[H-DS11] Geohazards in humid, tectonically active countries and their precursors

[HDS11-06] A Measurement Study on Dissolved Gas in Groundwater for Slope Stability

*TAKANORI OJIMA¹, AKITO KANAZAWA¹, HIROAKI NAKAYA¹, YOSHITO OGAWAUCHI² (1.National Institute for Land and Infrastructure Management, 2.Japan Conservation Engineers Co., LTD.)

Keywords:Dissoved Oxygen, Excess Pore Water Pressure, Sudden Collapse

In Japan archipelago, which is located in a humid climate to semitropical climate zone, slope failure (landslide, rock avalanche, and debris flow) occurrence tends to be increase in rainy season to typhoon season (June - September). Rainfall-triggered slope failure is generally considered to be the shear strength reducing processes and internal destruction due to suction in unsaturated soils or bedrock tissue fractures. On the other hand, as a precursory phenomenon, slope failure without rainfall occurs at a low frequency, and as with the disasters associated with earthquakes, the damages is likely to be enormous due to the difficulty of prediction and countermeasures. Regarding sudden slope failures, there are have been reported by several researches (e.g. Jitousono et al., 2006, and Kubota et al., 2018), it is suggested that the groundwater accumulated in bedrock may have contributed to the collapse.

Previous study in urban areas focusing on seepage failure of the sheet pile rooting part, one of the causes is that the dissolved gas in the groundwater precipitates due to the head difference and interparticle composition, which decrease the shear strength of soil and bedrock (Kodaka & Asaoka, 1995). Therefore, we focused on the groundwater flowing near the steep slope of the pyroclastic plateau and attempted to monitor the fluctuation of dissolved gas and several properties in the groundwater. Dissolved gas to be monitored was relatively easy to obtain, and dissolved oxygen was measured using a dissolved oxygen meter with many accuracies and models.
Dissolved oxygen in general groundwater is considered to be low due to biological consumption and geochemical action, and there are very few observation reports on mountain slopes area. In this study, the fluctuation characteristics of dissolved oxygen were investigated by comparing with other water properties(EC, pH, temperature), rainfall, groundwater level, groundwater flux (flow rate of drainage well).