The Ollachea tunnel is to be excavated in Section 4 of the Interoceanic Road Peru – Brazil, at 3,000 m altitude. The environment shows the presence of numerous hyperthermal springs and a thermal spa in exploitation, which requires to study the thermal conditions that will affect the excavation works.
A first set of information has been provided by 16 thermal springs, with temperatures between 21 ºC and 78 ºC. More detailed data has been obtained thanks to three research holes drilled from the left bank of the Ollachea River Canyon, 13 sub-horizontals holes (0 º to -5 º) and 3 descending holes (-13 º to 75 º); six of them with lengths between 150 m and 226 m. These holes have provided accurate information about the geological formations that will be present along the tunnel. The very abrupt morphology of the canyon, along which the road runs, has made it impossible to drill vertical holes on the path of the tunnel or its immediate surroundings.
Groundwater in this context is classified as mesothermal (30 ºC to 50 ºC), hyperthermal of low enthalpy (up to 70 ºC), and medium enthalpy (from 70 ºC up to more than 90 ºC).
The isotherms maps at the tunnel level (isolines of 30 ºC to 90°C) define the presence of major sub vertical faults, and evidence that the contributions of thermal water are concentrated in the quartzites of the Upper Ordovician and in the andesites of the Cretaceous.
All this information has enabled: the location of subvertical deep faults that act as thermal flow-way; the establishment of the planimetry of the water isotherms in the path of the tunnel and along its axis; the verification of thermal reservoir temperatures (at least between 138 °C and 190 °C); and the design of the geothermal conceptual model, consistent with the extensive collected information.
In summary, all this data has been fundamental to define the best location and path for the tunnel and to establish its construction project, taking into account the high temperatures of water and rock types to be crossed.