循環荷載下粗細砂混和土壤之勁度變化與水壓激發

Abstract

土壤勁度在大地工程領域是一重要的參數，而土壤勁度的改變也與土體變形量有關。本研究藉由超過二十組動力三軸試驗之施作，著重觀察循環荷載過程中土壤勁度的變化。其中探討包含反覆循環剪應力比、孔隙比、加載頻率及細顆粒含量在勁度改變的影響。其中觀察到第一個荷載循環下產生的平均軸向應變影響勁度折減曲線，並發現第一個荷載循環平均軸向應變因應反覆循環剪應力及孔隙比之改變而變化。而隨著的細顆粒含量改變，等效顆粒孔隙比較孔隙比更能有效反映第一個荷載循環平均軸向應變的改變。並於此研究中提出等效顆粒孔隙比、反覆循環剪應力比及第一個荷載循環平均軸向應變之關係。由於超額孔隙水壓激發與土壤弱化表現有關，藉由本研究中施作之試驗數據觀察結果，提出一簡單勁度變化與超額孔隙水壓比之模型。利用第一個荷載循環平均軸向應變與勁度折減比找出勁度變化過程的軸向應變量。並將第一個荷載循環平均軸向應變、勁度折減比、超額孔隙水壓激發比連結抗液化安全係數，建立室內試驗物理量與廣泛使用參數之關係。 土壤中超額孔隙水壓力的變化為一相當受重視的物理量。除本研究施作之動三軸數據外，額外蒐集多組已發表之動力三軸實驗數據，並針對反覆循環剪應力比、壓密應力及細顆粒含量等等因素對超額水壓激發之影響進行觀察。提出一數學模型模擬水壓激發曲線，此模型能有效描述超額水壓激發曲線，並觀察此數學模型中三個參數因應試驗條件改變所產生之變化，最後提出超額水壓激發上下界之參數選擇建議。

Stiffness is a fundamental property of soils. It governs the deformation of soil layers. More than 20 cyclic triaxial tests were conducted herein to investigate the stiffness degradation during loadings under undrained condition. These tests included four factors: cyclic stress ratio, void ratio, frequency, and fine particles. Through test results, the average axial strain in the first cycle seemed to serve as a factor that decides degradation path of soils. The average axial strain in the first cycle changed in response to void ratio and cyclic stress ratio. With the adding fine particles, the equivalent granular void ratio was found to be more effective in distinguishing the collected data of average axial strain in the first cycle among test results than void ratio. Relations among average axial strain in the first cycle, CSR and equivalent granular void ratio were proposed herein. Excess pore water pressure is a conventional factor that is considered to correlate with weaken behaviors of soils. A simple stiffness degradation model was proposed to be a function of excess pore water pressure ratio. The average axial strain in the first cycle and stiffness degradation ratio were used to predict the axial strain in the degradation process. The average axial strain in the first cycle, stiffness degradation ratio, and excess pore water pressure ratio were correlated with factor of safety against liquefaction. The test results from laboratory were connected to the general parameter by this way. The excess pore water pressure generation is an important issue. In addition to test results in this study, some cyclic triaxial test results from published papers were collected and analyzed in this study of excess pore water pressure generation. Cyclic stress ratio, effective consolidation stress and fines content were observed to be influential to excess pore water pressure generation. A model was proposed in this research to simulate the excess pore water pressure generation. This model, which needs three parameters, was proved to be able to model the excess pore water pressure generation curves effectively. The upper and lower bounds of the excess pore water pressure generation were also recommended herein.