台灣中南部粉土質細砂的壓縮性

Abstract

粉土質砂土為台灣中南部沖積平原常見的土壤形式，且在此等土壤上曾多次的發生地層下陷、基樁沉陷以及土壤液化等大地工程災害，顯示對於此類土壤現有知識存在著許多缺陷。作者在針對三種分布於台灣中南部的粉土質砂土(麥寮砂(MLS)、員林砂(YLS)、高雄砂(KHS))進行等速應變壓密試驗(CRS)、單向度壓密不排水三軸試驗(CKoU)，並依據過去的經驗公式標定出此等土壤的靜止土壓力與過壓密比(Ko-OCR)之間的關係、剪力模數與孔隙比以及應力狀態三者間的關係，且修改MIT-S1模式對壓縮曲線的結果進行闡述。試驗結果顯示，此等土壤的壓縮性高而剪力強度則低於一般常見的砂土，且壓縮量的貢獻並非來自於土壤顆粒的破碎(與過去的研究成果相牴觸)。另外，此等土壤受到細料含量的不同，其剪力波速、壓縮係數、剪力強度以及不排水剪強度與過壓密比(OCR)之間的關係也因此有明顯的不同。另外，研究結果也發現，台灣中南部的粉土質砂土在細料含量介於某個區間時會表現出介於砂土與黏土之間的過渡性行為(transitional behavior)。

The alluvial soils that consist of silty sand to sandy silt can be found in most parts of Central and Southern Taiwan. There have been continued and excessive settlement in the region due to compression of this alluvial deposit, foundation settlement and soil liquefaction have also been reoported. The exisiting theories that deal with the compressibility and srength of clean sands have been found to be inadequate to predict the behavior of silty sands in Central and Southern Taiwan. As part of a long term research effort, the author conducted a series of constant rate of consolidation (CRS) and CKoU triaxial tests with shear-wave velocity measurements on Mail Liao Sand (MLS), Yuan Lin Sand (YLS) and Kao Hsiung Sand (KHS). Based on the available data, the author calibrated the Ko-OCR and Gmax-e-stress state relationships. The MIT-S1 model was modified to describe the compressibility behavior of MLS, YLS and KHS. Test results revealed that MLS, YLS and KHS had higher compressibility and lower strength than unifrmlygraded quartz sand typically reported in literatures. The compressibility of MLS, YLS and KHS did not involved significant grain crushing as in the case of clean quartz sand. The fine content played a significant role in stiffness, compressibility, undrained shear strength and its relationship with stress history. Under certain range of fines contents, the compressibility of the studied silty sand had a transitional behavior.