The Effect of Forests in Slope Failure Prevention at the 1997 Kagoshimaken-Hokuseibu Earthquake

Abstract

The effect of forests on slope failure prevention at the 1997 Kagoshimaken-Hokuseibu Earthquake was estimated based on the comparison of failure distributions between a young forest and the surrounding mature forest.
Many slope failures have occurred in a young forest near the epicenter of the earthquake, where needle trees were planted several years ago. On the other hand, the density of slope failures in the surrounding mature forest was relatively low. It should be due to the differnece of roots in each forest.
Slope angle, topsoil depth, and soil type along a measurement line in the young forest were investigated. It was clarified that topsoils were relatively deep in the slopes where failure occurred compared with the slopes where failure did not occurred. The stable limit of soil depth is estimated 115cm for the young forest and 140cm for the mature forest in the same slope angle of 46 degrees. The difference of the soil cohesion of each site, that is the effect of tree roots, is estimated 2.65kPa in a case of 0.5 in horizontal seismic coefficient, 46 degrees in slope angle, and 30 degrees in angle of internal friction. This estimated value is similar with some data which have been experimentally obtained by previous studies, and this value corresponds to 0.15 in horizontal seismic coefficient. Tree roots can prevent slope topsoil from failures to some extent in earthquake as well as in rainfall.