路基土壤於最佳含水量附近(OMC)施工滾壓至開放交通服務期間，地下水位係分佈在地表下某一範圍深度內，而路基土壤通常位於地下水位面以上，呈不飽和狀態。土壤在不飽和狀態時，土壤吸力是一項重要的參數，因不飽和土壤有效應力會受到土壤吸力控制，而回彈模數為應力之函數。因此，本研究發展以土壤吸力及軸差應力所組合之回彈模數組成模式，於鋪面結構分析時，能考慮季節性含水量變化對路基土壤之影響。 另外，為評估路基土壤受交通載重作用下所累積之永久變形，而發展路基土壤永久變形模式，且為考慮含水量變化對路基土壤永久變形之影響，將模式發展成應力及載重次數之函數，如此可配合所發展之回彈模數組成模式，預測路基土壤在不同載重次數下之永久變形。 本研究首先係探討紅土及破碎泥岩土壤之不飽和吸力特性，並強調含水量及夯實能量對土壤吸力之影響。之後，於不同含水量下進行回彈模數試驗，且於回彈模數試驗完成後以濾紙法進行土壤吸力試驗，並探討兩者之關係。另外，以三軸反覆載重試驗評估路基土壤受到交通反覆載重作用下累積之永久變形量及回彈模數隨載重次數下之變化情形，深入探討路基土壤受反覆載重作用下之彈性及塑性行為，並據此發展回彈模數循環硬化/軟化行為及路基土壤永久變形模式。 研究結果顯示，夯實土壤之吸力特性會受到含水量和夯實能量之影響。基質吸力會隨著含水量降低而增大，然而在相同含水量下，夯實能量愈大，因土壤特性不同，基質吸力不一定會愈大，而與不飽和土壤之氣–水界面曲率有關，當曲率愈大，則基質吸力愈大。此外，試驗結果證實基質吸力較總吸力更適合作為預測路基土壤回彈模數之參數，而基於不飽和土壤有效應力觀念，本研究將傳統回彈模數軸差應力模式加以改良，納入基質吸力之影響，稱之為軸差應力–基質吸力模式，並建立土壤吸力與飽和度之關係，而可預測夯實土壤在不同含水量下之基質吸力與總吸力。 在反覆載重試驗方面，試驗結果顯示可由路基土壤三軸反覆載重試驗下之行為反應而界定出臨界應力水準，路基土壤受到高於臨界應力水準之反覆載重時，塑性應變會迅速累積，而處於不穩定的狀態，且回彈模數受反覆載重至一定次數後會發生循環軟化現象。本研究根據路基土壤三軸反覆載重作用下之塑性行為提出一多元迴歸塑性模式，以改善傳統模式之缺點，並合理預測路基土壤受到反覆載重作用下所產生之塑性變形。 Construction specifications generally require that subgrade soils be compacted in the field at or near optimum moisture content (OMC). As such, subgrade soils should be treated as unsaturated soils. Soil suction is a fundamental physical property of unsaturated soils, since soil suction dictates the state of stress in unsaturated soils. It was known resilient modulus model is a function of stress state. Hence, this study developed a resilient model based on deviator stress and matric suction. This study was undertaken to evaluate the variations of soil suction with the moisture content for two soils. In particular, the effects of compaction energy on soil suction were investigated. The matric suctions were determined by the filter paper method on soil specimens after the resilient modulus test. Findings from these tests indicated that soil suction of compacted soil is influenced by the compaction energy and water content. Soil suction increases with decreasing water content. However, high compaction energy does not necessarily lead to high soil suction. Test results demonstrated that resilient modulus correlates better with the matric suction than with total suction. Matirc suction was found to be a key parameter when predicting the resilient modulus of cohesive subgrade soils. Also, a prediction model incorporating deviator stress and matric suction for subgrade soil resilient modulus, called the deviator stress-matric suction model was established. Based on the response of subgrade soil under repeated loading tests, a critical stress level can be defined. At stress levels higher than the critical level, the plastic strain of subgrade soil accumulates rapidly and an unstable condition occurs, and soil tends to soften after a specific number of load applications. Using the plastic strain data obtained at stress states below the critical level, a multiple regression plastic model for cohesive subgrade soils was developed. Taking both the deviator and confining stresses into account, the model can be used to predict the permanent deformation in the subgrade of flexible pavements under repeated loading in a reasonable way.
