Mechanical characteristics of undisturbed coral gravel soils: The intergranular void ratio as a common governing parameter

Coral gravel soils are composite soils comprised of large fragments of finger coral and fine particles from silt matrixes. When the amount of coral fragments is small, the mechanical behavior of the coral gravel soil is mainly governed by the silt matrixes, and when the amount of coral fragments is large, the mechanical behavior is mainly governed by the coral fragments. Undisturbed samples, called high-quality samples in the present study, were collected at large-scale coastal construction sites in Okinawa Prefecture and Kagoshima Prefecture, Japan. A series of triaxial CD-tests was conducted in the laboratory to evaluate the mechanical characteristics of coral gravel soils. The test results were examined from various viewpoints to find a common governing parameter for the mechanical behavior of coral gravel soils. It was anticipated that it would be difficult to interpret the mechanical behavior of the high-quality samples in a uniform manner because undisturbed coral gravel soils, in the form of natural sediments, are generally very heterogeneous. To provide a reference for the parametric interpretation of the test results showing the remarkable features of coral gravel soils, such as interlocking and particle crushing, the test data newly obtained for the high-quality samples were compared to the previous test results obtained for reconstituted mixtures having various volumetric percentages of coral fragments. It was found that an intergranular void ratio corresponding to 0.075 mm, in which particles finer than a grain size of 0.075 mm (i.e., particles of clay and silt) are regarded as voids, can be employed as a useful parameter in evaluating the shear strength of both reconstituted and undisturbed coral gravel soil samples

Suffusion in densely compacted Satozuka pumice sand and its impact on static loading undrained shear strength and dilation behaviour

Pumice sand of volcanic origin contains a high fraction of non-plastic fines (>40 % for Satozuka pumice sand in Sapporo, Japan). Suffusion in such soil can wash away a portion of the fine particles and alter the soil microstructure. The moisture content and degree of compaction can affect the suffusion characteristics of soil deposits, however their effect has not yet been evaluated. Future construction sites in growing Sapporo City, consisting of pumice sand, will require a high degree of compaction (over 90 % and preferably over 95 %) as this sand is prone to suffusion in spite of its dense state. The aim of this study is to assess the impact of suffusion on densely compacted pumice sand with a high proportion of fines, based on its mechanical properties, with an emphasis on shear strength and dilatancy. Firstly, the suffusion characteristics of Satozuka pumice sand were evaluated. Subsequently, undrained triaxial tests (CU¯ tests) under monotonic loading were conducted on high-density specimens, with suffusion and without suffusion, to study the impact of suffusion. It is seen in the results that the hydraulic conductivity, shear strength, stress paths, and dilatancy are all noticeably affected by suffusion. The specimens with suffusion exhibit an increase in residual shear strength and maximum deviator stress under shearing and experience an earlier occurrence of phase transformation from contraction to dilation during shearing. This tendency implies that suffusion has no significant negative impact on the deterioration of earth fill made from pumice sand and non-plastic fines, and that it persists at degrees of compaction between 80 % and 100 %

Ground movements and damage in Satozuka district, Sapporo due to 2018 Hokkaido Eastern Iburi Earthquake

This report describes the damage and ground movements observed in the residential area of Satozuka 1-Jo, Sapporo, Japan during the 2018 Eastern Iburi Earthquake as triggered by the liquefaction of pumice sand with which a valley had been filled. The geographical setting and the on-ground and aerial reconnaissance and soil characterization results are reported, leading to a possible scenario of the state of the pumice fill from its formation to the earthquake. This report focuses on the liquefaction disaster in Satozuka 1-Jo and summarizes it as follows: (a) The Satozuka residential area is located at the end of a plateau consisting of pyroclastic flow deposits, pumice sand called Spfl, deriving from the Lake Shikotsu caldera (30 to 40 thousand years ago), and some valleys that formed by erosion. (b) To develop level residential land, the pumice sand ridges were cut and the valleys were filled in with the pumice sand in the 1970s. The gradual loss of drainage capability might have led to a rise in the long-term groundwater level. (c) Liquefaction of the pumice sand fill was triggered and the fluidized fill flowed downwards along the sloped topography. Where the slope angle became steeper, asphalt pavements seem to have been pushed upwards and broken through by the liquefied soil, and the liquefied soil flowed out to the ground. (d) Physical and mechanical tests were conducted on the pumice sand fill, and its characteristics were reported with particular emphasis on its volumetric behaviour upon compaction, wetting, and pre- and post-liquefaction compression. Undrained cyclic triaxial tests showed the low liquefaction resistance of the pumice fill despite its high fines content (around 40%, non-plastic). (e) The laboratory test results and field evidence suggest that the potential key factor of the liquefaction at this site was the loosely compacted pumice sand fill, which easily forms a loose soil structure with macropores even when compacted on the wet side of optimum