Relationship of the faulting to the creep movement of Iwakura landslide in Saga, Japan.

Abstract

A huge creep type slow moving landslide was identified near Saga City of Kyushu Island in 1999. It has been evidenced that this landslide was activating through out several years since the quarry was opened up at the bottom of the valley. Quantitative and qualitative analyses were implemented to investigate the rate of movement, to demarcate the extent of the affected area and to find the major cause of this sliding.Periodical monitoring together with field and laboratory investigations has been carried out for nearly 2 years . Analysis of the extensometer measurement data, results of the surface displacement monitoring survey and the surface geological and structural investigation revealed that this sliding is taking place along the fault planes. Stereographic projection data combined with the sur-face movement monitoring suggested that the sliding is taking place in three directions along the fault planes and along the inter-section of two extensive fault planes sub-parallel to the slope concerned. Subsequent borehole drilling and the pipe strain gauge installation confirmed the location of the slip surface at 27 m and 29 m below the surface along these faults . The average move-ment rate of this sliding mass has been approximated to 6 cm/day, which has occasionally increased to a value more than 12 cm/day, just after heavy rainfall.