Takaji Kokusho

Current state of research on flow failure considering void redistribution in liquefied deposits

Abstract In liquefied ground, lateral flow is sometimes much larger than surface settlement and may exceed several meters even in a gentle slope of less than a few percent. It occurs not only during but also after earthquake shaking. Conventional laboratory soil tests using uniform sand cannot reproduce this phenomenon. Its mechanism is still poorly understood. In this paper, there is a major focus on the mechanism involving void redistribution or water film effects in layered sand deposits using recent findings obtained by different researchers on void redistribution and the associated lateral flow movement that potentially occurs in layered sand deposits. 1G shake table tests, 1D tube tests, torsional simple shear tests, in situ soil investigations, case history studies, etc. are used to develop an understanding of the lateral flow mechanism during liquefaction. Some of the major findings are; sand deposits in the field consist of sublayers with different particle sizes and permeability and readily develop water films by post-liquefaction void redistribution at sublayer boundaries. The water films may have served as sliding surfaces for large flow during the 1964 Niigata earthquake without the constraint of the dilatancy effect because the water films serve as shear stress isolators. The potential of this type of flow failure will be high for loose sand with relative density around 40% or less.

Liquefaction Strengths of Poorly-Graded and Well-Graded Granular Soils Investigated By Lab Tests

In order to understand differences in liquefaction behavior of well-graded gravelly soils compared to poorly-graded sands, a series of lab tests was performed on granular soils with different particle gradations or fines content having different relative densities reconstituted in laboratory. Large soil container tests indicated that SPT N-value of well-graded gravels of relative density higher than 50% is considerably larger than that of sand of the same relative density, resulting in lower liquefaction strength of gravelly soils than that of poor-graded sand under the same corrected N-value, N1, for N1 > 25−30. Cyclic triaxial tests on reconstitutes specimens indicated that relative density can serve as a proper index to uniquely evaluate liquefaction strength corresponding to 5% DA strain for variety of granular soils having different gradations. In contrast, post-liquefaction undrained residual strength for larger strain is not uniquely determined by relative density but largely dependent on particle gradations. Also found was that the liquefaction strength clearly reduces with increasing fines content Fc both in well-graded and poorly-graded soils but the reduction occurs in a smaller range of Fc in accordance with smaller critical void ratio for well-graded soils than for poorly-graded sand. Increase in Fc also reduces post-liquefaction residual strength of granular soils particularly for higher relative density. Greater reduction occurs in smaller Fc range for well-graded soils than for poorly-graded sand because of the difference in the critical void ratio.

Liquefaction at Strong Motion Stations and in Urayasu City during the 2011 Tohoku-Oki Earthquake

The 2011 MW = 9.0 Tohoku-oki earthquake generated a large number of unique soil liquefaction case histories, including cases with strong ground motion recordings on liquefiable or potentially liquefiable soils. We have compiled a list of 22 strong motion stations (SMS) where surface evidence of liquefaction was observed and 16 SMS underlain by geologically recent sediments or fills where surface evidence of liquefaction was not observed. Pre-earthquake standard penetration test data and borehole shear wave velocity (Vs) profiles are available for some stations, but critical information, such as grain size distribution and fines plasticity, are often lacking. In the heavily damaged city of Urayasu, we performed post-earthquake cone penetration testing at seven SMS and Vs profiles, using surface wave methods at 28 additional locations to supplement existing geotechnical data. We describe the liquefaction effects in Urayasu, the available site characterization data, and our initial data interpretations.

Geotechnical aspects of the 2004 Niigata Ken Chuetsu, Japan, earthquake

The Niigata Ken Chuetsu earthquake induced significant geotechnical and geologic failures throughout the affected region. The most prevalent geotechnical observations from this earthquake are related to ground failure, including landslides in natural ground, failures of highway embankments and residential earth fills, and limited liquefaction in alluvial deposits. The absence of considerable levee deformations and surface faulting was noted. This paper documents the geotechnical aspects of the Niigata Ken Chuetsu earthquake as related to earth structures, liquefaction, and surface faulting; landslides are discussed in an accompanying paper.

Sand Liquefaction Observed During Recent Earthquake and Basic Laboratory Studies on Aging Effect

During the 2011 Tohoku Pacific Ocean earthquake (M9.0), liquefaction occurred extensively in reclaimed land in Kanto area more than 200km far from the earthquake fault. The liquefied sand generally contained a lot of non-plastic fines with fines content more than 50% in some places. Almost all sand deposits along the Tokyo bay area reclaimed in 1960s or later liquefied, while in a good contrast, those older than that did not.

Sailing Solar-Cell Raft Project and Weather and Marine Conditions in Low-Latitude Pacific Ocean

AbstractDevelopment of a huge wind-sailing solar cell raft (SCR) with dimensions of 5×5  km is proposed, which can generate electricity comparable to a 1,000-MW nuclear power plant in low-latitude Pacific Ocean. Solar energy of 8  k·Wh/m2/day or more is targeted because the SCR navigates in fine weather using weather satellites. The generated electricity will be transported by battery tankers loaded with a tremendous number of high-energy-density batteries. Studies based on available data indicate that there are vast open seas in the tropical Pacific Ocean, where the maximum solar energy attains 7  k·Wh/m2/day annually on average and conditions of winds, waves, and sea currents are favorable for the solar energy system to operate. Three major technologies for breakthrough to realize this system are discussed from their future perspectives.

Earthquake-Induced Submarine Landslides in View of Void Redistribution

Submarine slides occurring near-shore in non-cohesive sandy deposits are first discussed in view of void redistribution effect. Model tests demonstrates that formation of water film beneath silt seam plays a key role in liquefaction-induced sliding failure in gentle slope. Considering that soil deposits are naturally stratified with sandwiched low permeability seams, it seems quite reasonable to recognize the water film effect as a major mechanism for seismically induced submarine slides in gently sloped sandy sea-bed near coasts. Analogous void redistribution effect on the submarine slides occurring in clayey sea floor far from coasts is then studied by cyclic loading tests to find out large volumetric strain after strong cyclic loading. Simple consolidation plus seepage analysis considering the earthquake-induced large volumetric strain indicates that excess pore water may accumulate beneath less permeable seams sandwiched in relatively permeable clayey layer, possibly causing delayed shear strength reduction due to excessive swelling just beneath the seam.

Effects of Fines and Aging on Liquefaction Strength and Cone Resistance of Sand Investigated in Triaxial Apparatus

Innovative miniature cone penetration and subsequent liquefaction tests were carried out in a modified triaxial apparatus on sand specimens containing fines. It has been found that one unique curve relating cone resistance qt and liquefaction strength RL can be established, despite the differences in relative density and fines content, the trend of which differs from the current liquefaction evaluation practice. In order to examine an aging effect on the relationship, sands containing fines added with a small amount of cement are tested to emulate a long geological period in a short time. The addition of cement to the fines tends to increase the liquefaction strength RL much more than the penetration resistance qt, resulting in obvious upward shift of the qt ∼ RL curve from the curve obtained for specimens without cement. Thus, it is revealed that the cementation effect by aging which dominantly occurs in fines can explain why higher fines content leads to higher liquefaction strength for the same cone resistance.

Site Amplification Formula Using Average Vs in Equivalent Surface Layer Based on Vertical Array Strong Motion Records

Seismic site amplification formula was developed for virtual surface arrays to be consistent with vertical arrays using a number of KiK-net records during recent 8 destructive earthquakes. A correlation between peak spectrum amplification and S-wave velocity ratio (base V s /surface V s ) improved much better if the surface V s was evaluated from fundamental mode frequency combined with a thickness of equivalent surface layer in which peak amplification is exerted, rather than using the conventional V s30. Also found was that soil nonlinearity effect during strong earthquakes has only a marginal effect on the surface array amplification observed and evaluated by the formula using small-strain Vs-values. The theoretical basis for the minor nonlinear effect has been discussed by conducting a simple study on a two-layered system with strain-dependent soil nonlinearity. The proposed amplification formula seems also applicable with a satisfactory accuracy to sites where soil liquefaction is involved.

Case Studies on Seismically Induced Slope Failures in Terms of Energy

An energy approach proposed to make a simple evaluation of travel distance of debris has been applied to seismically induced slope failures during recent two earthquakes. Mobilized friction coefficients have been back-calculated, revealing their strong dependency on initial slope inclinations. The friction coefficients are found to become smaller than the initial slope inclinations in gentler slopes presumably due to seismic shock. The friction coefficients also tend to decrease with increasing volume of failed slopes, which is consistent with previous case studies on huge landslides. Most importantly, the average thickness of failed soil mass seems to serve as a good indicator for specifying the mobilized friction coefficient to predict the travel distance using the energy approach.