Abstract: The caves not tamped back of vault lining of the excavated metro would destroy the soil layer above the metro tunnel, which can develop to ground and became the big dangerous to the traffic and buildings. The prediction aquifers and interface between rock and soil is important thing to construction safety in excavating subway tunnel. The landsonar method has been used to solve those problems and got good feedback. Three caves of 2 m long were found in the Landsonar time-section of 60 m long survey lines in metro line-6 in Tongzhou, Beijing, which cross with Jing-Ha railway under ground surface of 21 m depth . The left and right survey lines are along the median of subway tunnels, surrounding rock of which are alluvial land and miscellaneous fill soil. On the Landsonar time-section of 50 m long survey line along the median of subway tunnel of Line-4 subway of Nanjing, it can be found that 9 caves and looseness region whose depth were around 8 m, which lead the soil body above them to loosen, and five developed to the asphalt pavement. There was another survey line with a collapse. From the Landsonar time-section, it can be seen that the collapse only developed to the clay layer of 4m depth, but the looseness already developed to the asphalt pavement.

Key words: metro, vault lining, landsonar, caves, looseness region of soil