The initial state of stress is an important parameter in underground excavations. From its knowledge, it is possible to calculate the stresses induced around the excavation and the necessary support for stabilization. This task becomes critical when the excavations have great dimensions such as caverns and when rock overburden is significant. Different methodologies are discussed, including simple regional stress maps, the survey of surface paleostress and hydraulic fracturing executed in borehole between 500 and 800 m depth. These methodologies have been applied in the Andean Cordillera Frontal, where the Agua Negra tunnel will be built between Argentina and Chile. This area is affected by the subduction of the Nazca plate beneath the South American plate. This setting generates that the greatest principal stress σ1 has a W-E orientation and relationship with the vertical stress (σ2) reaches about 1.4.

1. Introduction

The Agua Negra Tunnel (TAN) site is located between El Elqui (Chile) and San Juan Province (Argentina), under the Andean Cordillera. The tunnel will pass through volcanic massifs mainly composed of andesites and rhyolites [1]. This region is characterized by its high seismic activity due to the stresses induced by the subduction of the Nazca Plate under the South American plate (Figure 1).

The 13.9 km long tunnels will substitute the existing pass road, avoiding the difference in altitude between 4085 m asl and 4765 m asl and shortening the distance by more than 40 km. The tunnel system is formed by two nearly parallel single tubes, W-E oriented, containing 2 lanes each, with a constant longitudinal slope of 3.36 %, falling towards West, from Argentina to Chile [2].

Starting at the West portal there is a rather steep ascent of the overburden. Below the Chilean/Argentinian border which occurs approximately at km 4, there is 1750 m of overburden. The Eastern branch of the tunnel runs almost parallel to the Quebrada de San Lorenzo with overburden between 300 and 600 meters. The tunnels will cross the San Lorenzo fault that is a major structural feature (Figure 3).

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