Abstract: Carbonate sand, a geomaterial with special structural and mechanical properties, is the main material of the South China Sea Island Reef blow-fill project. A series of drained triaxial tests was carried out on carbonate sand that had previously been extensively researched. This paper presents a modified model referred to as the Shen Zhujiang's elastoplastic model that is capable of predicting the experimental data and simulating the stress-strain behaviour of carbonate sand over a wide range of confining pressures and regions. A functional relationship between the tangent modulus and the ratio of tangential volume to the stress ratio of carbonate sand is proposed. These functional relationships enable the model to describe the characteristic transitions from contractive to dilative shear response of carbonate sands as the stress increases under low confining pressure. Meanwhile, predictions based on the modified models are obtained and compared with the experimental data of the carbonate sand. It is found that the characteristic dilatancy and contraction of carbonate sand, as well as the stress-strain softening curve can be reasonably described compared with the drained triaxial tests data to illustrate the modified model capability for describing the shear behaviour of carbonate sands. For the un-drained triaxial tests, the pore pressure coefficient was introduced into the expression of pore pressure, and the results show that the data is well simulated. Compared with the existing Sun Jizhu's elastoplastic model, the proposed model expression is easy to understand and the parameters can be obtained easily.

Key words: carbonate sand, south water double yield surface model, tangent modulus, pore pressure, stress-strain relationship