Raiden Coast Rock Block Slide

Location

Raiden Kaigan, Katanakake Iwanai-Cho, Hokkaido, (42*55'N; 140*24'E).

Date of Slide

July 12, 1993

Size of Slide

size of failed block; Height; 25 m; Width; 10 m; Depth; 11 m; volume: 2.9x103m3.

Damages

Although no lining was damaged by the failure, national Highway Route 229 was closed for one month because of the hanging rock block.

Topographic and Geologic Conditions

The Raiden coastal area consists of a continuation of steep eroded sea cliffs up to 200 m high, and is composed of Pliocene volcanic breccia beds. There beds consist of hyaloclastite, pyroclastic breccia and massive lava flows. Rocks near the failure consist mainly of breccia and subrounded pebbles of valcanic conglomerate within a sandstone matrix. This volcanic conglomerate dips gently to the north with vertical joints of several meters apart.

mechanism of Failure and Type of Movement

Arock block glide occurred along the vertical joint and bedding plane slide. However, the 25x108 m rock block remained hanging n the steep slope.

Causes of Failure

Off shore southwest Hokkaido Earthquake of July 12, 1993 (Magnitude 7.8).

Mitigation Measures

The mitigation measures include reinforcement of the lining below the hanging rock block with H-beams. On October 7, 1993, the hanging rock block was blasted off. Later, additional rocks were removed and wire mesh was applied over the exposed rocks.

Raiden Coast Rock Block Slide