In the case of grain-size analysis test of soils smaller than 2,000 microns-particle size, a sedimentation process using a hydrometer is used. In this procedure, the dispersing agent is used for securing the necessary and accurate data of grain sizes. In JIS procedure-A 1204-1969, usually, sodium silicate (Na₂Si0₃9H₂0) or sodium hydroxide (NaOH) is used as the agent. However, in Okinawa it is thought that, sodium triphosphate (Na₅P₃0₁₀) is more effective for the certain soils to disperse. This paper clescribes a test results of grain-size analysis using some other dispersing agents for Shimajiri Formation soils to get a suitable agent and its procedure. It is shown that sodium hexametaphosphate (NaP0₃)₆ is more effective than others used before for Shimajiri Formation soils.
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