加勁擋土牆在設計時是以透水性佳的優良級配砂作為回填材料。但在施工現場常為了節省經費而直接使用現地透水性差的黏性土壤回填，這種作法顯然地違反當初設計時的假設。透水性差的黏土有可能因連日大雨而使含水量上升，造成土壤不排水剪力強度減低而側向土壓力增高的情況，甚至因而崩塌。 本研究主要利用大地工程離心機進行黏土加勁擋土牆的離心模型實驗，討論以改變加勁長度對改善高含水量下黏土加勁擋土牆穩定性之效果。 研究結果可得下列結論：1.土壤經水洗後，性質仍與未水洗的土壤相同。2.適當地增長加勁長度對擋土牆的穩定性幫助很大，但有極限值存在；以中大紅土為背填土的加勁擋土牆為例，在含水量為41%時，其臨界加勁長度為牆高的1.35倍。3.加勁擋土牆的加勁區末端會有沈陷區產生。4.當加勁長度為臨界加勁長度時，可改善69%的牆面頂沈陷及64%的牆面頂前傾。5.在高含水量黏土加勁擋土牆穩定性的改善對策中，以增長加勁長度的方式，在臨界加勁長度以下時，較陳柏文(2003)提出減少加勁間距的方法節省材料的使用量。 Mechanically Stabilized Earth Wall (MSEW) is required to use the high quality granular soil as reinforced backfill materials. However, in many cases the low quality in-situ clayey soil is frequently used instead to cut down the cost, which obviously violates the design assumptions. In addition, the clayey soil backfill may increase in water content during heavy rainfall, and that will result in decreasing of undrained shear stress of clayey soil and increasing of lateral earth pressure, sometimes leading to collapse of wall. A series of centrifuge modeling tests have been performed to study the effectiveness of reinforcement length in improving the stability of MSEW with clayey soil backfill of high water content. From this study, the conclusions can be drawn as following: (1)There is no significant difference in mechanical properties between water-purged soil sample and non-water-purged soil sample; (2)Appropriately increasing reinforcement length can improve the stability of MSEW. The critical reinforcement length of MSEW with clayey soil backfill is 1.35H at 41% water content; (3)Settlement zone will occur at the end of the reinforcement on the top of the wall; (4)Increasing reinforcement length to the critical length can reduce 69% settlement and 64% horizontal displacement of crest of the wall; (5)The way with increasing reinforcement length under the critical length can save more quantities of material than the way with reducing reinforcement spacing to achieve the same stability.