Kazuya Yasuhara

Effective use of fly ash slurry as fill material

A lot of effort has been put into increasing coal ash utilization; however, 50% of total amount is disposed of on land and in the sea. Several attempts have been reported recently concerning slurried coal fly ash use for civil engineering materials, such as for structural fill and backfill. The authors have studied this issue for more than 15 years and reported its potential for (1) underwater fills, (2) light weight backfills, and (3) light weight structural fills, through both laboratory tests and construction works. This paper is an overview of the results obtained for slurry, focusing on the following. (1) Coal fly ash reclaimed by slurry placement shows lower compressibility, higher ground density, and higher strength than by the other methods. This higher strength increases stability against liquefaction during earthquake. (2) Higher stability of the fly ash ground formed by slurry placement is caused by higher density and its self-hardening property. (3) Stability of fly ash reclaimed ground can be increased by increasing density and also by strength enhancement by cement addition. (4) Technical data obtained through a man-made island construction project shows the advantages of fly ash slurry in terms of mechanical properties such as higher stability against sliding failure, sufficient ground strength, and also in terms of cost saving. (5) Concentration in leachates from the placed slurry is lower than the Japanese environmental law. (6) In order to enlarge the fly ash slurry application toward a lightweight fill, mixtures of air foam, cement and fly ash were examined. Test results shows sufficient durability of this material against creep failure. This material was then used as lightweight structural fill around a high-rise building, and showed sufficient quality. From the above data, it can be concluded that coal fly ash slurry can be effectively utilized in civil engineering projects.

Swelling characteristics of bentonites in artificial seawater

Bentonite is currently designated for use as a buffer material for the repository of high-level radioactive waste because such a material requires swelling characteristics to seal the waste. A high-level radioactive waste disposal facility may be built in a coastal area of Japan because transportation of this waste by ships is feasible. Therefore, it is important to investigate the effects of seawater on a bentonite-based buffer. This study is intended to investigate the influence of seawater on the swelling pressure and swelling deformation characteristics of five common types of bentonite. The experiments described herein clarify the relations between the influence of seawater grade and compaction density, type of exchangeable cation, montmorillonite content of the bentonite, and vertical pressure condition. Based on experimental results, suitable specifications have been defined for a bentonite-based buffer that can withstand the effects of seawater.

Direct simple shear testing for post-cyclic degradation in stiffness of nonplastic silt

Using a sequential procedure of cyclic and postcyclic direct simple shear tests, the stiffness degradation characteristics of nonplastic silt are studied during and immediately after cyclic loading by direct simple shear (DSS) tests. The results from cyclic and postcyclic DSS tests were interpreted using methods modified from those adopted for cyclic and postcyclic triaxial tests. In particular, the effect of initial static shear stress (ISSS) on postcyclic behavior is considered for nonplastic silt. The findings obtained from the sequential DSS testing on nonplastic silt are: (i) during cyclic loading the tendency of the stiffness to decrease with increasing normalized pore pressure is different for plastic and nonplastic silts, (ii) since the relation between equivalent stiffness ratio and single amplitude shear strain for nonplastic silt does not filt the Hardin-Drnevich Model, new modified relation is needed, (iii) the larger the ISSS is, the faster the decrease in stiffness ratio becomes, and (iv) the decrease of stiffness ratio for nonplatic silt is less marked than that for plastic silt when plotted against the normalized pore pressure after cyclic load.

Investigation of tire chips-sand mixtures as preventive measure against liquefaction

This paper presents results of a series of element testing and model testing in which tire derived geosynthetics such as tire chips are utilized as liquefaction preventive backfill material. Undrained cyclic shear tests were conducted on tire chips and sand mixed tire chips for various percentages of mixtures, and the liquefaction potentials of the mixtures were evaluated. The best mixing percentage of tire chips was found to be close to 50% by the total volume of sand. Also, a model shaking table test on a caisson type quay wall was conducted on liquefaction prevention measures for backfill sand reinforced with tire chips. The test results have demonstrated that, despite the fact that the tire chips reinforced composite backfill has a very low relative density, there was no liquefaction in the backfill. Also, the earth pressure on the wall and its residual displacement could be substantially reduced, implying a good performance of the soil-structure system during earthquake loading.

Reducing flood risks in rural households: survey of perception and adaptation in the Mekong delta

The Mekong Delta, one of the most vulnerable regions to climate change, is the focus of international action for adaptation. Improved understanding of the exposures and perceptions at the local level will help residents to cope with natural disasters and future climate disruptions. In this study, we assess the exposure to floods that rural communities across the Mekong Delta are facing. Among the various natural disasters threatening the delta, extreme flooding is by far the most prevailing concern. By carrying out a field survey on 1350 residents, we characterize the flooding patterns and vulnerabilities of households in three geographically diverse provinces. The aim of this study is to assess the effectiveness of current flood adaptation in these households in conditions of regular flooding, as well as to extreme flooding events, which become more intense and frequent due to climate change. While the findings suggest varying degrees of coping with seasonal flooding by raising the ground floors of their homes, their capacity to prepare for extreme floods is limited in spite of the residents’ awareness of the changing dynamics of natural disasters in their locality. We also examine their perception to the effects of climate change and identify factors that influence their awareness.

Batch leaching test focusing on clod size of drinking water sludge and applicability to long-term prediction using column leaching test

For environmental impact assessment, the Japanese leaching test (JLT) is frequently used in Japan. The grains in JLT are ordinarily smaller than 2.0 mm, but the grain size might be impractical. Therefore, this study investigated the influence of controlled clod size on the leaching characteristics of represented crushable waste; drinking water sludge (DWS). First, compaction tests were conducted using DWS. Second, batch and column leaching tests were performed using various clod size fractions of DWS. The Al concentration was analyzed to investigate the effect on plant growth. Consequently, 81% of DWS was crushed to less than 2.0 mm clod size by compaction, so the clod size in JLT is reasonable for environmental impact assessment of DWS utilization. Furthermore, results of batch leaching tests using clods smaller than 0.25 mm substituted for the maximum Al concentration in column leaching tests.

Adaptation to Climate Change-Induced Geodisasters in Coastal Zones of the Asia-Pacific Region

Results of recent investigations suggest that climate change tends to accelerate geodisasters. Therefore, adaptation to climate change has rapidly become and urgent issue. In comparison to those examining water disasters, few studies have examined climate change-induced geodisasters. This study aims to focus on climate change-induced geodisasters in various countries of the Asia-Pacific region, especially in Japan and Vietnam. Sea level rise is accounted for about 2 mm/y on average in the region. This amount is much larger in some places due to groundwater extraction. Moreover, we should prepare for the worst case in which climate-induced severe rainfall, wave attacks, storm surges and a great earthquake might take place simultaneously or almost simultaneously with each other in the coastal zone, although this worst case might be very rare. As a possible compound geohazard caused by climate change, we propose solutions with emphasizes on using geosynthetics and ecological engineering measures.

Opening Traffic for a Temporarily Remediated EPS Road After the 2016 Kumamoto Earthquake: A World First

In 2016, two successive gigantic earthquakes struck the Kumamoto area of Japan, damaging many private residences and some local roads. An EPS embankment in the Mashiki area near the earthquake epicenter also collapsed. In this EPS embankment, the EPS blocks were shifted sideways because of a landslide behind the EPS fill caused by the earthquake. Nevertheless, only part of the embankment collapsed. The road surface was temporarily remediated and was maintained as it was, for the opening of traffic. This case proves that EPS gives rise to great benefits: most importantly, the collapsed EPS road was put into practical use as a temporary road.

Development of Joint Metal Binder for Improving Earthquake Resistance of EPS Embankment

EPS embankments have been found to be able to ensure earthquake resistance by using joint metal binder between blocks. However, since the conventional joint metal binder is rectangular, it may act eccentric to a seismic force. There were also parts with no spikes around the metal edges, and it was pointed out that it provided a weak connection between blocks. This paper reports on the effectiveness of a new joint metal binder developed and improved on a shear testing machine.

Shaking Table Test of Scaled 1/5 EPS Embankment Model

A large earthquake in the Kumamoto district in the south of Japan in April 2016 occurred twice within 28 h with a magnitude class 6.5. Due to this earthquake many houses and roads collapsed. In order to investigate the behavior of EPS embankments when a large-scale earthquake acts consecutively, an EPS embankment model with a scale of 1/5 was built and a shaking table experiment was conducted using the shaking table (3 m × 2 m) at the University of Tokyo. The EPS embankment was found to cause rocking phenomenon due to seismic motion. As a countermeasure, in addition to the effect of an improved Joint Metal Binder (JBM), the effect of applying a larger number of JMBs was also investigated.