The Southern Interoceanic Highway corridor is a toll road that links Peru with Brazil, crossing the Andes range, in the Central Volcanic Zone. Between km 231 + 700 and km 232 + 820, the road runs along the river Ollachea, through a narrow and deep canyon, with very steep slopes (45 º to 60 º). The road is often cut by landslides and rocks avalanches, which has led to project a tunnel to avoid this conflicting section of the canyon.
The projected tunnel will pass through rocks formation which are very affected by the subduction of the Nazca Plate under the South American Plate. This is a very active geodynamic setting, affected by thrusting faults, volcanism, erosion, ... Given the presence of hyperthermal waters accompanied with high pressure gases, the hydrogeological characterization has demanded the implementation of innovative research techniques.
The tunnel will cross geological formations that correspond to the Upper Ordovician (sandstone quartzites with interbedded shales), Silurian (siltstones and sandstones / slates and shales), Cretaceous - Miocene (andesitic lavas) and Quaternary (colluvial debris). The formations have developed a tectonized complex, affected by overthrust faults and very deep sub-vertical faults.
The hydrogeological research was supported by data obtained mainly through sub-horizontal holes. The drills are performed from the existing road, crossing faults, fractures and brechia rocks, producing continuous upwelling of hot water.
Those research holes, together with hydrochemicals, thermometric and hydrodynamics studies, have allowed to locate very penetrative deep faults, with thermal up-flow (temperature above 90 ° C), spanning the thrust surfaces (reverse faults of low-angle). The fissured rocks associated with faults, appears filled by crystallization of calcite along the strip where gases are released. Orthogonal faults to the later have also been found. Thermal waters mixed with storm water infiltration, discharge to the Ollachea River through these orthogonal faults.
The hydrogeological environment (lithology, faults, fractures, RQD, filling of fractures, weathering, ...) has been characterized by sub-horizontal research holes. Hydrodynamics parameters (mainly permeability) have been determined through depressurization test (Lugeon) in sub-horizontal holes, very complex in its execution, because of the pressure of water and the high temperatures.
The rock compound shows, in general, low primary permeability, which is overlapped by a very heterogeneous secondary permeability caused by fracturing. Finally, permeability is reduced by calcite deposits, originated by the depressurization and degasification of hyperthermal waters of deep origin.