A newly-built tunnel passes through a heavy-loaded road and a viaduct at the same time, which is 1.65 m away from the bridge piles, with a large tunnel section and shallow depth, as well as a large amount of surface traffic and difficulties in diversion, which makes the engineering risk factor extremely high. In order to ensure the safety of tunnel construction and existing structures, relying on the actual engineering, we analyse the force and deformation principle of the tunnel, surface roads and bridges under different grouting reinforcement measures through three-dimensional numerical calculations and on-site monitoring methods. The results show that: the tunnel arch subsidence, bottom bulge and surface subsidence under full-section curtain grouting are significantly reduced compared with the non-grouting condition, and are much lower than the control value; the bridge pile is located between the two tunnels, which is more unfavourable to subsidence and force than when it is located on one side of the tunnel, so it is recommended to increase the construction spacing between the left and right holes; the use of full-section curtain grouting or bridge pile reinforcing grouting can significantly reduce the subsidence and axial force of bridge abutment, however the longitudinal and transverse differential subsidence values of bridge piers can meet the requirements when combined with the above two measures; the on-site bridge pile reinforcement grouting process will cause vertical displacement fluctuations of the bridge abutment, and the overall trend is bulging, which needs to focus on controlling the grouting pressure; numerical calculations are in agreement with field monitoring data, which effectively verifies the accuracy of the numerical calculation results.