Abstract: To investigate the stress transfer path and the pattern of stress system transformation during the underpinning process when using a castinsitu tunnel structure to support an operational bridge, a specific case is examined, focusing on the cutandcover tunnel beneath the Fangzhong road of the Xiamen 2nd East Passage(Xiang′an bridge), which passes beneath an operational BRT viaduct. The investigation involves the establishment of an automatic monitoring system for vertical displacement in bridge abutments, tunnel structures, and reinforcement stress on piles. This system is designed to assess the settlement of bearing platforms and abutments as well as the forces acting on the top plate, middle sidewall, bottom plate, and endbearing piles at the middle wall of the tunnel. Then, the transfer paths of the underpinning load and the stress transformation pattern of the tunnel structure are revealed. Subsequently, a simplified loadstructure calculation model incorporating prestress considerations is developed to analyze internal force changes in the tunnel structure under the underpinning load and assess the prestress effect. The key findings are as follows: (1) The settlement of the two underpinned piers is measured at 3.30 mm and 4.22 mm. Notably, the neighboring pier experiences settlement before the underpinned piers due to the influence of bridge deck stiffness. Post underpinning, the bridge pier may undergo continued slow settlement for a certain period before stabilizing. (2) Following underpinning, the entire bridge load is not borne by the entire tunnel; instead, the primary load is supported by the top slab of the tunnel on the underpinned side, subsequently transferring to the pile foundation through the middle wall and adjacent sidewalls. (3) The deformation and force results are calculated by the simplified calculation model of tunnel loadstructure which considering prestress align well with the monitoring outcomes. In comparison to scenarios neglecting prestress, the inclusion of prestressed cables leads to an approximate 25.7% reduction in settlement.

Key words: Xiamen 2nd East Passage, BRT bridge, pile foundation underpinning, cut-and-cover tunnel, stress system transform, prestressed concrete, in-situ monitoring, load-structure model