Ohori, Narusawa, Kohma-Cho, Iwaki City, Fukushima Prefecture, (37*05'N;
140*48'E).
Size of Slide
Length: 500 to 700 m; Width: 650 m; Area: 45 ha; Volume: 1.1x107m3.
Damages
The failure ocurred on August 12, 1988, and caused the destruction
of one residential structure and closing of a city street.
Geology, Mechanism of Failure and Type of Movement
The geology of the area consists of Lower Tertiary sandstone, mudstone,
conglomerate and coal overlying the granitic rock basement complex. The
areas behind the slide are separated by the Nojima Fault and are underlain
by gabbro. Early sliding of the Narusawa Landslide could be quite old as
fragments of lacustrine deposits (indicating the low energy environment)
and large blocks of gabbro, thought to have toppled or failed rapidly from
the higher elevations, are incorporated into the slide mass. The causative
factors of sliding include: 1) the existence of the fault at the scarp
area, which acted as the plane of separation; 2) the presence of tuffaceous
siltstones, which acted as the lubricant; and 3) drag folds that were formed
within the Lower Tertiary beds near the fault as a result of faulting.
The inclination of the tuffaceous siltstone which became the slide plane
is about 20 degrees. The contributing factors of slide movement include
the increased water pressure from infiltration into the open cracks along
the fault separation plane, and slope instability near the toe of the slide
due to erosion.
Mitigation Measures
The landslide control works include large scale soil removal at
the head area, installation of a 3-dimensional dewatering system (tunnel
conduit and drainage wells) to drain the depression at the head area, and
construction of dams to prevent sediment discharge at the toe area. The
restraint works included installation of piers and anchors along the cut
slopes and the toe area.
