Kamenose, Kashiwara City, Ohsoka-Fu, (34_ 35'N; 135_ 39'E).
Size of Slide
Relief: 175 m; Average slope: 20 degrees; Area: 87.2 ha. The Kamenose
Landslide consists of two areas, the Ridge Area and the Shimizutoni Area.
Each slide directly influences the other.
Damages
DAMAGES: In recent years, large scale landslide movement occurred in
1903, 1931, and 1967, resulting in the clogging and flooding of the Yamato
River, destruction of the National Rail Tunnel, closing of the National
Highway, and destruction of structures. Among the slides that have occurred
in the Ridge Area (which covers about 32 ha) between 1932 and 1982, damages
included uplifting of the river bed of the Yamato River, and the National
Rail Tunnel which passes through the landslide area was crushed. In 1967,
a landslide measuring 15 ha, 800 m long and 200 to 300 m wide, occurred
in the Shimizutani Area. This failure in turn triggered sliding in the
Ridge Area. Finally, the sliding developed into a large scale landslide
covering the entire Kamenose region.
Geology, Mechanism of Failure and Type of Movement
The geologic structure of this region includes two formations, the
granitic basement rocks and volcanic deposits and lava (andesite). Surface
deposits blanket both of the formations. Generally, the depth of the surface
slide mass, on average, is 30 to 40 m with the deepest part of the slide
exceeding 80 m. Most of the slide planes consist of thin tuff beds with
intrusive tuff breccia above the tuff and andesite beds.
Mitigation Measures
The early stages of mitigation works have been implemented by the Ohsaka-Fu
government. The comprehensive mitigation works began in 1962 under the
jurisdiction of the National government. It should be noted that almost
all of the mitigation measures developed in Japan have been utilized at
this site. The major landslide mitigation measures began under the jurisdiction
of the National government in 1962, and included soil removal works, drainage
tunnel works, horizontal gravity and vertical drain works, drainage wells,
surface drainage control works and steel pile works. The major mitigation
measures were finally completed in 1986.
Current Conditions
A total of 18.5 billion Yen (U.S. $ 176 million, based on the January,
1996 exchange rate) has been spent up to 1986. The mitigation works in
the Shimizutani area have been progressing well and is nearing completion.
The stability of the upper slopes (the Ridge Area) wa been significantly
improved, and appeared to be in a stable state. However, some areas in
the lower slopes indicated signs of movement and warranted that mitigation
measures were needed. The mitigation (now in progress) includes installing
gigantic caisson piles, which had never been tried in Japan. The caissons
have a maximum diameter of 6.5 m and a total length of 100 m. Considering
the social and economical importance of the Kamenose Landslide, the actual
mitigation measures that were implemented include hardware type measures,
and software type measures which consisted of an automatic monitoring system
that was introduced to the local construction office in 1986. The system
was designed to obtain real time data from instruments such as the tiltmeter,
extensometer, rain gauge and other instruments that are strategically positioned
on the landslide so that the behavior of the slide can be understood in
a timely manner.
