國內外捷運系統有許多路段之隧道工程採用潛盾工法施工，在潛盾機之發進及到達地段，可能發生地下水滲入開挖面或崩塌現象。為使潛盾機能順利掘進，通常於連續壁外側預先施以地盤改良。另外，現今於軟弱土層中進行深開挖的工程逐漸增加，為使開挖工作順利進行，克服隆起破壞、減少擋土壁變位及提高土壤強度與勁度的考量下，常進行噴射灌漿改良以減少地表沉陷量和增加擋土支撐系統的穩定性。以噴射灌漿改良軟弱地盤雖有助於工程之進行，但剛改良後的複合地盤卻因噴射樁未固結而呈現不穩定狀態。 本研究嘗試以數值分析，探討噴射灌漿後樁體強度未發展前地盤的穩定性，以供後續各種深開挖工程或地下工程之開挖防護工程灌漿設計之參考。分析工具採用ABAQUS有限元素法分析程式，於軟弱土層中使用SJM（Super Jet Midi）工法進行地盤改良，藉由不同噴射樁數、不同覆土厚度及不同土壤之地盤條件，探討地盤的穩定性。研究結果顯示噴射施工剛完成時樁數越多，地表沉陷量越大；當覆土厚度越小時，地表沉陷量越大，影響範圍較小；不同土壤強度的情況下，強度越高，地表沉陷量越小。因此，基於安全考量，噴射樁不宜緊鄰連續施作。 Shield machines are used frequently in tunnel construction works of rapid transit system in Taiwan or foreign countries. The soft soil layer around the construction shaft usually needs to be improved to prevent construction disasters during the shield machine passes through the slurry wall. In the other hand, many deep excavations of soft ground were performed for constructing new buildings. In order to prevent heaving of ground, to decrease settlement of ground surface or to increase strength of soil, the soft soil at both sides of retaining wall should be improved also. Jet grout method is a useful method for ground improvement. Although this kind of improvement is helpful to the construction work, but the jet grout composite ground maybe unstable before the cementing of jet grout columns. To investigate the stability of composite ground with uncemented jet grout columns, this research tried to use FEM program ABAQUS to carry out a series of numerical analysis. In this analysis, it is supposed that a new jet grout method SJM, i.e. Super Jet Midi, is used. From the results of the analysis, it is shown that a larger settlement of ground surface will occur for three jet grout columns which were not cemented. For the condition of small overburden layer, a larger settlement of ground surface will occur, but its influence area is smaller. Furthermore, a smaller settlement of ground surface will occur for the soil layer with higher strength of soil. Therefore, for the consideration of safety, it is suggested that the jet grout columns should not be set in succession.