Coral gravel soils are composite soils comprising of finger coral fragments and silt matrix. In a case of small amount of coral fragments, the mechanical behavior of the coral gravel soil is governed by silt matrix, and in a case of large amount of coral fragments, mechanical behavior of the coral gravel soil is governed by coral fragments. As the first step, the authors prepared an artificial coral gravel soil mixtures with various volumetric percentages of coral fragments, and conducted a series of triaxial tests. It was clarified that the contact effect (or interlocking) between coral fragments was observed for the samples with volumetric percentage of coral fragments larger than a certain threshold value (20% in the authors’ previous study). For undisturbed samples collected from the field, however, the mechanical behavior might be different from that for the reconstituted samples, because the silt matrix is non-uniform and the coral fragments have various dimensions and shapes. In the present study, high-quality samples collected as undisturbed samples from various sites were examined through a series of triaxial CD tests. The test results were discussed with various points of view to find an integrated governing parameter for the mechanical behavior of coral gravel soils. It was found out that intergranular void ratio corresponding to 0.075 mm, in which particles finer than a diameter of 0.075 mm (i.e. particles of clay and silt) are regarded as void, is very useful parameter in evaluation of the shear strength of both the reconstituted and undisturbed coral gravel soil samples.
Keywords: Coral gravel soil, undisturbed sample, coral fragment, silt matrix, particle crush, triaxial test