Abstract: The stability of diversion tunnel’s carrying capacity jointly provided by the surrounding rock mass and plug is essential for the power generation and safe operation of hydropower projects. The diversion tunnels of Wudongde hydropower plant use column plugs instead of traditional expanded wedge plugs. Whether the safety of the new plug structure interacting with specific surrounding rock mass and subjecting to high water head conditions meets the engineering requirements is an important issue that needs verification. The paper analyzes different types of plugs, including straight column, curved column and wedge shape plugs under the action of high water head by using geophysical exploration, testing and other means in combination with numerical calculation that simulates the process of progressive failure of the interface between the surrounding rock and plug. The main results obtained include: 1) the calculation formula of anti-sliding force under the interaction of different types of plugs and surrounding rock is given, and the wedge-shaped plug is proposed. The ‘clamping effect’ of wedge-shaped plug and the ‘bending section effect’ of curved column plug are analysed. 2) The contact state between the surrounding rock and the concrete in the plug section of the Wudongde diversion tunnel is obtained showing that the interfaces of the plug section between the lining, shotcrete and surrounding rock are tightly contacted and well cemented without any weak filling or opening on the contact surfaces. The shear strength parameters of these contact surfaces are also suggested. 3) Under the action of high head, the deformation law of different types of plugs and surrounding rock mass interaction is basically the same, and the impact range of plug compression deformation approximately halves the plug length. The deformation range caused by water pressure is approximately 1 times the tunnel span, and the shearing effect between the concrete and the surrounding rock is not fully exerted. The effective shearing impacting area is approximately 1 time the tunnel span. 4) When the mechanical properties of the contact surface between the plug and the surrounding rock are poor, the safety ranking of plugs from high to low is wedge-shaped plug, curved column plug, straight column type. When the contact surface between the plug and the surrounding rock mass is favourable, the column plug can provide the same safety margin as the traditional wedge-shaped plug. Finally, the safety evaluation on the interaction of the plug and surrounding rock mass in the diversion tunnel of Wudongde hydropower plant during construction and operation periods is achieved, indicating that the safety load borne by the column plug structure can range between 3.1 and 7.4 times the design head, obtaining a safety factor greater than the required specification. Therefore, a column structure is feasible and can be used as a reference for similar projects.

Key words: Wudongde hydropower station, large-scale diversion tunnel, straight column plug, curved column plug, surrounding rock mass and plug interaction, mechanical response, safety evaluation