Development of effective and economical technologies to evaluate liquefaction potentials and to prevent liquefaction damages is one of the most pressing issues, which were strongly addressed by the Japanese Geotechnical Society after the serious liquefaction damages occurred in the 2011 Tohoku earthquake. In this paper, a series of large-scale shaking table tests is proposed to obtain the valuable database of case histories for the theme of liquefaction of young reclaimed ground. Using the world largest shaking table facility called E-Defense, this plan aims the new idea of ‘ prototype tests’ rather than the conventional ‘ model tests’.

抄録全体を表示

Soil liquefaction is a common geo-hazard in active earthquake zones. Regular measures against soil liquefaction such as soil densification and soil cementation only have limited scope of applications because of their high costs and complicated implementations. An alternative approach to the mitigation of soil liquefaction is desaturation, that is, to lower the degree of saturation of originally saturated soil. However, the key challenge is to develop a reliable technique to achieve desaturation effect in liquefiable soil ground in a uniform manner. In this paper, an innovative technique, microbial soil desaturation, is introduced and evaluated. Denitrifying bacteria are adopted to produce tiny gas bubbles in-situ from nutrients. The nutrients are dissolved in water and thus can flow like water in sand. The performance of the technique is evaluated using shaking table model tests. It is found that when the degree of saturation of the specimens has decreased from 100% to 90%, the excess pore water pressure generation and the liquefaction potential of sand under seismic loading can be greatly reduced.

抄録全体を表示

Past studies show that soil liquefaction due to a long duration earthquake with low acceleration, for example as observed in the Tokyo Bay area in the 2011 Great East Japan Earthquake, is largely affected by liquefaction strength for a large number of loading cycles. It is, therefore, extremely important to know the liquefaction resistance for a large number of loading cycles. This paper describes the results of the hollow cylindrical cyclic torsion shear tests that were conducted to ascertain the effects of grain size distributions on liquefaction resistance for a large number of loading cycles. The test results clearly show that the tendency of a decrease in liquefaction resistance for a large number of loading cycles is influenced by mean grain size, inclination of grain size distribution, and so on. Finally, the effect of liquefaction resistance curve characteristics on the assessment of liquefaction potential is examined using the cumulative damage theory.

抄録全体を表示

Several methods have been used for the evaluation of liquefaction potential, among which the simplified method is mostly used. In this method, which is mainly based on standard penetration test (SPT), cone penetration test (CPT) and shear wave velocity (Vs) measurement, a boundary curve is provided to separate the liquefiable and non-liquefiable soil zones. Vs measurement is a good alternative method of penetration-based methods (SPT and CPT). This is especially true in micro-zonation of liquefaction potential. Although relatively large studies have been carried out to establish the correlation between Vs and liquefaction resistance for sands; there are uncertainties about the effects of non-plastic fines on the correlation. The objective of this research is to study the effects of fines on the correlation of Vs and liquefaction resistance. In this regard, the cyclic triaxial and bender elements tests have been performed, and Vs and liquefaction resistance of a clean sand and four combinations of this sand with non-plastic fines up to 25% have been measured. A simple, semi-empirical, semi-analytical method is proposed to establish a correlation between Vs and liquefaction resistance. The effects of non-plastic fines on Vs and liquefaction resistance of silty sands have been examined in this study, and the effects of non-plastic fines on the correlations between these two parameters are considered. The results in this study show that an increase in the ratio of silt to sand will result in a decrease in Vs and liquefaction resistance. Based on these results, it is argued that the correlation between Vs and liquefaction resistance of various combinations of sand and non-plastic fines are soil specific. Also based on the results of this study, it is found that the existing methods of liquefaction potential evaluation which are based on Vs may underestimate or overestimate the liquefaction resistance of silty sands.

抄録全体を表示

Mechanical properties of subsoil are frequently subject to temporal change and liquefaction resistance is not an exception. The experiences during the 2011 gigantic earthquake in Japan illustrated that more aged soils did not develop liquefaction. Since this effect of soil age is not explicitly accounted for by existing codes, the authors attempted to interpret cases where subsoil of known age liquefied or not liquefied during past earthquakes. It was found that liquefaction resistance increases after hundreds of years and that the increase of 40% is reasonably supposed to occur after 400 years. Further study was conducted on the mechanism of ageing in which grain dislocation was focused on.

抄録全体を表示

Liquefaction resistance changes with the presence of certain amount of fines in sand but literature related to laboratory and field investigations and the ground behaviour under earthquakes have clearly highlighted the conflicting evidence of the presence of fines on the liquefaction behaviour of sand-fines mixtures particularly low plasticity clay fines. Some studies have indicated decrease in liquefaction resistance with increase in plasticity and vice-versa. The postcyclic undrained shear behaviour of sand and low plasticity are controlled by excess pore pressure ratio and the behaviour is independent of density and confining pressure. In this paper, a series of undrained stress controlled cyclic triaxial shear tests and monotonic undrained shear loading were conducted on reconstituted sample of sand-fines mixtures to evaluate the liquefaction and postliquefaction behavior. Sand was procured from Cauvery river bed, Karnataka, India and locally available low plasticity clay fines were used to prepare sand-fines mixtures. The results indicate that Cyclic Resistance Ratio (CRR) increases with relative density and the post cyclic monotonic strength show increased stress at Ru = 0.

抄録全体を表示

The influence of fines on undrained behavior of sand was evaluated through a series of tests conducted in silty sand retrieved from Tokyo Bay Area after the 2011 Tohoku Earthquake and an artificial mix of low-plasticity fines with sand. Since the use of density measures as void ratio, relative density or sand skeleton void ratio has led to divergent results, a different approach was taken in this research, by using compaction energy as the parameter of comparison. Stress-strain curves and stress paths for non-plastic and low-plastic fines are given. Excess pore pressure development and liquefaction curves are provided and compared. It can be seen that the addition of fines decreases liquefaction resistance either if they are of non-plastic or low-plastic nature. After conducting cyclic shearing, drainage was opened to measure volumetric deformation and results are also reported. Relevant conclusions regarding the role of fines in liquefaction potential and post-liquefaction behavior are provided

抄録全体を表示