The Yui landslide has been divided into 26 distinct blocks. Since the
initial sliding in 1781, the slide has been continuously active. The size
of the slide is increasing due to the formation of a new slide immediately
adjacent to the failed areas. The recent failures include the Terao Block
in 1961, and the Nigorisawa Block in 1974. These slides inflicted damages
to the Tokaido Line of the Japan Railroad, Natural Highway Route 1 and
many residential structures.
Geology, Mechanism of Failure and Type of Movement
The site area is located east of the Itoigawa-Shizuoka Tectonic Line,
and southwest of the Fossa Magna and Iriyama Thrust Faults that extend
in a north-south direction along the Yui River. The geology of the site
area includes the Miocene Epoch Okouchi Formation, and the Pleistocene
Epoch Hamaishidake Conglomerate. The upper slopes are locally underlain
by alternating beds of conglomerate mixed with agglomerate, sandstone and
mudstone. The surface area of the mid-slope and up to the summit is covered
with sandy to gravelly loam layers and loam layers, while in the lower
half of the slope the mudstone beds are overlain by loam layers and deposits
of sand and gravel.
Mitigation Measures
The design criteria of the mitigation works is based on the Tokai Earthquake,
and the torrential rains of July, 1974 (amount of rainfall: 546 mm/day
and 78 mm/hr). In the past, the mitigation measures were implemented on
an individual slide block basis. At the same time, earthquake resistance
was incorporated as a special mitigation measure to maintain the integrity
of the structures, and to prevent secondary landsliding along the toe areas.
The mitigation measures implemented to date include soil removal works,
erosion prevention works, surface drainage control works, subsurface drainage
works (culvert, horizontal gravity and vertical drains, drainage wells
and interceptor under drains), steel pile works, anchor works and revegetation.
Since 1987, large diameter cast-in-place pile works have been constructed.
