Abstract: The construction of new subway stations under the existing subway tunnels has high risks and strict requirements for settlement control. The ceiling of Satellite Square station, which belongs to Changchun metro line No.1, contacts tunnel baseplate of light rail line No.3 directly. There is no soil layer between the upper tunnel and lower station, under such circumstances it is described as dual-layer formation. The engineering approach is introduced in this paper. Soil was reinforced firstly by grouting before any excavation, then ten hydraulic jacks were arranged in pilot tunnels No.1 & No.4 right beneath baseplate of upper tunnel to mitigate adverse settlement. Then two groups of concrete-filled tubes, together with longitudinal beams, were forged in pilot tunnels No.3 & No.5 and pilot tunnels No.2 & No.4 as permanent supporting. As soon as these tubes were operative, hydraulic jacks and preliminary supports of pilot tunnels were removed orderly while ceiling of station was cast piece by piece, and the rest of station was cast conventionally at last. In the second part, bearing capacity and settlement of upper tunnel are discussed by numerical simulation. It shows that the maximum bending moment on the baseplate of upper tunnel is allowable, and the settlement on central of baseplate is variable from w-shaped to v-shaped. Analysis of filed monitoring about this case is introduced in the third part, the settlement curves fall into two distinctive categories and highly depend on physical positions of monitoring points. The observed settlements on central of baseplate are consistent to numerical results both in maximum value and overall profile. Due to effective control of hydraulic jacks, the differential settlements on movement joints are also manageable.

Key words: tunneling, subway undercrossing, dual-layer formation, jack uplifting, settlement