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

在1985年春季反应性滑坡发生后的1986年和87年，利用开发的自动观测系统对新泻县松原山村滑坡的地下水进行了调查。该系统即使在冬季大雪纷飞的情况下也能自动将几个月的观测数据记录在一张IC卡上。所得结果如下：在地表以下厚度达6 m左右的老滑坡体中，无侧限地下水位在近地表四季几乎保持不变，几乎不受降雨供水和钻孔排水的影响。反之，随排水工程的进行，无承压地下水下伏的风化泥岩中的地下水压力减小。滑移区外地下水压力受降雨影响较大，而滑移区内地下水压力受降雨影响较小。从以上和其他结果来看，可以假定在前一次滑移中产生的滑移面粘土构成了一个几乎不透水的层。通过对滑坡体滑坡体滑面周围地下水主要成分的分析，探讨了引发滑坡体滑动的地下水行为。在滑面下伏的风化凝灰岩中发现了源自古海水的氯化钠型水。通过对地下水压力的调查和对水的追踪，得出了排水是控制滑坡复燃的必要条件。

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.