不飽和夯實紅土經乾化或濕化，土體內之含水量及基質吸力皆會改變，外載重如靜載重、反覆加載作用所引致之土體應變也會影響土壤模數，此外，不同深度下的土壤會有不同的圍壓應力，因此，同時考慮基質吸力、應變及圍壓應力對土壤模數之影響應是一值得深入探討之課題。本研究亦針對基質吸力對不飽和夯實紅土之強度參數進行探討。本研究以靜壓夯實法製作三種不同初始含水量之試體分別為乾側、最佳含水量(OMC) 及濕側，並利用環境模擬設備進行乾濕化以改變是體內部之含水量。接著再進行一系列之室內試驗包括:壓力鍋試驗、無圍壓縮試驗、不飽和三軸試驗及彎曲元件(無圍壓狀態及有圍壓狀態)等試驗，分別量測基質吸力、土壤強度及土壤模數。並根據試驗結果探討土壤組構、基質吸力、視凝聚力及應變相依土壤模數之相互關係。本研究針對試驗需求研製了一組改良的不飽和三軸試驗系統，此系統可以同時進行不飽和三軸強度試驗及彎曲元件試驗。試驗結果顯示，乾側試體經濕化到飽和再乾化後其土壤模數大幅降低且乾化時土壤模數隨吸力提高其增加之趨勢也較緩慢。OMC與濕側試體兩者之吸力特性較相近，但因OMC試體飽和後其膨脹較大，組構弱化程度比濕側大，因此土壤模數隨基質吸力提高之趨勢較濕側遲緩。此外，試驗結果亦顯示出圍壓應力之大小對土壤模數有著顯著的影響。由不飽和三軸試驗之結果可觀察到基質吸力與視凝聚力和摩擦角 呈現一非線性關係，且會隨基質吸力增加其變化量會趨於平緩。

The water content and matric suction of the unsaturated compacted lateritic soils are affected by the wetting and drying process. The strain induced by the external loading may influence the soil modulus. In addition, the confining pressure effect on the soil modulus is also an important issue. Therefore, the soil modulus varying with the matric suction, strain and confining pressure is an interesting issue worthy of further study. In addtion, the matric suction effect on the unsaturated shear strength parameters of compacted unsaturated lateritic soil is also investigated in this study. Soil specimens were compacted at three different initial water contents including the dry side, OMC and the wet side. They are then subjected to designated drying and wetting paths using the environmental simulation apparatus. Subsequently, a series of laboratory tests were conducted including the pressure plate test, the unconfined compression test, the unsaturated triaxial test and the bender element test (confined and unconfined condition) to assess the relationship among soil structure, matric suction, apparent cohesion and strain-dependent soil modulus. Moreover, a new unsaturated triaxial testing system was developed in this study to satisfy the testing purpose. Test results demonstrated that the soil modulus of dry side decreased substantially after wetting and increased less significantly as suction increase upon drying. Matric suction characteristics of the wet side and the OMC are similar However, soil modulus of OMC increased less rapidly than wet side upon drying due to the weakening of fabric upon saturation. Moreover, the test results indicate that the soil modulus is significantly affected by increasing the confining pressure. The test results also exhibit that the apparent cohesion increase with matric suction with a nonlinear relationship. In addition, the apparent cohesion increase become milder when the soil suction reaches high matric suction.

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