Geotechnical properties of submarine sediments in the Seto Inland Sea

Abstract

Sampling of submarine sediments by an improved piston corer and a bucket dredger has been carried out since 1973. The length of the core samples ranged from 0.9 m to 5.4 m. The recovery ratio ranged from 39.1 percent to 98.9 percent. The physical and engineering properties of 16 sediment cores and the physical properties of 125 dredged samples were determined, and variation of these properties was analyzed.

Clayey silts and silty clays cover the floor of relatively wide bays and sea areas. Sands and sandy silts blanket the floor of channels that have swift currents, and are also found in the vicinity of estuaries.

The piston core samples showed considerable sample disturbance, which should be estimated quantitatively in the future.     

Slope failures and tailings dam damage in the 1978 Izu-Ohshima-Kinkai earthquake

The Izu-Ohshima-Kinkai earthquake (M = 7.0 on the Richter scale) occurred at 12h24 Japan Standard Time on 14 January 1978 in Sagami Bay, and caused hazards in the eastern to central Izu Peninsula. The meizoseismal zone extended 15 km from east to west by 20 km from north to south.

The aftershock zone shifted westward to the interior of the peninsula. The largest aftershock (M = 5.8) struck the central to western part of the peninsula on 15 January, causing severe damage in its mountainous terrain.

Most of the hazards related to slope failures of sea cliffs, river cliffs and road cuttings of convex vertical section with the upper parts sloping gently and the lower parts steeply.

Large slope failures occurred close to the activated faults, indicating that the shocks and displacement were locally amplified along the activated faults.

In the meizoseismal zone, high embankments of roads and housing sites on natural slopes were damaged by slides and tension cracks between the embankment and natural ground.

Subsidence and tilting of retaining structures were generally observed. Damage to high concrete retaining walls of gravity type was due to the insufficient bearing capacity of the foundation soil. Any type of retaining structure, even if seismic-resistant, would be considered ineffective in case of failure of the back slope.

Buildings in the meizoseismal zone were damaged due to sliding of the underlying embankments and ground displacement due to the activated faults.

Falling rock blocks and sliding soil masses struck cars and houses close to the foot of cliffs and steep slopes, causing many deaths and injuries.

A number of tension cracks in the middle to upper parts of steep slopes and on the shoulders of road embankments threatened to cause secondary failures, rockfalls and avalanches during heavy rain.

At a gold and silver mine, the tailings dam of a concentration plant failed due to liquefaction of the tailings.

The above experience may throw some light on the possibility of seismic failures in mountainous countries in seismically active regions of the world.