Abstract: This paper presents a case history of adverse settlement evaluation of the existing railway roadbed and its mitigation details. The factors that could result in the roadbed settlement and feasible settlement mitigation method are briefly discussed. The process of using the jet grout mitigating the roadbed was also numerically simulated with the sitespecific soil parameters. The following results are obtained: 1) if the significant permeability differences exist between the subgrade structure and the original soil, the presence of silty soils will likely induce in the differential settlement within the roadbed; 2) the raise of groundwater will soften the subgrade soils and increase the compressibility of the subgrade soils and then result in additional settlement; 3) When applying the jet grout method, the best result is at time the column jetting angle is 25 degrees; 4) When the  replacement ratio of the jet groutsoil is reached 20%, the best mitigation results can be obtained.

Key words: Existing Railways, Subgrade Settlement, Highpressure Jet Grout Columns, Mitigation Plan