An Investigation into Workability of the Cover Layer Materials

Ninomiya, Koji*; Yoshizawa, Hideaki*; Sato, Yasushi*; Onishi, Toshimitsu*

Layer to be applied for the capping of uranium mill tailings and waste rock yard at Ningyo-toge.In consideration of the business scale, operation efficiency and cost performance, etc, we selected the decomposed granite as a base soil, bentonite as an additive, and a Twister as a mixer for this research. Based on those materials and a mixer, we actually made the cover layer and measured the permeability, N2 ventilation, strength of the layer, using as a parameter different types of bentonite and different bentonite/sand mixture ratios.According to the permeability test results, permeability coefficient proved to be stand at below 1x10-9 m/s, regardless of any combination of bentonite/sand mixture ratios made with the twister. Through a series of laboratory tests, base soil and additive, we found out the optimum phase of combination, which are the 7wt% bentonite/sand mixture in case of Volclay; and 16wt% in case of Redhill. N2 ventilation tests were also carried out, using as a parameter the degree of moisture saturation of cover layer material. Test results showed that the gas ventilation is sensitive to changes of the degree of the saturation, and that under the conditions of moisture saturation of over 90%, the coefficient of N2 ventilation stands at below 1x10-10 m/s, under which conditions the radon barrier will work out in an efficient way.Lastly, we described the directions of future investigations and studies, including the necessity of gathering technical data on the quality variation of cover layer materials, the same caused by continuous mixing process, and the need of further investigations and designing into/of the comprehensive cover layer systems, including the capillary-break layer, etc.