傳統三軸應力路徑下石英砂之臨界狀態與其力學行為

Abstract

臨界土壤力學在發展過程中，經過理論的推導與許多試驗結果，已建立一套完整的理論架構。從此架構下發展出彈塑模型-Modified Cam Clay model，廣泛的應用於數值分析上，來預測黏土受力後變形行為；此外，臨界狀態概念下所衍生之狀態參數 (state parameter)，也可用於預測現地土壤的力學參數與變形行為，由上述可知臨界土壤力學確實能在工程分析與設計帶來幫助。 但許多文獻的試驗結果顯示，砂土之臨界狀態其孔隙比對平均有效應力呈現不唯一的線性關係，與理論架構下的唯一的線性關係有所差別，造成砂土之試驗結果與臨界土壤力學不符合的情形，因此有必要釐清砂土之臨界狀態真實情況。 為了解砂土臨界狀態之真實面貌，本研究採用石英砂進行一系列的三軸壓縮試驗，以相對密度、排水條件、有效圍壓為試驗變因，並加入潤滑端座 (lubricated end platen) 與解壓再壓試驗 (unloading-reloading) 方式，以探求試驗變因與試驗方式對石英砂之臨界狀態分布與其力學行為之影響。 試驗結果顯示：砂土之相對密度為影響三軸試驗結果重要的試驗變因，剪力強度、體積應變、應力應變、臨界狀態分布皆受其影響。而三軸試體高徑比大於2.4時，可忽略端座效應，試驗所得剪力強度與臨界狀態分布並不受端座效應所影響，此外，應力路徑與試體配置方式並不會影響臨界狀態線之形式。而試驗最後所得石英砂之臨界狀態，孔隙比與平均有效應力的關係與土壤初始孔隙比有關，與臨界土壤力學所建立之關係式不相同，因此建議砂土之臨界狀態有必要做進一布的探討。

The framework of the critical state soil mechanics has been estabilished through theoretical derivation and many experimental test results. Under this framework, the elastic-plastic Modified Cam Clay model is capable of predicting the deformation behavior of clay after loading. In addition, the development of state parameter under the concept of critical state can also be used to predict in-situ soil properties and deformation behavior. Therefor, critical state soil mechanics can bring many help in engineering analysis design. However, it is indicated in the literatures that the relationship between the void ratio and the mean effective stress at critical state of sand does not uniquely exist, which differs from the fundamental definition of critical state soil mechanics. Hence, we need to clarify the critical state of sand. For clarifying the critical state and constitutive behaviors of qartz sand, this research carried out a series of triaxial compression tests. Test variables include relative density, drainage condition, effective confining pressure, and different test methods, namely the triaxial tests with lubricated end platens and unloading-reloading triaxial tests. The test results show that relative density affected shear strength, volumetric strain, stress-strain curve, and critical sate. It is the most important variable in triaxial test of sand. When the specimen height-to-diaimeter ratio is more than 2.4, the end platens have minimum effects on influencing shear strength and critical state. Further -more, sample preparation method and stress paths have little effect on critical state line. Test results also indicated that the relationship between the void ratio and the mean effective stress at critical state of quartz sand is not unique, where the initial void ratios may have effect on critical state conditions. According to test results, critical state of sand needs to be further studied.