台灣邊坡及台地等淺層土壤，長時間位於地下水位面以上，土壤常處於不飽和狀態，以傳統飽和土壤力學無法詮釋其行為。現地夯實土壤可能因降雨日曬而改變其土壤含水量，進而影響土壤的基質吸力、強度及勁度等。因此本研究為探討不飽和夯實土壤的力學特性，進行一系列室內試驗，探討土壤波速、強度及模數之間的關係，以供工程應用參考。本研究採用之試驗主要為超音波波速量測試驗，不飽和無圍壓縮試驗，與嵌入彎曲元件之不飽和三軸多階加載試驗。試驗結果顯示，土壤波速與土壤強度明顯受基質吸力影響，不過不同初始夯實狀態，其變化趨勢不同。造成此差異的原因除了土壤組構不同外，試體於濕化過程膨脹程度也不同，但無論初始夯實狀態如何，其土壤波速與土壤強度增量皆呈一線性關係。以不飽和無圍壓縮試驗之應力-應變關係求得之應變相依割線模數，整體變化趨勢合理，但有低估初始勁度的現象。本研究提出一個修正方法，修正後之結果相當合理，可做為推估不飽和土壤之應變相依割線模數之參考。

Surficial soils of slopes and terraces in Taiwan are often below the groundwater table and belong to the unsaturated soil. The traditional saturated soil mechanics cannot interpret its behavior. In addition, the compacted soil in the field may change its water content due to rainfall or sun shine, resulting in changes in matric suction, strength and stiffness. In order to investigate the mechanical properties of the unsaturated compacted soil, this study conducted a series of laboratory experiments to investigate the relationship between soil velocity, strength and modulus. This study will conduct ultrasonic wave velocity measurement test, unsaturated unconfined compression test and unsaturated multistage triaxial test with the embedded bender element. Test results show that soil velocity and soil strength are significantly affected by the matric suction. However, the variation trends vary with the initial compaction conditions. The difference is probably due to the difference in soil structure and the different volume expansion of the specimen in the saturation process. However, regardless of the initial compaction condition, the relationship between soil velocity and soil strength increment is linear. The trend of the strain-dependent secant modulus obtained from the stress-strain relationship of the unsaturated unconfined compression test is reasonable. However, the initial stiffness is underestimated. Therefore, this study proposes a correction method. The results after the correction look reasonable and may be used to estimate the strain-dependent secant modulus of unsaturated soils.

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