Assessment of slope failure risk by evaluating water quality on site

Abstract

The calcium and magnesium ion concentrations of stream water are greatly affected by chemical weathering and may thus predict the risk of slope failure. The on-site measurements of these ions might enable on-site risk evaluation and improve the efficiency of the field survey. We measured calcium and magnesium ion concentrations of 22 spring water sources in the 2009 Hofu disaster region using portable water-quality analyzers. We found significant differences in calcium ion concentrations between collapsed and non-collapsed regions, but no significant difference in magnesium ion concentrations. We analyzed the terrain of sites exhibiting high calcium ion concentrations that did not in fact collapse. We confirmed that the slopes were greatly affected by the heavy rainfall of 2009, increasing the risk of collapse. Therefore, the on-site measurement of calcium ion concentration was a useful tool to assess the risk of slope failure.