Geotechnical and rheological characteristics of waste rock deposits influencing potential debris flow occurrence at the abandoned Imgi Mine, Korea

Abstract

Mass movements associated with abandoned mines are dangerous and destructive. Local communities near abandoned mine sites are often threatened by the effect of extreme precipitation events on these sites. In this study, the geotechnical and rheological characteristics of mine deposits were examined to delimit the hazardous areas near abandoned mines. Waste rock materials were collected from a pyrophyllite mine deposit in the mountainous region of Korea, and a sieve analysis was performed to determine the grain size distribution. The yield stress in the debris flow modeling was determined using fall cone and rheometric tests. Rheological tests were performed on materials finer than 4.75 mm using a ball-measuring system rheometer. These samples were mainly composed of gravels and sands with small clay content. The waste materials typically exhibited a shear-thinning behavior within a given shear rate ranging between 10−5 and 102 s−1. The yield stresses obtained from the rheometric tests agreed well with those obtained from the fall cone tests. For the input parameter, we selected the yield stresses ranging from 100 to 500 Pa. As a case study, the debris flow mobility associated with the abandoned mine was examined using a one-dimensional flow dynamic model. The runout distance and velocity of the waste materials are strongly dependent on the yield stress; both factors significantly increase with decreasing yield stress. However, even under the highest yield stress (i.e., 500 Pa), the failed mass can flow into the mountain stream. Thus, the physico-chemical contaminations of areas downslope of abandoned mines may be unavoidable with respect to extreme rainfall events.