不飽和夯實紅土視凝聚力與剪力強度特性研究

A Study of the Apparent Cohesion and Shear Strength Characteristics of Unsaturated Compacted Lateritic Soil

不飽和土壤視凝聚力是最能反應基質吸力影響之強度參數，但此參數求取不易，必須進行複雜之不飽和土壤強度試驗。本研究藉由一系列不飽和無圍壓縮強度和三軸試驗，探討以無圍壓縮強度，進行不飽和紅土視凝聚力評估的可行性研究。無圍壓縮強度試驗則配合濾紙法試驗，求得試體經環境模擬設備改變含水量後之無圍壓縮強度和基質吸力，再據此探討乾濕化路徑對基質吸力及強度變化的影響。接著應用延伸莫爾庫倫破壞準則概念，以無圍壓縮強度推估不飽和土壤視凝聚力Cuc，並與不飽和三軸試驗求得之真實視凝聚力C進行比較。研究結果顯示，以無圍壓縮試驗結果推估之視凝聚力與真實視凝聚力呈線性遞增關係，比例函數α值約介於0.65-1.04之間，只要將無圍壓縮視凝聚力稍作修正後，即可合理評估不飽和土壤真實視凝聚力，這對工程實務上不飽和土壤的強度分析有很大的助益。

Apparent cohesion is the most widely used strength parameter of unsaturatedsoils that reflect the influence of matric suction, but it is not easyto obtain. A rather complicated strength test is needed. Hence, this studyinvestigates the feasibility of adopting test results of the unconfined com- pression strength to assess the apparent cohesion of unsaturated lateriticsoil. A series of laboratory tests were conducted, including unconfinedcompression strength and a triaxial test of unsaturated compacted lateriticsoil. In specific, the unconfined compression test was combined with filterpaper test to obtain the unconfined compression strength and matric suctionof the samples. Soil samples were first compacted at a designated water contentand then subjected to drying and wetting using an apparatus developedby the authors. As a result, the correlations among the matric suction, theunconfined compression strength and the apparent cohesion can be studied.Based on the extended Mohr-Coulomb criterion, a simple analysis method isestablished, which uses the apparent cohesion of unconfined compressionCuc to evaluate the unsaturated triaxial apparent cohesion C value. Test resultsshow that the relationship between C and Cuc value is an approximatelinearly increase, the proportion function  value is about 0.65 to 1.04.Therefore, the apparent cohesion can be reasonably assessed using the apparentcohesion derived from the unconfined compressive test with propermodification. The method presented in this paper exhibits high practicalpotential.