水力抽砂回填所形成之土層通常都相當疏鬆，地震發生時可能會引起砂土液化的問題。如果能在回填過程中利用添加水泥方式形成無液化顧慮的土壤材料施工，則事後即不再需要採取防止液化之措施。為避免改良土進行水中回填時發生水泥與砂土分離現象，故於改良土中添加離子型聚丙烯醯胺作為抗分散劑，以自由沉降模式模擬改良土之水中回填。因此本研究探討添加抗分散劑水泥改良土之水中沉降機制與改良成效。本研究之試驗重點在於以不同抗分散劑添加量配合各種水泥配比，將波特蘭水泥第Ⅰ型與砂土拌和，利用回填水槽進行水中沉降製作改良土試體，經7天或28天之養治後，進行無圍壓縮試驗與動力三軸試驗。 試驗結果顯示，試體之無圍壓縮強度會隨水泥配比之加大而增強，然強度之變化則因抗分散劑添加率及離子型態之不同而有所變化。由動力三軸試驗結果得知，在抗分散劑添加率為344 mg/kg之試驗條件下，添加中陽離子型聚丙烯醯胺與中陰離子型聚丙烯醯胺之改良土，皆在水泥配比為6 %，養治時間為28天時，液化阻抗可獲得改善。因此，添加離子型聚丙烯醯胺及水泥確可有效地改善砂土的動態特性。 Sandy soil layers formed by hydraulic filling are usually very loose. It may cause liquefaction problems during earthquake. If a kind of non-liquefaction soil material mixed with cement were used in the process of backfill, the latter soil treatment will not be necessary for preventing liquefaction. In order to avoid the separation of cement particles from cement treated sands during the backfilling in water, the cement treated sand was improved by adding various types of coagulant (cation and anion of polyacrylamide) to simulate the backfill of free sedimentation mode. This research focused on the sedimentation mechanism and discussed the effects of cement treated sands with coagulant in water. A series of unconfined compression tests and dynamic triaxial tests were carried out by adding various contents of coagulant and cement. According to the experimental results, the unconfined compression strength of sand tended to be improved while the cement content was increased. The types of content and coagulant influenced the unconfined compression strength of sand. Moreover, the cyclic resistance would be significantly increased by adding 344 mg/kg coagulant, 6 % cement. Therefore, cation and anion of polyacrylamide and cement can improve the dynamic properties of sands.