不同加載條件下水泥改良土之力學性質存在著明顯的差異，當在以水泥改良工法所構築的人工島或海埔新生地進行深開挖時，因大幅解壓可能使周圍土體應力場產生變化。因此本研究採用雲林西螺砂為主要材料，以不同水泥配比作為變因，進行無圍壓縮試驗、動力三軸及靜力三軸試驗，探討未改良砂土和水泥改良土在不同加載條件下之剪力強度特性，以期未來能正確且經濟地應用在工程實務上。 由於改良土之改良強度之作用，致使在低壓範圍內，土體呈現類似過壓密現象，而在高壓範圍內，則呈現正常壓密土壤之特性，因此本研究使用兩階段方式整理水泥改良土之破壞包絡線。在圍壓低於改良強度之範圍內，不同水泥配比改良土之有效內摩擦角皆有遞減現象，而其有效凝聚力則略增；在圍壓高於改良強度之範圍內，軸向解壓試驗所得之有效內摩擦角大於加壓試驗之有效內摩擦角，故在設計時若能考量應力路徑情況的不同，將改良土剪力強度作合理的增減，便有可能進行較經濟的設計，因而降低工程成本。 There exists some apparent differences in the mechanical properties of cement treated sands under different loading conditions. In the process of deep excavation, the massive unloading will change the stress states of in-situ soil. This study used Shiluo sand as the soil material and the Portland cement typeⅠas the improvement material. A series of unconfined compression tests, axial compression tests (AC test), axial extension tests (AE test) and cyclic triaxial tests were performed to investigate the properties of shear strength of untreated sand and cement treated sands under different loading conditions. Due to the improved strength of cement treated sands, the soil shows a trend of over consolidated properties within the low pressure area, and in the other hand it shows normally consolidated properties within the high pressure area. Therefore, the failure envelopes of cement treated sands were figured out by some broken lines in this research. The experimental results demonstrated that the effective internal friction angles decreased and the effective cohesions increased in the area of pressure lower than the improved strength. In the other hand, the effective friction angles obtained from the axial extension tests are greater than those of axial compression tests.
