麥寮砂應力歷史正常化行為之量測

Abstract

過去研究曾針對員林粉土質砂進行動態三軸試驗，發現隨著細料含量（Fines content, FC）的增加，其動態行為由類似砂土轉變為類似黏土的行為；並使用Laval Sampler取得非擾動員林砂試體，與重模試體進行比較，結果顯示非擾動試體之細料含量對反覆阻抗比（Cyclic Resistance Ratio, CRR）與剪力波速（Shear wave velocity, Vs）的影響，遠小於重模試體，因此提出應力歷史會影響粉土質砂強度之假設。為驗證此假設，作者製作過壓密麥寮砂重模試體進行靜態及動態三軸試驗，試驗結果顯示過壓密試體的臨界狀態之軸差應力與CRR皆高於正常壓密試體，且FC<30%時，其應力－應變關係在除以有效圍壓後，存在正常化行為（Normalized behavior）。壓密試驗顯示麥寮砂在細料含量為50%時，表現出介於砂土與黏土之間的過渡性行為。另一方面，作者利用不排水靜態三軸試驗，製作麥寮砂臨界狀態曲線（Critical State Line, CSL），使用狀態參數（State paremeter, Ψ）觀念作比較基礎，結果顯示麥寮砂之CRR－Ψ與Vs－Ψ關係都存在著清楚的相關性。

Cyclic triaxial tests on Yuan Lin silty sands have indicated that as the fines content (FC) increases, the cyclic behavior of Yuan Lin sand gradually changed from sand-like to clay-like. Tests on natural Yuan Lin sand retrieved by Laval sampler showed that the effects of FC on cyclic resistance ratio (CRR) and shear wave velocity (Vs) are signifcinatly less than those obtained from tests using the reconstituted specimens. It was thus hypothesized that the effects of fines were also age or stress history dependent. The author performed a series of monotonic and cyclic triaxial tests on reconstituted Mai Liao Sand specimens under overconsolidatewd state to verify the above hypothesis. Results from monotonic triaxial tests on over consolidated specimens with FC < 30% showed higher deviator stress at critical state than those sheared under normally consolidated condition and similar void ratios. The stress-strain relationship from these over consolidated specimens did show a normalized behavior, where the relationships between deviator stress divided by the respective confining stress and axial strain tend to collapse into a narrow range. As the FC reached 50%, the consolidation curves showed a transitional behavior between sand and clay. Based on this series of tests, the author was able to establish the criticla state lines and determine the state parameters (Ψ) for the sepecimens tested. Results showed that for Mai Liao Sand, both normally and over consolidated specimens shared consistent and unique CRR－Ψ and Vs－Ψ corrleations when FC < 30%.