1.2. Properties of nickel ore
The samples of nickel ore from Gebe, Pomalaa, Boakaine, Tontouta and Rio Tuba were used in this research. Table 2 shows solid density, i.e., the density of solid part not including void consisting of moisture and air, of the samples and Fig. 2 shows the grain size distributions of them. The grain size distributions were measured after removing particles larger than 100 mm. In the figure, the abscissa and ordinate denote the grain size in mm and percentage of finer particles in dry weight, respectively. The graphs are the grain size accumulation curves of the samples. The figure shows that nickel ore varies in grain size distribution.
Table 2. Solid density
Fig. 2. Grain size distribution of the samples
1.3. Limitation of moisture content 3)
Sliding failure of nickel ore can be prevented by limiting the moisture content. To limit the moisture content of nickel ore, the upper bound of moisture content should be determined for each type of nickel ore, because it varies so wide depending on properties of nickel ore, in particular on grain size distribution. In laboratory, it is possible to determine the upper bound of moisture content based on shear tests and slope stability calculations. It takes, however, a few days to determine the upper bound of moisture content by such method. There is, therefore, no appropriate method for determining the upper bound of moisture content in a short time which can be employed in the practice of transportation of nickel ore.
For cargoes which may liquefy, the methods for determining the transportable moisture limit are described in the Code of Safe Practice for Solid Bulk Cargoes 4). These methods, however, cannot be applied to the determination of the upper bound of moisture content of nickel ore, because sliding failure of nickel ore is the phenomenon caused by the shortage of static shear strength irrelevant to liquefaction which is caused by cyclic loading and increase of pore water pressure.
2. New test procedure
2.1. Outline of the new test procedure
Considering the above issues, we developed the new in-situ test procedure for evaluating shear strength of nickel ore in view of prevention of sliding failure. The proposed test procedure can be conducted at places of loading for nickel ore in a few hours. By applying the test for nickel ore to be shipped, it is possible to avoid the shipment of dangerous nickel ore which contains excessive amount of moisture.
The new test procedure mainly consists of three steps, i.e., removal of large particles from a sample, insertion of the sample in the mould and cone penetration test which is employed for evaluating foundation in the practice of construction of houses and buildings. 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, which is called cone penetrating force, is measured. Insertion of the sample and cone penetration test are repeated three times. The results of these tests are represented by the minimum value of three maximum cone penetrating forces which is called representing cone penetrating force hereafter. When the representing cone penetrating force is less than 300 N, it is judged that the nickel ore should not be accepted for loading without taking special safety measures.
It should be noted that the criterion for judgment on prevention of sliding failure in the test procedure is determined under the premise that nickel ore is trimmed as illustrated in Fig. 1 and that increase of moisture content during loading operation is negligible. In other words, nickel ore should be trimmed as far as practical and increase of moisture content due to rain or ingress of sea water into the cargo on a barge should be prohibited for safe transportation, regardless of the judgment based on the new test procedure.
2.2. Removal of large particles 1)
When the cone hits a large particle during the cone penetration test, the cone penetrating force increases considerably. On the other hand, macroscopic shear strength of nickel ore depends on the characteristics of fine particles and effect of large particles on the shear strength of the material is not significant. The large particles should, therefore, be removed from the sample to keep its consistency. Considering the maximum grain size described in JIS A 1210: 1999 (c.f. 2.3) and the diameter of cone (c.f. 2.4), we determined the maximum grain size at 19 mm.
