Field Investigations on Seasonal Variations of the Groundwater Level and Pore Water Pressure in Landslide Areas

In order to take a knowledge on the seasonal variations of groundwater level and pore water pressure in landslide areas, the field investigations were carried out during five years. Based on the field investigation, the following conclusions can be drawn: 1) Groundwater level and pore water pressures at the slip surface in landslide areas have changed seasonally, with marked increases after the snow-melting and heavy rainfall seasons. 2) Pore water pressures at the slip surfaces do not always coincide with hydraulic pressure values calculated from the groundwater level. Therefore, when making slope stability analyses and taking preventive measures against reactivatings, pore water pressures should be measured directly all along the slip surface with electric piezometers. 3) Landslide is liable to occur during two weeks after heavy rainfall because of time lag between groundwater level and rainfall, therefore, the careful attention should be necessary for these period. 4) The slope stability analysis should be conducted on the shearing strength considered the overconsolidated states of soils due to the increasing of pore water pressures.