چكيده به لاتين
Skeletal carbonate soils are formed from the remains of marine organisms, and the type of these organisms depends on the characteristics of the depositional environment; therefore, the same behavior cannot be defined for all carbonate sediments worldwide. This study investigates the physical and mechanical properties of Konarak carbonate sand, located in one of the most strategic regions of southern Iran along the northern coast of the Oman Sea, and examines the effect of marine silt on these properties. To this aim, a systematic experimental program was designed, including basic index tests, one-dimensional compression, triaxial compression, and strain-controlled cyclic triaxial tests. These were conducted on Konarak carbonate sand–silt mixtures containing a wide range of marine silt contents (0% to 100%) under simulated field conditions involving various relative densities (loose, medium, dense, and very dense) and effective confining pressures of 50, 100, and 200 kPa. The results of triaxial compression tests indicated that, under the specified stress levels, Konarak carbonate sand exhibits a strong dilation tendency during undrained monotonic shearing. This behavior is attributed to the unique grain shapes (rod-shaped and disk-shaped), high resistance to particle breakage, and relatively low extreme void ratios. The effect of marine silt on the behavior of carbonate sand depends on both the silt content (SC) and the relative density of the mixture. For instance, adding 10% silt to sand (SC=10%) reduces the undrained monotonic shear strength by approximately 29% and 16% in loose and medium states, respectively, while it has little effect on the shear strength in dense and very dense conditions. Adding 20% silt to the sand enhances the supporting role of silt particles for the sand grains, leading to an increase in monotonic shear strength in all states of the relative density. However, once the silt content exceeds a threshold value (26%), the behavior of the mixture gradually shifts toward that of pure silt, characterized by reduced shear strength and increased tendency to compression. When the silt content is below this threshold, the use of the equivalent intergranular void ratio successfully eliminates the scatter in the test results, aligning them closely around the response of the host clean sand. The results of cyclic triaxial tests also indicated that the trend of variation in capacity energy (i.e. the cumulative dissipated energy required to initiate liquefaction) of sand–silt mixtures versus variation in silt contents was highly dependent on the relative density. For instance, at a relative density of 40%, the capacity energy of clean sand is approximately 19% lower than that of pure silt. However, at a relative density of 80%, this trend reverses, and the capacity energy of clean sand becomes about 185% greater than that of pure silt. This also highlights the dependence of marine silt behavior on the loading type and the resulting strain conditions. Using the concepts of equivalent intergranular and inter-fine void ratios, a new relationship was proposed for estimating the capacity energy of the Konarak sand–silt mixtures under various field conditions. In addition, the performance of several excess pore pressure ratio models—based on dissipated energy—was evaluated for predicting the excess pore pressure generation in carbonate sand–silt mixtures. The calibration parameters of the most compatible model were modified to take into account the effects of silt content, which had been overlooked in previous studies. Similarly, as there exists a distinct relationship between energy dissipation and the excess pore water pressure generation during cyclic loading, a significant correlation is also observed between energy dissipation and stiffness degradation for carbonate soil. Furthermore, it was found that the trends in stiffness degradation under undrained cyclic loading and capacity energy variation with respect to silt content were remarkably similar.
