Engineering characteristics of the calcareous sand in Nansha Islands, South China Sea

Calcareous sand is widespread on the coral reefs of Nansha Islands, South China Sea. It represents a special soil type due to its compressibility which results from high void ratio sediments and crushable particles. In oceanic engineering, calcareous sand is commonly used as building foundations and backfill material for road embankments or airport runways. Site investigations of coral reef geology, geomorphology and sedimentation on Yongshu Reef, Nansha Islands, South China Sea indicate that calcareous sand is the main composition of the upper reef flat, the only available site for constructions. Therefore, it is very important to estimate the engineering characteristics of calcareous sand. This paper presents the site geology of calcareous sand on Yongshu Reef and results of corresponding laboratory plate load tests. The bearing capacity and deformational behavior of calcareous sand are studied. Results of this study show that: (1) the bearing capacity of dry sand exceeds 500 kPa when the relative density is 68%, so calcareous sand can be compacted to achieve greater bearing capacity and reduce settlement; (2) plastic deformation is the main part of the ground settlement; (3) the bearing capacity of saturated calcareous sand is 50% less than that of the dry sand; (4) square foundation can provide greater bearing capacity than circular one with the same area; (5) calcareous sand ground is obviously stiffer than quartz sand ground; (6) the effective depth of load transmission is about 1.0 to 2.0 times of the load plate width and increases with the imposed load.

Research highlights
► The bearing capacity of dry sand exceeds 500 kPa when the relative density is 68%, so the calcareous sand can be compacted to achieve greater bearing capacity and reduce settlement.
► Plastic deformation is the main part of the ground settlement.
► The bearing capacity of saturated calcareous sand is 50% less than that of the dry sand.
► Square foundation can provide greater bearing capacity than circular one with the same areas.
► The calcareous sand ground is obviously stiffer than quartz sand ground.
► The effective depth of load transmission is about 1.0 to 2.0 times of the load plate width and increases with the imposed load.