عنوان مقاله [English]
Knowledge of the dynamic behavior curves of soils is important for designing various geotechnical structures subjected to different kinds of vibrations, including earthquakes. Reviewing the previous studies on soil dynamic behavior shows that most of the studies have focused on the siliceous soils. Geological studies suggest that vast areas of the earth in the tropical regions are covered by calcareous deposits. Large areas of the southern Iran, which are seismic-prone based on the recent earthquake activities, are also covered by calcareous soils. Foundation problems associated with carbonate soil deposits, particularly as experienced by the offshore hydrocarbon industry have led to significant research focused on understanding the behavior of these soils. Origination of calcareous soils from processes and minerals different than those known for the silicate soils clarifies the necessity for comprehensive research on geotechnical properties of such soils. The wide variation of the origin of calcareous soils, due to their offshore locations and the related fauna that make their formation, merit more research into the behavior of these soils from various regions.
In the current research, shear modulus and damping ratio of Hormuz calcareous sand and Babolsar siliceous sand were measured and compared. An identical grains size distribution curve was synthetically attained for the tested sands. The soils are classified as poorly graded sand (SP) according to the USCS (ASTM D2487). The shear modulus and damping ratio values were calculated at small and large shear strains using resonant column and cyclic triaxial tests. The calcareous and siliceous soil specimens were tested by a fixed-free type of resonant column apparatus (SEIKEN model). By using the resonant column apparatus, shear modulus and damping ratio of the calcareous and siliceous sands were measured for the shear strain amplitude ranging from about 10-4 % to 10-2 %. The cyclic triaxial tests were conducted using a fully automated GDS triaxial testing apparatus. The cyclic tests were done on the samples with shear strain amplitudes ranging from about 10-2 % to 1 %. The procedure used to perform the resonant column and cyclic triaxial tests was the multi-stage strain-controlled loading under undrained condition.
The results for both calcareous and siliceous sands indicate that the shear modulus (G) decreases and damping ratio (D) increases with an increase in shear strain amplitude (γ), as expected. Moreover, the shear modulus of tested soils increases with the increase of effective confining pressure (σ'0). Further, the tests results indicate that the increased amount of effective confining pressure leads to the smaller damping ratio for the Hormuz and Babolsar sands. The shear modulus of Hormuz calcareous sand is higher than that of the Babolsar siliceous sand while grains size distribution, effective confining pressure, and void ratio of both soils were identical. It is evident from the resonant column and cyclic triaxial tests that the Hormuz calcareous sand has lower values of damping ratio (D) in comparison to the Babolsar siliceous sand. Finally, dynamic behavior curves of the studied sands are compared with the available relationships. These comparisons indicate that the available relationships are unable to accurately assess behavior of the tested calcareous sand.