Characteristics of paddy field landslide referring to groundwater and its preservation works.

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Investigation of groundwater at Matunoyamakoshi landslide in Niigata Prefecture were made in 1986 87 after the occurunce of the reactive landslide in the spring 1985 by the developed automatic observation system. This system can automatically recorded several months observation data into an IC card even under beep snow in winter.The results obtained are as follos.Unconfined groundwater level which exists in old landslide debris of thickness up to about 6 m below the ground surface has been nearly constant near the ground surface in all seasons and was scarcely influenced by both water supply due to tainfall and drainage due to boring works. Conversly, groundwater pressure in the weathering mudstone underlying below the unconfined groundwater decreased with the drainage works. Pressure of groundwater outside the area of the slip was easily influenced to the rise by rainfall, in contrast with groundwater whithin the area of the slip. From above and other results, it may be assumed that the slip surface clay which had been produced during the preceding slip constitutes a nearly impermeable layer.Main components of groundwater around the slip surface in the landslide were analyzed to trace the behavior of groundwater which acted as a trigger of sliding. The sodium chloride type water originated from the fossil sea water was observbed in the weathered tuff underlying the slip suoface.It is concluded that drainage of this confined groundwater is necessary to cotroll reactivation of landslide from the investigation of groundwater pressure and tracing the water.