Sediment Sluice Tunnel of Zengwen Reservoir and construction of section with huge underground excavation adjacent to neighboring slope

Abstract

To mitigate severe and worsening siltation, a series of management and improvement measures have been carried out at the Zengwen Reservoir. The Sediment Sluice Tunnel, built in the mountain to the left of the dam, is a crucial project to prevent inflows with high sediment concentrations that are caused by heavy rain, slowing the siltation to extend the service life of the reservoir. The tunnel has a length of 1220.3 m, with an elevation of 175.0 m at its intake and 103.5 m at its outlet. The designed maximum flow velocity is 30 m/s. A huge underground excavation (1058 m2) was conducted to construct an energy dissipation pool and connected twin-bore outlet tunnels were constructed adjacent to a neighboring slope to solve the problem of hinterland limitation and to mitigate scouring on the opposite river bank. During the construction of the twin-bore outlet tunnels, the middle wall between these tunnels suffered from overstress of support and many mutual locked bolts broke successively, causing a major emergency. Fortunately, successively scanned LiDAR point clouds provide not only the comprehensive displacements of rock wall surrounding the excavation and associated variation of spatiotemporal deformation patterns during tunneling, but also the geometric related characteristics of discontinuity sets in rock, including the attitude, spacing and persistency of bedding and two joint sets. The joint set (J2), that had poor persistence per the results of geological investigation performed during design stage, was disturbed by successive excavations of twin-bore outlet tunnels. The mobilization of shear displacement of the J2 joint set leaded to an increase in its persistence and a reduction in shear strength, and weakening the whole rock mass of the middle wall. Such an excavation-induced mobilization of joint set, accounting for the successively breaking of mutual locked bolts in the middle wall, can be hardly determined by conventional numerical simulation. Additional rock bolts were installed perpendicular to the J2 joint set to suppress its shear displacement soon after the mechanism of bolts breaking was understood, mitigating the construction crisis effectively.