A risk analysis method of an irrigation system in an earthquake is proposed. The irrigation system consists of irrigation tanks and canals. The damage probability of the irrigation tanks and canals was obtained from both numerical simulation and actual disaster data from the Mid Niigata prefecture earthquake. The losses due to reduced crop yield, restoration and secondary disaster were considered in the risk assessment. The probability of annual peak ground acceleration was introduced from the earthquake records in Niigata prefecture. To reduce the damage probability, an enhanced foundation of canals on flat land and widening of the embankment were applied. It was found that the countermeasures for the irrigation tanks were more effective than those for the canals. In the case of a large secondary disaster of the irrigation system on flat land, the countermeasures for the irrigation system were very effective.
An efficient and low cost method is required to improve the saline soils. Soil slaking has long been studied from the stand point of stability of aggregates. However, it has not been studied from that of salt removal. The objective of this study is to examine the contribution of slaking to desalinization of soil accompanied by land drying practice. A slaking test was carried out for evaluating the efficiency of slaking and their impacts on salt removal of salinized soil under various water contents. We prepared natural/virgin and air-dried soils to give different intensity of pre-drying. Those soils were resaturated (for air-dry soil) and well-mixed, then dried to different moisture contents (60, 50, 40, 30, 20 and 10% by weight). After 24 hours immersion in water, the soils never slaked at 60 and 50% moisture contents in natural soil whereas 88-89% of the specimens were slaked in air-dry soil under the same moisture contents. The slaking rate was highest under 30% moisture contents in natural soil. In air-dry soil 30 and 20% showed the higher slaking rate in compared to other water contents. The proportion of salt released into equilibrated water after 24 hours immersion was also high at the same water contents. Since the natural soil did not slake until 40%, drying below 30% moisture content will be effective for the removal of salt from these soils.