers was 20 degrees. The sand is classified into Sand with less-5% fine and coarse fractions, and the gravel is siliceous with typical particle size of 4 to 7mm. Dielectric probes ECH2O were employed to measure volumetric moisture contents at 15, 10, 5 and 1cm above and 1cm beneath the boundary surface between the soil layers. The volumetric moisture contents in the soil were measured during four months together with precipitation. The results showed effective waterproof by the capillary barrier of soil