Shigeyasu Okusa

Consolidation of Sand-Clay Mixtures

Consolidation properties of bentonite-sand mixtures, as reclamation materials, were investigated in the one-dimensional consolidation test. From the relationships between the void ratio of nonclay fraction and consolidation pressure was found the existence of a threshold void ratio at which the frictional resistance becomes dominant during consolidation. The threshold value is slightly greater than the maximum void ratio of sand used for the mixtures. A new standard, which utilizes values of clay content, dry density, and threshold void ratio, is proposed to clarify the boundary between sandy and clayey soils. The salt infiltration into the mixtures causes a rapid settlement but greatly reduces the swelling in unloading stage. As long as the void ratio of nonclay fraction is less than the threshold value, however, the influence of salt disappears.

Measurements of wave‐induced pore pressure in submarine sediments under various marine conditions

Abstract Wave‐induced pore pressures were measured at four sites around Shimizu Harbor, Central Japan. At Site 1 the measurements of the wave‐induced seafloor pressure and pore pressure at a depth of 0.5 m below the mud line were conducted in about 14 m of water. The sediment was silty sand with a permeability of 4 × 10−5 m/s. The pressures induced by wind waves with about a 10‐s period were transferred through the sediment to the pore pressure with damping of about 4% through 9% and time lag of 0.1 rad (0.2 s) through 0.16 rad (0.3 s). The wave‐induced seafloor pressure and pore pressure changed differently in the breaker sediment. At Site 3 absolute and differential piezometers were installed in sand and gravel sediment at depths of 0.7 m and 0.5 m, respectively, in about 1.7 m of water. The pressure attenuation was about 1% for the tide without time lag. The piezometers used at Sites 1 through 3 were a transformer type. At Site 4 a new probe of semiconductor type was tested in sand and gravel sediment ...

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.

Pore‐water pressure change in submarine sediments due to waves

Abstract The pore‐water pressure was measured in disturbed silty sand sediment at 1.5 m depth below the sea floor in about 12 m water depth, concurrently with monitoring of wave pressure in the water at about 10 m above the sea floor, over the period 1976–1978 in Shimizu Harbor, Central Japan. The piezometer was designed to measure perturbation of a minimum differential pore‐water pressure of 2 g/cm2 (0.196 kPa, 2 cm water head) and a maximum of 0.1 kg/cm2 (9.8 kPa, 1 m water head) from the wave pressure at the sea floor. The measured surface wave had periods of 4–20 seconds and heights of 50–100 cm. The wave pressure at the sea floor was transmitted to the pore‐water, being diminished to Vi to 4/s with a time lag at 1.5 m depth in the sediment. The damping was smaller for a long‐period wave than for a short‐period one. The time lag was relatively clear for long‐period waves. The forms of wave pressure were slightly different from the fluctuation forms of the differential pore‐water pressure. The damping ...

Measurements of Wave-induced Pore Pressure and Coefficients of Permeability of Submarine Sediments during Reversing Flow

Wave-induced pore pressure was measured at a location on the Pacific coast of central Japan with two types of probe. Analysis revealed that the damping of wave-induced pore pressure with respect to the wave pressure at the seafloor was greater than that predicted by published theories, even at shallow depths proximate to the mudline.

General characteristics of upper soil sediments

Abstract In this study, “averaged void ratio system” is introduced to generalize the self‐weight compression of sediments. In a steady state problem, upper sediment layers, which can be distinguishable from the subsoil, can be characterized in terms of three defined void ratios and a single constant parameter. In this study, surface void ratio, asymptotic average void ratio, and final void ratio of upper sediments are defined, based on the experimental results and observations, and the correlations between them are obtained. In other words, self‐weight compression processes can be characterized by initial and final void ratios and a constant parameter which determines the path of the process. The experimental results show that the constant parameters are almost independent of the types of soil system, as far as upper sediments are concerned. With this result, a method for in‐situ determination of upper sediment profile is proposed.

A New Technique to Identify Sea Sediments in Shallows

A simple technique for fast prediction of the grain size characteristics of sea sediments is developed. The device used is a small sled hinged to a detector which is made of steel blade. When the sled is dragged by a ship, the free end of the detector slides on the seafloor. In principle, the detector vibrates at different rates during sliding according to the type of soil encountered. In this study the oscillation characteristics shown by the vibration of the detector were recorded by an oscillator through a bridge amplifier, and were then correlated with the grain size of the identified soils.

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.