Quantification of slope stability afforded by mountain foot fixation to avoid deep-seated landslides

Abstract

Large-scale deep-seated landslides and landslide dam formation have become common, for example during the 2004 Mid-Niigata Prefecture and the 2008 Iwate-Miyagi Inland earthquakes, and Typhoon Talas of 2011. Countermeasures have been aimed at both prior and subsequent events. Most countermeasures feature standby-type erosion control dams that are expected to capture debris flow or reduce the scale thereof. Although mountain foot fixing of such dams improves slope stability, few quantitative data are available ; planning remains empirical. Here, we examine slope height, the extent of ground collapse, the microtopography of the collapsed slope, and the extent and scale of prior deep failure. We quantify the erosion control afforded by mountain foot fixation of dams aimed at controlling deep-seated landslides ; we explore whether foot-fixing stabilizes slopes subjected to rising groundwater levels. We analyzed the stability of model slopes when validating and seeking to apply our quantification method. When we examined earlier deep-seated landslides in the Kii Mountain range, we found that mountain foot fixation exerted a fixed effect in terms of scale reduction. We evaluated the slope stability afforded by such fixation using a simple numerical method ; we found that it was possible to evaluate quantitatively how foot fixation reduced the extent and frequency of instability when the landform and soil parameters were defined. In the future, it is necessary to accumulate continuous observation data of groundwater level at the time of collapse occurrence, to verify the validity of this method, and to examine the application method to actual countermeasures.