ABSTRACT:

During the underground excavations of the Alfalfal Hydroelectrical Plant, located in the Chilean Central Andes, severe evidences of rock mass stress release were detected in the Alfalfal Main Water Tunnel. They began as simple opening of joints in the walls, then popping and finally noisy rockbursting. The 2,250 m long affected zone presents a maximum overburden of 900–1,150 m at the 360 m long most critical area. A special constructive sequence and a flexible rock support based on the installation of systematic rockbolts and welded steel mesh reinforced shotcrete was applied here. Rock mass was here identified as La Gloria Granodioritic Batholith. On October 25/1990, a large and very noisy (explosion) rockburst occurred that involved a 15 m tunnel portion, which generated a localized 2 m maximum overexcavation; a 100 m3 volume of large rock blocks fall was induced. This accident forced the DAYG Group to define a more careful excavation procedure, a heavier flexible support and also to include the placing of polyurethane layers as shock absorbers and then a fiber reinforced concrete lining, in order to guarantee tunnel stability. Specific preparation and testing of this fiber reinforced concrete is described. Regular inspection after 8 years operation showed no damage due to rockbursting.

INTRODUCTION

The Alfalfal Hydroelectric Plant, owned by Gener S.A., is located approximately 50 km east of Santiago and was developed between the intakes of Colorado River (2,023 m a.s.l.)/Olivares River (2,016 m a.s.l.) and the Power Cavern, at elevation 1,330 m a.s.l. The Alfalfal hydropower plant comprises two main intakes and a 25 km long main water pressurized underground system, composed of the Olivares Tunnel, the Colorado Tunnel and the Alfalfal Main Water Tunnel, that convey a maximum water flow of 27 m3/sec to the Power Cavern.

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