Determination of Small-strain Dynamic Shear Modulus and Damping Ratio of Calcareous Sand by Resonant Column Test

鈣質砂是由碳酸鈣所組成，相較於石英礦物所組成的矽砂較為軟弱、孔隙比較大、顆粒形狀大多呈角稜狀。這種砂土大部分遍佈於熱帶與亞熱帶的淺海地區，此區域經常有地震的發生，許多離岸的結構物，像是港口設施、海上基礎、離岸風力發電機等，可能因為此種砂的特性而造成影響，故鈣質砂的動態性質將是研究的重點。 本研究以共振柱試驗求得不同鈣含量下土壤的動態性質:剪力模數與阻尼比。試驗材料為純石英砂土、純鈣質砂土與三種配比的矽鈣混合砂土，土壤的相對密度皆為35%，以霣降法重模試體。圍壓對剪力模數有很大的影響，每顆試體皆會分別承受四種圍壓，求得當時狀態下的剪力模數與阻尼比。 試驗結果顯示，每種砂土之剪力模數會隨著圍壓增加而增加；阻尼比則是隨著圍壓增加而減少。在小圍壓下，剪力模數隨著鈣含量的增加而降低。隨著圍壓的增加，鈣含量對剪力模數的影響逐漸變小。相同圍壓下，鈣含量對阻尼比與正規化後的剪力模數則無太大的區別。經驗公式中的常係數值並不適用於鈣質砂，故本研究會找出含鈣砂土的常係數值。Calcareous sands are composed of calcium carbonate, which is weak structurally compared to quartz sand. Calcareous sand has high void ratio and angular particle shape. They are mostly distributed in tropical and subtropical shallow waters where earthquakes occur frequently. It has been observed in past earthquakes that sites that have high content of calcareous sands experienced strong shaking and liquefaction. Hence, it is important to study the dynamic behavior of calcareous sands as their behavior may have an adverse effect on coastal and offshore structures during earthquakes. In this study, resonant column tests were performed to evaluate the small-strain dynamic properties of calcareous sands. Sands with different contents of calcium carbonate are considered. Specimens are prepared by pluviation method. Effect of confining pressure on small-strain shear modulus and damping ratio is also investigated. Laboratory test results indicate that small-strain shear modulus increases and damping ratio decreases with increasing confining pressure. Shear modulus decreases with increasing calcium carbonate content at low confining pressure. However, the influence of calcium carbonate content on shear modulus becomes smaller with increasing confining pressure. On the other hand, effect of calcium carbonate content on damping ratio and modulus reduction curves is insignificant at all levels of confining pressure. Predictions of maximum shear modulus and damping ratio from empirical models by Hardin & Richart’s equation and Menq are compared to the test data. The comparison indicates that there would be bias in the prediction if the effect of calcium carbonate content is not considered. Hence new coefficients for those empirical models, which are functions of calcium carbonate content, are proposed