1988 Fiscal Year Final Research Report Summary
Lequefaction of Seabed deposits and its effects on the submarine Iandslide and stability of marine structures.
| Project/Area Number |
60302065
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| Research Category |
Grant-in-Aid for Co-operative Research (A)
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| Allocation Type | Single-year Grants |
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| Research Field |
基礎・土質工学
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| Research Institution | The University of Tokyo |
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Principal Investigator |
ISHIHARA Kenji University of Tokyo, Professor, 工学部, 教授 (10010662)
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| Co-Investigator(Kenkyū-buntansha) |
OHKUSA Shigeyasu University of Tokai, Professor (30055909)
FUKUE Masaharu University of Tokai, Associate Professor
YOSHIDA Yoshitada University of Tokyo, Research Associate (60010815)
TOWHATA Ikuo University of Tokyo, Associate Professor (20155500)
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| Project Period (FY) |
1985 – 1987
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| Keywords | seabed deposit / waves / triaxial torsion test / pile / pore pressures / liquefaction / 間隙水圧 |
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| Research Abstract |
The cyclic stresses induced in the seabed deposit due to overhead travelling waves are characterized by the rotation of principal stress direction. To simulate such stress conditions in the laboratory tests, the existing triaxial torsion shear test apparatus was revamped and remodeled and multiple series of tests were conducted on hollow cylindrical specimens of Toyoura sand. The results of tests disclosed that the cyclic shear strength of the sand is lowered by 20-30%, if the continuous rotation of principal stress is imparted, as compared to the strength obtained in the cyclic stress application without rotation of principal stress direction.
The second prime subject was to provide a basic body of information on the possible reduction in shear strength of under-consolidated clay extant in the sea bottom. Several series of triaxial shear tests were performed on clay samples prepared under different degrees of under-consolidation. The results of tests disclosed the fact that a sharp red
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uction in shear strength can encountered if the extant seabed deposit is under-consolidated. This findings appear to prove the occurrence of lange-scale lendslides on gentle slopes in the ocean bottom.
The third item of the present study was to see the effects of the cyclic deformation of seabed deposit due to the travelling wave on underwater driven piles. The current design of piling considers only forces coming fromthe impulse of waves near the sea surface. It appears highly probable that, in addition to this force, there should be forces on piles exerted from the deformation of surrounding ground due to the wave transmission. The analysis results in this context disclosed that the contribution from the seabed deformation to the bending moment in piles is insignificant as compared to the moment induced by the impulsive forces of waves overhead. However, when the soil surrounding the piles is to be checked against failure, the cyclic shear stress induced by wave itself is as significant as the cyclic stress imparted from the piles to the soil being considered.
The fourth item of the study was an attempt to monitor insitu pore water pressures in the seabed deposits. Three pieces of piezometers were embedded, on the seabed surface and at 2.5m and 3.5m deep sand deposits, in the area of Shimizu harbor. The results of observation indicated that the transmission of wave-induced stress through the seabed deposits is governed largely by the deformation characteristics of soils. This fact indicated, in turn, that the deformation and stability of the seabed soils is dominated significantly by the magnitude of pore water pressures induced by the ocean waves. Less
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Research Products
(5 results)
