Abstract: The karst features and soil cavities are welldeveloped along Yongtai Station of North Extension of Guangzhou Metro Line 3. The potential ground collapses might have an adverse effect on the stability of the foundations and stability of the rail alignments. The soils between the limestone pinnacles and in the rock cavities have poor engineering properties and require strengthening. Based on the rock and soil cavity investigation, it is recommended two different improvement methods to be adopted: For cavities with a height greater than two meters, the structural sand is filled by sand blown method followed by the grouting with sleeve casing method; for smaller cavities, the cased grouting method is directly used. The core boring and SPT N values obtained after the construction indicated that the results are satisfactory.

Key words: City Metro, Karst Features, Grouting Improvements, Supporting Structures