Rock Mass Stress Release In The Alfalfal Main Water Tunnel: Evidence And Remedial Actions

ROCK MASS STRESS RELEASE IN THE ALFALFAL MAIN WATER TUNNEL: eVIDENCE AND REMEDIAL ACTIONS

Santiago O. Castro[1], Juan P. Soler[2], Carlos F. Andrade[3] and Hugo A. Delucchi[4]

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 WaterTunnel. 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. Rockmass 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 flexiblesupport 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 GenerS.A., is located about 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 Colorado River is a tributary of the Maipo River, the main drainage course of this region. (see Figure 1).
Figure 1: Location Map of the Alfalfal Hydroelectric Plant.

This plant has a160 MW energy generation capacity and has been in operation since 1991.
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. The hydraulic head of the system is almost 700 m.This underground system was complemented with additional 10 km of several Construction Adits, initial Chacayes Acueduct Tunnel, Tailrace Tunnel and Access to the Power Cavern Tunnel. The Alfalfal Main Water Tunnel cross section is approximately 22 m²; detailed geometry is defined in Figure 4.
The “design as you go” method was adopted for the engineering and construction of the undergroundworks, for the first time in Chile. Basically, this method involves the final design definition during construction, according to the encountered geological-geotechnical conditions, on the basis of a basic design that has served to bid and award the construction contracts. The DAYG Alfalfal Consultant Group, a consortia formed by Cade Idepe (Chile) and Norconsult (Norway), was responsible forthe design and inspection of the underground works during construction. Hochtief (Germany)-E.Gordo (Chile) consortia, from the NE ending, and Losinger (Switzerland), from the SW ending, were the Alfalfal Main Water Tunnel contractors. Authors 1 and 4 were in charge of the DAYG chilean geological team. Author 2 was the head of the Hochtief-E.Gordo Concrete Laboratory.
Rock mass stressrelease, consisting of fractures opening, popping and rockburst, occurred during the construction of approximately 2,250 m of the Alfalfal Main Water Tunnel, being critical at a 360 m long central portion.

CASE PRESENTATION

During the construction of the Alfalfal Main Water Tunnel, evidences of rock mass stress release were observed between km 7,000 and 9,250, consisting of fracture...