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The Norandino pipeline, build between 1998 and 1999, begins at the town of Pichanal nearby Oran in Argentina where it is connected to Argentinean Gas Transport System, It runs towards west crossing the Yunga in Salta province, climbs the Sierras Subandinas and the Cordillera Oriental through highly erosive environment, descends to Humahuaca city in Jujuy Province, crosses the Altiplano plateau, and reaches Chile through Paso de Jama then climbs up to 4.980 mas, crosses the Atacama desert to finally arrives at Tocopilla, Mejillones and Coloso cities. With 1070 km long and 20" diameter, it's capable to carry 4,6 million cubic meters per day expandable up to 8. The most hazardous part of the alignment is the Sierras Subandinas area, due mainly to geology and heavy rains. The east slope of the Cordillera Oriental, where the risk arises from the intense geodynamic and strong erosions in river crossings and landslides because of heavy rains that reach 1.500mm in four month, from December to March. In Cuesta de Mal Paso site, in front of San Andres village in Salta Province, the pipeline should be laid along the hillside on a slope formed by the anticline limb, whose exposure to heavy erosion of the San Andres river at the base of the slope, the bedding to the river and seismic conditions of the region, constitute a geological and geotechnical risk to the pipeline integrity.

The evaluation of deformation measurements of the slope, measured along several years, provided the necessary arguments to undertake the construction of tunnel in rock of 750 meters long and 2.5 meters wide and high, allowing the pipeline to pass below the main slip surface and therefore protecting it from a possible slide of the superficial layers of the slope. This paper describes the geological conditions, the geostructural situation of Cuesta de Mal Paso, the deformation monitoring system, the evaluation of the auscultation measurements, the design of the tunnel, the meaningful geological and geotechnical aspects of the tunnel construction and, finally, criteria for definition the auscultation system to monitoring the behavior of the tunnel and the Cuesta de Mal Paso slope.