Examining effects of tree roots on shearing resistance in shallow landslides triggered by heavy rainfall in Shobara in 2010

Abstract

The long-term accumulation of precipitation was accompanied by a seasonal rain front (known as the Baiu front) covering wide areas of western and central Japan during the period 11–16 July 2010. In Shobara city, Hiroshima prefecture, more than a thousand shallow landslides occurred within a very limited area (9 km² = 3 × 3 km) during the afternoon of 16 July 2010, claiming one fatality. In the source areas on steep slopes, meanwhile, volcanoclastic soil slid due to the heavy rainfall, then fluidised to flow over long distances. Landslides appeared to occur on all slopes; not only young forests but also mature forests. A field investigation revealed a case where slides occurred in young Japanese cypress (Chamaecyparis obtusa) forests, but surface water only flowed over the ground in mature Japanese cypress forest sandwiched by the young forests, while the side ends of the slides reached the edge of the mature forest. To examine the differences between both forests, simplified cone penetration tests, tests on tree root distribution in trenches, tree root pull out tests, soil samples, and geotechnical soil tests were all conducted. Although the number of lateral roots found in the trenches differed little, the root diameters in the mature forest exceeded those in the young forest. The thicker and stronger root in the mature forest would, thus, help prevent shallow landslides.