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Excavating the ground which consists of the marine deposits formed during Neogen and/or Quaternary period, the acid sulfate soil occasionally appears on the execution basis. As this soil includes sulfuric acid produced with the oxidation of sulfide, the growth of plants is disturbed and it becomes difficult to improve the soil by adding a stabilizer. In general, the calcic materials such as stone powder and zeolite have been employed to neutralize the acid sulfate soil from the past. Therefore, the shells which are composed mainly of a calcium carbonate can also be considered to be useful as a material for the neutralization process. We have ascertained the stabilization effect due to the using of crushed oyster shell by comparing the amount of swelling and CBR value for soil-stabilizer-shell mixture with those for soil-stabilizer mixture. Mixing crushed oyster shell into the soil, pH value of the mixture gradually shifted to neutral with elapsed time and its water content decreased immediately. Furthermore, the amount of swelling which occurred during submergence curing was extremely small for soil-stabilizer-shell mixture and CBR value of this mixture was 1.5 to 4 times larger than that of soil-stabilizer mixture. These results indicate that both surplus soil and non-industrial waste will be utilized effectively as embankment and subgrade materials at a time.

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In the oyster producing district, the shells are being crushed to maintain the capacity of dump yard. Though the crushed oyster shell has been diverted to feed, manure and soil conditioner from a past, the shell pieces are possible to also use as the construction materials. The purpose of present study is to verify the usefulness of crushed oyster shell as Subgrade materials by executing improvement of existing road. The construction work was carried out in March, 2002 and the widening subgrade (70m in length, 3.5m in width and 0.35m in thickness) containing crushed oyster shell was built under three mixing conditions. The surface treatment was immediately paved over the base course after base materials were compacted. Allowing passage of vehicles, we have recently investigated the deflection shape of road surface by using "falling weight deflectometer" (FWD). According to the relationship between the load and the detected deflections, the maximum deflection at loading point tended to decrease with an increase in service time. Particularly, CBR value estimated from the deflection (D1500) has exceeded an object level of improvement for section including the almost dried shell pieces. These results show that the crushed oyster shell can be effectively utilized as Subgrade materials.

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