Abstract: To guarantee the safe shield tunneling of overlapping tunnel with ultrasmall clear distance in waterrich sandy stratum, a case study is conducted on the tunnel between Taikang road station to Erqi square station of Zhengzhou metro line No. 3. Then, the safety control techniques for overlapping tunnel are proposed according to complex and severe conditions such as the limited site conditions, waterrich siltyfine sandy stratum, ultrasmall clear distance, and the superposition of secondary settlement. The results show the following: (1) The horizontal platetype freezing wall for freezing reinforcement and short steel sleeve launching technology are adopted to improve the stability of the end soil and to reduce the risk of water leakage of shield launching. (2) To ensure strength of the soil layer and reduce the secondary settlement, the middle layer in the lower tunnel is grouted first, and the middle layer in the upper tunnel is grouted after tunneling. (3) Shield driving parameters are reasonably controlled, and synchronous grouting and monitoring inside and outside the tunnel are strengthened. (4) The lower tunnel is supported by roller trolley when tunneling in the upper tunnel, and the trolley is moved synchronously with the upper tunnel shield tunneling to ensure the continuous support force. (5) The analytical results of monitoring data of the tunneling of the lower tunnel and the superimposed tunneling of the upper tunnel show that the maximum ground settlement, pipeline settlement, and tunnel deformation are 14.74 mm, 2.44 mm and 3.14 mm respectively, which can control the ground, pipeline, and tunnel deformation within the safe range and achieve good results.

Key words: shield, overlapping tunnels, waterrich sandy stratum, freezing, interlayer grouting