台灣北部地區之紅土邊坡分布相當廣泛，邊坡表層之土壤長期處於不飽和狀態，若以飽和土壤力學觀點較無法完整地詮釋其行為，因此本研究進行一系列室內試驗以探討不飽和夯實紅土之吸力及強度特性。接著應用延伸莫爾庫倫概念求得無圍壓縮試驗下所對應之視凝聚力，並與不飽和三軸試驗結果比較後發現，於相同基質吸力下無圍壓縮視凝聚力與不飽和三軸視凝聚力有明顯之正向關係，但有高估之趨勢。差異原因可能為無圍壓縮試驗過程中，不飽和土壤會因受壓導致內部孔隙氣壓產生變化，進而影響應力莫爾圓之大小及位置。本研究根據試驗結果及不同假設，建立三種以無圍壓縮強度推估視凝聚力之方法及其關係式。初步研究結果顯示，上述三種方法皆能夠合理地推估基質吸力範圍介於0~350kPa所對應之視凝聚力，這些方法頗具工程應用價值，但各有其優點及限制，值得進一步之研究。

Lateritic soil slope are widely distributed in northern Taiwan, and most of time they are under unsaturated condition. Therefore, traditional saturated soil mechanics cannot fully explain the behavior of complex unsaturated soil. This research conducted a series of laboratory tests to study the characteristic of suction and strength of unsaturated compacted lateritic soils. And then, the apparent cohesion was estimated by unconfined compression test according to extend Mohr-Coulomb concept, and compared with the actual apparent cohesion obtained by the unsaturated triaxial test. The comparing results show a obvious positive relationship of apparent cohesion between both tests. However, the apparent cohesion was overestimated using unconfined compression test results. The difference may be attributed to the pore air pressure change of unsaturated soils during compression. As a result, the size and location of Mohr circle at failure are changed. Hence, this research develope three methods that can assess the apparent cohesion using the unconfined compressive strength according to the test results and some assumptions. The analysis results show that above-mentioned methods reasonably estimated the apparent cohesion when the matric suction was in the range between 0 and 350 kPa. These methods exhibit potential for engineering applications. However, each method proposed in this research had its own advantages and limitations. Further study is warranted.

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