Abstract: The bridge settlement induced by undercrossing of fourline overlapping smallspacing tunnels should be controlled. As a result, the numerical simulation is adopted to study the deformation settlement of Chaoyangxi Bridge and internal force variation of overlapping tunnels on Nanning Rail Transit Lines 1 and 2. And then the effectiveness of the numerical simulation and reinforcement measures is verified by monitoring data. Finally, the measures of steel cofferdam diversion, rotary jet pile reinforcement for riverbed, Soletanche reinforcement for pile foundation, temporary support system and shield tunneling parameters control are put forward to ensure the safe construction of fourline parallel shield tunnels with small spacing crossing underneath Chaoyangxi Bridge. The results show that: (1) The fourline tunneling will have a great impact on the stability of the bridge if no measures are taken, causing the bridge foundation to tilt seriously and the collapse of some tunnel segments. (2) Chaoyangxi Bridge was built long ago with complex stress structure system and is sensitive to strata disturbance, which indicates that the wrapped grouting reinforcement for bridge foundation and hardening method of the riverbed with rotary jet pile and concrete plate should be carried out to prevent the soil above tunnel from sweeping by river.

Key words: shield tunnel, four lines overlap, foundation, settlement control, numerical simulation