Abstract: The friction is a crucial factor that affects the mechanical behavior of an existing structure induced by undercrossed shield tunnels. To investigate the influence laws of soil chamber pressure, grouting pressure, and friction on the deformation of existing tunnels and provide rational tunneling parameters, a friction calculation formula is deduced considering shield deadweight and soil pressure distribution characteristics. Then, the distribution law of friction is analyzed and the results are written into a finite difference software, FLAC 3D, through Fish language. Finally, a systematic study is conducted on technical parameters of shield tunneling underneath a metro tunnel and the feasibility is verified by field monitoring results. The results show the following. (1) The friction between shield shell and soil is unevenly distributed around the shield shell and the value increases with increasing friction angle, gravity, and tunnel depth. (2) In the process of shield tunneling, the vertical displacement of the existing structure first uplifted and then settled, soil chamber pressure and friction are the main reasons for the existing structure to be uplifted, and the uplifting increases with the increasing soil chamber pressure and friction. (3) Settlement of the existing structure is mainly affected by grouting pressure and friction after shield tail passes through, and the settlement decreases with the increase in grouting pressure and decrease in friction. (4) Compared with the numerical calculation model without considering friction, the calculation results of the model considering the friction agrees better with the engineeringmeasured data, and the structural deformation of the existing metro tunnel in the operation meets the control standards, which verifies the rationality and adaptability of the results.

Key words: shield tunneling underneath existing tunnel, shield shell , soil friction, shield tunneling parameters, deformation monitoring, FLAC 3D