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31.
The undrained shear behaviour of sands has been a key topic after the devastating geo-disasters during the 1964 Niigata Earthquake
in Japan. Extensive geo-technical soil tests, especially undrained triaxial compression tests, have revealed that the liquefaction
phenomenon was the major cause for the disaster expansions. To numerically reproduce the liquefaction phenomenon, the pore-water
pressure was coupled with a distinct element method. In this model, the dynamic changes in pore-water pressure were taken
into consideration by the changes in volumetric strain and modulus of compressibility of water in the respective measurement
spheres. Fluid-flows among the measurement spheres were controlled by Darcy’s law. The effective stress paths and steady state
strengths in undrained triaxial compression tests associated with the wide ranges of initial void ratio were investigated.
The effective mean stresses of medium-dense to dense numerical specimens at the steady state were negatively proportional
to the initial void ratio. Loose numerical specimens reproduced quasi-liquefaction with the effective mean stresses that were
less than 25% of the initial value. The medium-dense numerical specimens reproduced the phase transformation that was a typical
characteristic of granular materials. The rolling restraints did not much influence of the effective angle of internal friction
but strongly affected pore-water pressure behaviour within a certain range of initial void ratio. 相似文献
32.
Excess pore pressure and grain crushing of sands by means of undrained and naturally drained ring-shear tests 总被引：5，自引：0，他引：5
An investigation of excess pore-pressure generation of a weathered granitic sand, taken from the source area of a typical landslide caused as a result of liquefaction, and a fine silica sand was conducted, in which grain crushing within the shear zone of ring-shear test specimens was examined as the key phenomenon of rapid long-runout motion of landslides. In order to investigate and explain the low average apparent-friction angle, mobilized in a liquidized landslide, speed-controlled ring-shear tests were conducted under undrained conditions on weathered granitic-sand specimens, formed under a wide range of initial void ratios. It was revealed that very small steady-state shear resistances were obtained irrespective of the initial void ratios, which can explain the low average apparent-friction angle. In addition, two series of ring-shear tests on weathered-granitic and fine-silica sands were conducted under naturally drained conditions by keeping the upper drain valve of the shear box open during the tests. The first series of tests was performed under differing total normal stresses, but at the same shear speed, and the second series was conducted at differing shear speeds, but under the same total normal stress. In order to investigate and analyze excess pore-pressure generation and dissipation within the shear zone that is associated with grain crushing, permeability analyses were conducted by passing water through the sample box of the ring-shear test apparatus before and after shearing. In addition, grain-size distribution analyses of samples taken from the shear zone after shearing were carried out. For the weathered granitic-sand samples, a significant change in bulk permeability and large amount of grain crushing were observed. In these tests on the above soil, a considerable reduction of shear resistance, which increased proportionally to the total normal stress and shear speed, were obtained. It was observed that due to grain crushing, finer grains that lowered the permeability of the soil in the shear zone, were formed. It is likely that the decrease in permeability facilitated the generation of high excess pore pressures by reducing the pore-pressure dissipation rate from the shear zone; thus, flow behaviour was exhibited even under naturally drained conditions. 相似文献
33.
Abstract— Demagnetized samples of cobalt precipitates in a copper matrix were shocked to 5, 10, and 20 GPa in a weak magnetic field of 7.7 μT to elucidate the origins of the natural remanent magnetization of meteorites and the magnetic anomalies of impact craters on the moon and Mars. The samples placed in the target acquired shock remanent magnetization (SRM) whose intensity increased up to 21.3 times compared with the demagnetized state, but SRM intensity and shock intensity were not correlated. The SRM direction was in most cases approximately perpendicular to the shock direction. The samples placed 4.8 mm from the impacted surface did not acquire significant magnetization, suggesting no plasma‐induced remanent magnetization (PIRM) up to 20 GPa. When the samples were divided into 8 sub‐samples, the SRM intensities of sub‐samples increased up to 40 times compared with bulk ones and their directions were scattered. Higher coercive force grains were magnetized perpendicular to the shock direction for shocks of 5 and 10 GPa, but at 20 GPa the directions were less systematically oriented. These results suggest that the proposed plasma‐induced magnetization of impactites should be reconsidered. 相似文献
34.
Variable microstructure of peridotite samples from the southern Mariana Trench: Evidence of a complex tectonic evolution 总被引：2，自引：0，他引：2
Katsuyoshi Michibayashi Miki Tasaka Yasuhiko Ohara Teruaki Ishii Atsushi Okamoto Patricia Fryer 《Tectonophysics》2007,444(1-4):111-118
We retrieved samples of peridotite from a dredge haul (KH92-1-D2) collected during Cruise KH92-1 undertaken by the research vessel (R/V) Hakuho in 1992 at the landward trench slope of the southern Mariana Trench (11°41.16′N, 143°29.62′E; depth 6594–7431 m), which is the deepest ocean in the world. Ten of 30 retrieved samples possessed both a foliation and lineation, as assessed from 46 thin sections of various orientations and observations of hand samples. The samples showed marked variation in microstructure, ranging from coarse (> 5 mm) equigranular and intensely elongated textures to finer (< 1 mm) porphyroclastic and fine-grained equigranular textures. Olivine fabrics also varied among the different samples, with (010)[100] and (010)[001] patterns (termed A- and B-type, respectively) observed in samples with coarse textures and no clear patterns observed in samples with fine textures. Even though the peridotite samples were retrieved from a single dredge site, some contain primary tectonic microstructures and some contain secondary microstructures. Recent bathymetric and topographic analyses indicate that the lithosphere in this region is as thin as 20 km. Such a thin lithosphere may have been intensely deformed, even perhaps in the ductile regime, during fore-arc extension; consequently, the observed variations in microstructure within the peridotite samples probably reflect the complex tectonic evolution of the southern Mariana region. 相似文献