Norimasa Yoshimoto

A comparative analysis of the mechanical behavior of carbon dioxide and methane hydrate-bearing sediments

Abstract Understanding the mechanical behaviors of carbon dioxide/methane hydrate-bearing sediments is essential for assessing the feasibility of CO2 displacement recovery methods to produce methane from hydrate reservoirs. In this study, a series of drained triaxial compression tests were conducted on synthetic carbon dioxide hydrate-bearing sediments under various conditions. A comparative analysis was also made between carbon dioxide and methane hydrate-bearing sediments. The stress-strain curves, shear strength, and the effects of hydrate saturation, effective confining stress, and temperature on the mechanical behaviors were investigated. Our experimental results indicate that the newly formed carbon dioxide hydrate would keep the reservoir mechanically stable when CH4-CO2 gas exchange took place in a relatively short period of time and spatially well distributed in the pore space. Experiments of CO2 injection in methane hydrate-bearing sediments are necessary to confirm this hypothesis.

Seismic Response Characteristics of Saturated Sand Deposits Mixed with Tire Chips

With the objective of better and more environmentally friendly recycling methods, many researchers are now examining the use of scrap tires as a new geomaterial. Based on past research, it was clear that tire chips reduce the rise of excess pore-water pressure when subjected to earthquake shaking. Based on such characteristics, online pseudodynamic response tests were conducted in this study on model grounds consisting of either tire chip-mixed sand or alternating layers of sand and tire chips with the aim of clarifying the seismic response characteristics of tire chips and tire chip-sand mixtures. Online testing is a method of feeding soil response characteristics directly from soil samples into a one-dimensional modeling algorithm. The test results showed that when tire chips with low stiffness were either mixed with sand or placed as layers, more significant damping and seismic isolation effects were observed. The presence of tire chips also reduced the accumulation of excess pore-water pressure in the layer, preventing the occurrence of liquefaction. In addition, when tire chips are installed as layers beneath the sand, liquefaction is not generated in the upper sandy layer because the amplitudes of the seismic waves are attenuated. Finally, the effectiveness of tire chips mixed with sand increased as the mix ratio was increased. When they were installed as pure layers, tire chips were more effective when placed at a deeper location or when the layer was thicker.

Bonding Strength by Methane Hydrate Formed among Sand Particles

The mechanical properties of methane hydrate‐bearing sand were investigated by low temperature and high confining pressure triaxial testing apparatus in the present study. The specimens were prepared by infiltrating the methane gas into partially saturated sand specimen under the given temperature and stress condition which is compatible with the phase equilibrium condition for the stability of methane hydrate. The tests were firstly performed to investigate the effect of temperature on the shear behaviour of the specimen. Then the effect of backpressure was investigated. The strength of methane hydrate bearing sand increased as the temperature decreased and the back pressure increased. The bonding strength due to methane hydrate was dependent on methane hydrate saturation, temperature and back pressure but independent of effective stress. Dissociation tests of methane hydrate were also performed by applying the temperature to the specimen at the various initial stress conditions. The marked development o...

Dynamic Behavior of Granulated Coal Ash during Earthquakes

AbstractCoal ashes discharged from coal-fired power plants have recently been gaining attention as a new form of geomaterials. They have been popularly used as ground materials for the improvement of unsuitable soil and for foam-mixed solidified soil. However, development of more applications is needed in light of the enormous amount of coal ash generated. The current study was performed to confirm the applicability of granulated coal ash (GCA) as reclamation material with adequate resistance against liquefaction during an earthquake. In this study, the liquefaction characteristics of GCA were investigated through cyclic triaxial tests and online pseudodynamic response tests, and the results were compared with those of natural sands to examine the cyclic shear properties of the material. The triaxial test results revealed that the slope of the cyclic shear strength curve of GCA was gentle under high confining pressure, but it was still higher than those of natural sands. It was observed that GCA underwent...

Material Characteristics of Clinker Ash and Examination of Applicability for Embankment

The clinker ash is a coal ash generated by the coal‐fired power station. It is characterized by its lightweight particle with asperity due to foaming during production process resulted in high shear stiffness and strength and high permeability. It is, therefore, increasingly used for the embankment material, the base course material, and the drainage material. Moreover, the application has started to be investigated for the vertical drain method and the compaction pile method. However, the physical characteristics and the mechanical characteristics of clinker ash have not been understood yet. It is expected that the stock yard for coal ashes will be insufficient as production of the coal ash will increase. It is necessary to use the coal ash more to maintain the global environment. The design parameter for clinker ash to apply to the embankment material was experimentally investigated in this research.

FUNDAMENTAL STUDY ON A CONSTRUCTING METHOD OF THE HIGH DENSITY SAND PILE BY IRRADIATION OF THE ULTRASONIC WAVE BY THE PRACTICAL SIZE

超音波を利用した高密度砂杭工法の開発を最終目的にしている．本研究は既に効果が確認された室内要素試験1)，2)と同等レベルの高密度化が可能で，かつ実用サイズに反映できる超音波の照射方法，および必要な水量を調査することを目的にした．加えて，ポータブルコーン貫入試験を実施し，相対密度と強度の関係についても調査した．そのため，実用サイズの土槽実験装置を試作し，実験を実施した．その結果，実用サイズについても室内要素試験と同程度の密度に豊浦砂が高密度化できること，豊浦砂の高密度化に土槽内の水位が影響しないこと，高密度化した豊浦砂の相対密度とポータブルコーン貫入抵抗の間に良好な相関関係があることが確認できた．

FUNDAMENTAL STUDY ON A CONSTRUCTING METHOD OF THE HIGH DENSITY SAND PILE BY IRRADIATION OF THE ULTRASONIC WAVE

超音波を利用した，騒音，振動，消費電力を大幅に低減可能な高密度砂杭工法の開発を目的にしている本研究は，開発した超音波供試体作製装置を使って，まず2つの供試体作製法について優位性と超音波の音圧が豊浦砂の高密度化に与える効果を検証した．次に超音波照射と水中落下打撃法および超音波を照射せずに作製した供試体の三軸圧縮試験を行い，それぞれのせん断特性を比較した．最後に超音波照射による相対密度と内部摩擦角の増加量を確認した．その結果，超音波照射による豊浦砂の高密度化特性およびせん断特性は水中落下打撃法と比較して同程度以上であることが確認できた．

Earthquake-Induced Deformations of Partially Saturated Embankments

The amount of water in soil plays very important role in defining its shear strength. Thus, when considering partially saturated slopes subjected to earthquake-induced ground shaking, it is important to quantify how such slopes respond to the induced dynamic stresses. For this purpose, undrained cyclic triaxial tests were performed on decomposed granite soil in order to quantitatively investigate the effects of degree of saturation and initial static shear stress on its earthquake response characteristics. The experimental results obtained were then used in conjunction with pseudo-static slope stability analysis and modified Newmark sliding block model to estimate the deformation of embankments which were under different degrees of saturation. The example calculations clearly showed the effect of saturation on the magnitude of seismic-induced displacements of embankments.