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General attention to carbonate sandshas been begun since the early 1960 in Iran. When the first bore drilled the Persian Gulf’s bed, it drew out layers of this soil type along with sand and shell. However, the large amount of carbonates in this soil was not first diagnosed. The first knowledge of the unique behavior of this type of soil was obtained during pile driving in the construction of an oil platform in Lavan Island, Persian Gulf, in 1968. In this operation, after a number of 30 inch pipe sank into the cemented soil for about 25 feet, they suddenly had a free fall with no resistance up to depth of 50 feet. Carbonate materials and aggregates can be found in nature in different shapes. The majority of them are made of calcium carbonate or other types of carbonate with soft and loose grains that can break easily. These soils are of remarkable importance in offshore engineering. Since early 70s, a great amount of studies have globally been conducted for identifying the unique engineering characteristics of carbonate sands. Different behavioral parameters including strain-stress behavior, shear strength, internal friction angle, volumetric behavior, elastic and shear modulus, Poisson’s ratio, carbonate content, shape of carbonate grains andetc have been studied. No certain laboratory methods or field study plan have yet been provided to obtain appropriate parameters for designing foundations in carbonate soils. Despite this, performing a number of laboratory and field experiments can be helpful. Executing at least amount of experiments to determine the following matters are essential: • Material composition, especially calcium carbonate content • The origin of the materials to distinguish between skeletal and non-skeletal deposits • The properties of the gains such as angularity, porosity, and initial void ratio • Compressibility of materials (using consolidation test) • Strength parameters of the materials, especially internal friction angle • Cementation, at least quantitatively In this paper, shear behavior of four skeletal carbonate sand samples obtained from the northern coasts of Persian Gulf and Oman Sea are investigated. Regarding some parameters of samples such as particle size distribution, physical properties, microscopic images of grains, compressibility, drained triaxial shear behavior, elastic modulus and internal friction angle it was attempted to promote geotechnical knowledge and improve civil engineers understandings of carbonate soils in this part of the world. On the basis of experimental tests like one-dimensional consolidation and triaxial tests, it was determined that despite a number of similarities between the shear behavior of carbonate sands, behaviorally they depend on the grains shape and size, initial particle size distribution and void ratio, calcium carbonate content, confining stresses and applied strains. Although all specimens were selected from the southern coasts of Iran, but a large number of difference was observed between them in terms of shear behavior and strength

Keywords: Carbonate sands, Triaxial experiment, Shear behavior, Persian Gulf, Oman Sea

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