Case Study on Heavy Rainfall-Induced Reactivation of Seismically Disturbed Slope Caused by the 2011 off the Pacific Coast of Tohoku Earthquake

Abstract

The stability of slopes that have developed cracks because of strong seismic activity is thought to be vulnerable to rainfall as their soil strength is reduced. Nonetheless, it is difficult and technically challenging to estimate such risk. This paper attempts to offer insights into this problem by presenting a case in which a seismically struck slope failed after intense rainfall. The Kuragasaki district of Sakura City, Tochigi Prefecture (Fig. 33.1), is situated on a 70-m-high terrace cliff, where Pliocene sediments are overlain by unconsolidated pyroclastic flow deposits and volcanic ash soil. On March 11, 2011, the date of the off the Pacific coast of Tohoku Earthquake, significant cracks and vertical drops appeared, which extended to approximately 600 m along the edge of the terrace cliff. Six months after the earthquake, the cracks extended further because of intense rainfall, which led to extensive slope failure, discharging 10,000 m3 of debris downslope. The cause of the failure is thought to be a combination of factors and events. First, the impermeable pyroclastic flow deposits were fractured by the shaking. Second, the intense rainfall, which had infiltrated the ground, accumulated above the pyroclastic layer. Finally, the pore water pressure increased within the fracture zone, which eventually led the slope to fail.