Search

The liquefaction-induced river levee failures observed at micro-scale landforms of an old river course during the 2011 Tohoku earthquake were investigated by means of weight sounding and soil sampling. An empirical procedure to evaluate the liquefaction resistance of soils using weight sounding and soil sampling is proposed, which explicitly incorporates the effect of the fines content of soils. Case studies were carried out at three locations along Kokaigawa River. The use of weight sounding and soil sampling proves effective in discerning the geotechnical reasons behind the occurrence of the river levee failures observed at the old river course. [ABSTRACT FROM AUTHOR]

We examined variations in age at seaward migration and sea age for the anadromous form of red‐spotted masu salmon (Oncorhynchus masou ishikawae) in two Japanese rivers. The anadromous form of red‐spotted masu salmon expressed only two sea migration patterns in the two rivers: (a) the majority of the salmon (95%, n = 81) were of age‐0, and age‐1 migrants were rare (n = 4); and (b) all the salmon examined (n = 22) made a return migration within a year, with 23% of the salmon exhibiting potamodromy in the river. Owing to low variation in their sea migratory patterns, the anadromous form of red‐spotted masu salmon is likely vulnerable to environmental fluctuations. [ABSTRACT FROM AUTHOR]

This paper is a study of determination of representative elementary volume (REV) size suitable for pore-scale flow simulation (PFS) and evaluation of permeability anisotropy for sand based on Kozeny–Carman (KC) equation. We extracted small digitized samples for PFS from the 3D structures of Toyoura sand and glass beads obtained by X-ray computed tomography. Using the digitized samples with various sizes, we performed a series of flow simulations. We characterized the sample size using the ratio of the porous length L to the median grain diameter D 50 . The L / D 50 of REV was determined on the basis of statistical evaluation on coefficients of variations of the simulated values of void ratio, specific surface, hydraulic conductivity and hydraulic tortuosity. We confirmed that the hydraulic properties obtained from the PFS using the REV size determined agreed well with the past experimental studies. We also conducted PFSs in the horizontal direction to evaluate the permeability anisotropy. We clarified that the shape factor in the KC equation has a strong influence on permeability anisotropy even though previous studies treated hydraulic tortuosity as the only factor that affects the permeability anisotropy. [ABSTRACT FROM AUTHOR]

Gas hydrates in sediments are forecast to become an unconventional natural gas resource. In 2013 and 2017, an offshore natural gas production test in subseabed gas hydrate‐bearing sand layers was carried out in the eastern Nankai Trough area of Japan. However, permeability evolution with progressive shearing of these faulted reservoirs has remained enigmatic. It is necessary to understand fault behavior under shear, as well as how normal stress affects permeability evolution. Here, we examine the relationship between fault development and permeability reduction in unconsolidated turbidite sandy sediment from Japan's eastern Nankai Trough area using a laboratory ring‐shear experiments. At each stress condition, the permeability decreased significantly following shear displacements of ~0.2–0.5 m, gradually decreasing further with shear displacements up to 10 m. Permeability after shearing was reduced by 3 to 4 orders of magnitude compared to that before shearing. Grain crushing and compaction within the shear zone and surrounding host sediment are dependent on stress and mineralogy. The findings of the role of shear zone development in modifying fault permeability for turbidite sand are expected to contribute directly to rational evaluation of gas‐hydrate reservoir, production simulations, and development strategies in future studies. Key Points: Reduction of fault permeability in unconsolidated turbidite sandy sediment is found by 3 to 4 orders of magnitudePermeability evolution could be classified into two groups in shear displacementStress and mineralogy dependencies are related to grain crushing and compaction [ABSTRACT FROM AUTHOR]