In September 1998, a natural slope failure in weathered sedimentary rocks occurred in Mt. Keummu, which is located at the southeast part of the Korean peninsula, due to the heavy rainfall. To find general characteristics of the natural slope failure, detailed geological survey, ground penetration radar (GPR) investigation, two boring tests, laboratory tests, and analyses using commercial computer programs were performed. The site is composed of sandstone, conglomerate, and shale, and the sliding surface is developed along the bedding plane. The ground penetration radar prospecting and boring tests confirmed that the location of the sliding surface near the head of sliding mass is at 3.3m. Based on the result of computer analyses it is concluded that the slope becomes unstable when the ground water table rises near the surface by heavy rainfall. A planar failure along one of the joint sets is also expected when the angle of internal friction is less than 20°. The effect of construction of an access route for forest watch is minor, and high water table is found to cause considerable displacements, especially at the toe of slumped mass.
In Korea, natural and excavated slope failures are one of the leading natural disaster types that result in the loss of human lives; 103 persons died by landslides in 1998. This number indicates that 26.8% out of a total of 384 natural-disaster-related deaths were caused by the slope stability related problems. Most of the landslides occurred during the period between July and August; part of the monsoon season. The majority of the landslides during this period can be categorized as flow type. A geological survey, ground penetration radar (GPR) prospecting, two boring tests, laboratory tests, and analyses using commercial computer programs were performed.