別紙2　日本造船学会論文原稿(平成12年春期講演会にて発表予定)

　

Study on Prevention of Sliding Failure of Nickel Ore in Bulk

- New Procedure for Evaluating Shear Strength -

by Sususmu Ota, Member^* Tamaki Ura, Member^**

Masaki Murayama, Member^*** Masato Tanaka^***

　

Summary

Nickel ore is natural clay containing large lumps. Every year, around four million tons of this material is transported to Japan from Indonesia, New Caledonia and Philippine by bulk carriers of around twenty thousands to forty thousands tons dead weight tonnage. When nickel ore contains much moisture, slopes of its cargo piles in cargo holds may slide, because its shear strength is low.

To prevent sliding failure of a bulk cargo, it is necessary to evaluate the shear strength of the cargo prior to shipment. Shear strength of nickel ore can be measured by direct shear tests or triaxial compression tests in a laboratory, and the stability of the cargo pile can be evaluated by slope stability calculation which is utilized in the field of soil mechanics based on the measured shear strength. Such method for evaluation, however, cannot be conducted at the time of shipment of nickel ore, because it takes at least a few days. Taking these issues into consideration, we developed a new in-situ test procedure for evaluating shear strength of nickel ore which can be conducted at places of loading for nickel ore in a few hours in view of prevention of sliding failure. To develop the criterion for judgment on sliding failure of nickel ore, we carried out direct shear tests and slope stability calculations for nickel ore from five ports.

The new test procedure mainly consists of three steps, i.e., removal of large particles, insertion of the sample in the mould and cone penetration test. First, particles larger than 19 mm are removed from the wet sample by passing through an appropriate sieve. Next, the sample is inserted in the mould and compacted to make a consistent specimen. Then the cone is pushed down into the specimen and the maximum value of reaction force is measured. When the force is less than 300 N, the nickel ore should not be accepted for loading without taking special safety measures.

　

1. Transportation of nickel ore and its properties

1.1.　Transportation of nickel ore ^{1), 2)}

Nickel ore is natural clay containing large lumps. It is a product of open cast mining and transported to Japan from the ports listed in Table 1, around four million tons per year, by bulk carriers of around twenty thousands to forty thousands tons dead weight tonnage.

When nickel ore contains much moisture, its shear strength is low and the shortage of shear strength may result in sliding failure of a cargo pile. On August 26, 1998, a bulk carrier carrying nickel ore capsized near Okinawa Island. By this accident, ten people were missing and eleven people were rescued. It was supposed that one of the causes of the accident was sliding failure of the cargo. Furthermore, extraordinary lists of bulk carriers resulted from sliding failure of nickel ore were reported several times.

　

Table 1. Ports of loading for nickel ore

　

While loading at the most of all ports listed in Table 1, nickel ore is carried to a bulk carrier anchoring off the pier by barges and is loaded in the cargo holds by ship's grabs. To prevent sliding failure, nickel ore is trimmed as level as practical using ship's grabs. Slopes of a cargo pile, however, still remain after trimming as illustrated in Fig. 1. Usually, cargo handling is suspended in the rain to avoid increase of moisture of the cargo.

　

Fig. 1. Shape of trimmed nickel ore

　

^*　Ship Research Institute

^**　Institute of Industrial Science, The University of Tokyo

^***　Research Institute of Marine Engineering

　

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