Yashio Dam is an asphalt concrete facing rockfill dam. Relatively large acceleration was observed at Yashio Dam in the 2011 Touhoku earthquake. Cracks occurred near the left and right abutments respectively. We had studied about the factor of generation of the cracks, and assumed that cracks occurred at the concrete joints in the dam crest because of the concentration of the strain during the earthquake. But, the length of cracks are about 70 to 80 m long in spite of the concentration of strain occurred at the limited area. Therefore, we put forward three hypotheses for the cause of development of the cracks. The one of them was the influence of the accumulated damage during the earthquake. We studied the past results of the cyclic bending tests to estimate it. Another was the influence of the strain occurred during the impoundment. We conducted the bending tests with the pre-bended beam to estimate it. The last one was the decline of the flexibility of the asphalt concrete by cracking. We conducted the bending test of the asphalt concrete beam with cutout and studied about the thermal stress. We finally found that the factor of the development of the cracks was the decline of the flexibility of the facing by the cracking during the earthquake and subsequent thermal stress.

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In dam construction, the grain size distribution of the dam body materials is especially important in quality control. However, in the quality control by the usual method of JIS, the sampling of test pieces is intermittent and there is a time-lag until we catch the grain size of it. On the other hand, because the grain size distribution of Trapezoidal CSG dam and fill dam materials is changing constantly, the new method of quality control to catch it in real time, continuously as well is necessary when we watch it. We applied the analysis system by digital image analysis with recent ICT technology to develop the continuous grain size distribution analysis system to check the grain size of materials on the conveyor belt in real time, continuously as well. And we confirmed the validity of the system by experiment with CSG materials and the concrete rubble.

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The suctioning method of discharging sediments using water head difference has been studied as one of the countermeasure against sedimentation at Yahagi Dam. Onsite substantiative experiment for the operation of discharging sediments by the mobile deck barge using the principle of Siphoning was performed to prove its effectiveness. Five different cases of operation were analyzed in terms of the different soil types of sediments, the current velocity in the suction pipe, and the feature of suctioning operation. Based on the results of proven tests of each case, the functions of equipments were confirmed and the several parameters for design of equipments were analyzed.

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