摘要 路基土壤受過大應力反覆加載作用下，塑性變形逐漸累積，終至面層產生過大的車轍變形。然而，至今仍缺乏一合理方式界定路基土壤臨界應力，及預測路基土壤於不同載重次數下之塑性變形。因此，本研究應用反覆載重三軸試驗，根據路基土壤於反覆載重下之回彈、塑性、遲滯及shakedown等行為加以界定路基土壤臨界應力，進而發展一多元迴歸塑性模式預測路基土壤塑性變形。此外，對於回彈模數行為預測，選擇常用之軸差應力模式及雙折線模式做一探討。 研究結果顯示，雙折線模式預測回彈模數行為較軸差應力模式為佳。而路基土壤破壞可由反覆載重下之消散能變化情形，及塑性應變發展型態加以判定，低於臨界應力水準時，路基土壤於反覆載重下之塑性行為係處於穩定累積狀態，且消散能持續遞減；而高於臨界應力水準時，塑性應變累積行為會呈現突然增大，有別於低應力水準下之行為，且消散能於載重若干次數後有復而升高之現象。 依shakedown觀點，路基土壤係由塑性潛變shakedown行為進入至增量崩潰階段，而臨界應力水準則存在於兩種行為之分野。研究結果發現，路基土壤臨界軸差應力隨含水量增加而下降；隨圍壓提高而上升。此外，所發展之多元迴歸塑性模式可有效地預測路基土壤於不同載重次數下之塑性應變。 ABSTRACT The subgrade soil failures due to excessive permanent deformation produced by high stresses in the pavement. However, lack of reasonable mode distinguishes the critical stress in subgrade soil. This paper describes the shakedown behavior of cohesive subgrade soil under repeated loading. The main goal is to define the critical stress level of subgrade soil under repeated loading. According to the shakedown concept, this level is termed the shakedown limit, and it can be distinguish on bases of resilient, plastic, hysteretic, and shakedown behavior by repeated triaxial test. Besides, to select the deviatoric stress model and bilinear model predict the resilient modulus of subgrade soil. The test results show the bilinear model is better than the deviatoric stress model. The determination of critical stress can rely on the behavior of soil under repeated loading, such as dissipated energy and the type of plastic strain accumulation. Below the critical stress, the plastic strain is steady to accumulate, and the dissipated energy is more and more small as the number of load cycles increase. As the stress level advancing above the critical stress, the plastic strain is suddenly increases, and the dissipated energy rises again after specific load applications. Besides, the method of description for plastic strain rate versus plastic strain also can effectively distinguish the shakedown behavior. According to the shakedown concept, subgrade soil is from plastic creep shakedown to incremental collapse. The critical stress level is between two behaviors, and it decreases with increasing water content, increases with increasing confining pressure.