Development of high-pressure low-temperature plane strain testing apparatus for methane hydrate-bearing sand

AbstractA high-pressure low-temperature plane strain testing apparatus was developed for visualizing the deformation of methane hydrate-bearing sand due to methane hydrate production. Using this testing apparatus, plane strain compression tests were performed on pure Toyoura sand and methane hydrate-bearing sand with localized deformation measurements. From the results, it was observed that the methane hydrate-free specimens, despite their relatively high density, showed changes in compressive volume. Marked increases in the initial stiffness and strength of the methane hydrate-bearing sand were observed (methane hydrate saturation of SMH=60%). Moreover, the volumetric strain changed from compressive to dilative. For the specimens with methane hydrate, a dilative behavior above SMH=0% was observed. An image analysis showed that the shear bands of the methane hydrate-bearing sand were thinner and steeper than those of the host sand. In addition, the dilative volumetric strain in the shear band increased markedly when methane hydrate existed in the pore spaces

On-line Response Tests on Case History of Earthquake Induced Deformation of River Dykes Founded on Saturated Sandy Deposits

River dykes and road embankments are frequently damaged during earthquakes. The liquefaction of foundation, the behavior of which is not yet well realized, is considered to be the main cause of the damage. Based on the results of past studies, the foundation of an embankment was divided into three zones to examine the failure modes. One-dimensional on-line earthquake tests, which were conducted by a combination of element tests and computer earthquake response analyses, were performed for such zones of actual river dykes damaged during earthquake. The cumulative horizontal displacement values obtained by the tests were compared with the measured embankment-crest settlement data, which showed that the liquefaction sliding failure under the toe of slope of such an embankment is found to be the most detrimental of all failure modes

Tire-Chip Reinforced Foundation as Liquefaction Countermeasure for Residential Buildings

To prevent vibration-induced and liquefaction-induced damage to residential buildings during earthquakes, a low-cost technique has been developed and described here. It utilizes a mixture of tire chips and gravel as the horizontal reinforcing inclusion under the foundation of residential houses. The horizontal reinforcing inclusion refers to a layer of tire chips and gravel which is placed horizontally beneath the foundation. This mixture of tire chips and gravel provides sufficient bearing capacity to the foundation. In this research, a series of small-scale 1 g model shaking table tests was performed to evaluate the effectiveness of the technique. In addition, cyclic undrained triaxial tests were performed to evaluate the liquefaction susceptibility of tire chip-gravel mixtures. The results of the model tests indicated that when the thickness of the reinforced layer is 10 cm (2 m in prototype) and the gravel fraction (percentage by volume of gravel in the mixture) is 50%, the technique yields the best performance. The element tests also indicated that the gravel fraction plays an important role. A gravel fraction of 50-60% by volume was found to be the best mixing percentage, at which the rise in excess pore water pressure could be significantly restrained without compromising the stiffness of the reinforcing inclusion

Strength reduction mechanism of cement-treated soil under seawater environment

Improving soft grounds with cement or lime is commonly used to increase their strength and deformation characteristics. However, the properties of cement/lime-treated soil deteriorate in seawater because magnesium salts accelerate calcium leaching. In this study, changes in the unconfined compressive strength of cement-treated soil samples with various water contents, amounts of added cement, and curing times were investigated after immersion in a highly concentrated Mg solution. Subsequently, a thermogravimetric-differential thermal analysis and scanning electron microscopy were used to determine the strength reduction mechanism based on the changes in the hydrate composition as the cement-treated soil deteriorated. The results indicate that the cement-treated soil lost more than 80% of its strength after immersion in the Mg solution. The initial conditions strongly influenced the strength of the deteriorated soil, and higher strength was observed in the samples with larger amounts of added cement and longer curing times. Furthermore, calcium silicate hydrate (C-S-H) and ettringite were not present in the deteriorated soil, implying the presence of magnesium silicate hydrate (M-S-H). Therefore, it was postulated that the loss in strength of the cement-treated soil in a seawater environment was caused by the transformation of C-S-H to M-S-H

Influences of Particle Characteristic and Compaction Degree on the Shear Response of Clinker Ash

Clinker ash is regarded as a granular waste of coal combustion but potentially employed as a recycled and light-weight backfill material for retaining wall and embankment in recent years. The physical, particle, compaction and mechanical properties of six selected types of clinker ash were thoroughly examined in this experimental investigation. Clinker ash owns very complex and angular shapes and it gives rise to the larger difference between the maximum and minimum void ratios. The single particle crushing strength of clinker ash is around 1/5–1/10 lower than natural sands and it indicates high crushability in nature. With similarity to natural sands, the mean crushing strength of clinker ash displays a decreasing tendency with the increase in grain diameter. Aseries of triaxial compression tests were performed on different types of clinker ash to examine the influences of particle characteristics, degree of compaction and effective confining pressure on their shear behaviour and deformation characteristics. Test results demonstrate that clinker ash possesses a higher peak friction angle at low effective confining pressures and gradually loses its shear strength with the rise in effective confining pressure. The great shear strength dependence on the stress level for clinker ash is confirmed. The stress-dila-tancy behaviour of a given type of clinker ash is minimally affected by the degree of compaction and level of effective confining pressure. The stress ratios at the critical state of clinker ash are well correlated with the mean crushing strengths. A larger N value for clinker ash determined using Nova's rule indicates its higher crushability. The downward shift of critical state line of clinker ash due to grain crushing is identified on the void ratio and logarithm of effective mean stress plane. Additionally, grain crushing was confirmed by the comparison of the variation in grain size distribution curves and the observation of colored particles previously seeded in the tested samples before and after shearing

Effect of fines on the compression behaviour of poorly graded silica sand

A series of high-pressure isotropic compression tests were performed on four types of poorly graded silica sand that were artificially prepared based on representative grading curves and similar mineralogy composition of seabed sediment containing different fines contents existing in the Nankai Trough. The addition of fines steepens the initial compression path and increases the decrement of the void ratio after loading. The transitional behaviour of the poorly graded sand with a larger amount of fines content was identified. The slope of the normal compression line shows a slight decreasing tendency with the level of fines content. The bulk modulus of silica sand with fines was lower when compared with the published results of silica sand without fines. A small amount of particle crushing of the four types of poorly graded sand with variable fines content levels was noticed, and the results indicated that the degree of particle crushing tended to decrease as the fines content increased