Abstract
The rainfall infiltration process on steep slopes mantled with a pumice layer was studied by artificial rainfall experiments. Two areas in central Hokkaido were selected as study sites: the Shiraoi site on a pyroclastic flow plateau and the Monbetsu site on Neogene Tertiary sedimentary rock. On both sites, the slopes were covered with 50cm thick Us-c layers, which were pore rich volcanic-gravel brought from Mt. Usu in 1663 A. D., and having underlying low permeable humus layers. On the Shiraoi site, the pore pressure in the pumice layer was raised less than 10 gf/cm2 by intensive rainfall of 100 to 200mm per hour. The seepage was collected mainly from the ground surface and humus layer but very little from the pumice layer. An attempt to pour water directly into the pumice layer yielded no seepage from any horizontal layer. On the Monbetsu site, the pore pressure was raised to 30 gf/cm2 by hourly rainfalls of more than 150mm, and a large amount of seepage was observed from the regolith layer overlying the bedrock, The seepage flowed out through the matrix in the early stage, but it was produced from the soil pipe which was formed after the slumping on the cut slope of the pit. These results imply that most of the rain-water infiltrated downward through the pumice and the buried humus layer without forming a throughflow. The difference of the regolith thickness and the structure of the overlying bed-rock contributes different patterns of infiltration in the two regions.
