The Sierra de Valle Fertil is located in the west of Argentina (San Juan province). It is crossed by the National Road 150, that has 40 km in length including six bidirectional road tunnels with a total length of 2000 meters. The geological formations are various Carbonic-Permian (Paganzo Basin) and Triassic (Ischigualasto-Villa Union Basin) sedimentary rocks, in an area adjacent to the Ischigualasto Natural Park. The Sierra has been formed by the reactivation of the basin during the formation of the Andes (Pliocene-Pleistocene) with a thick-skinned tectonics. The trace of the route follows a creek that cuts anticlinal folds. The first three tunnels are developed through soft sandstones with siltstones and shales (middle Triassic). The two following slightly cemented sandstones and conglomerates (Lower Triassic) and the last is in rigid greywackes (upper Carbonic). The support provided by the design was estimated by using RMR and Q method. During construction geological surveys were conducted on the front as it was excavated, and expected support was adjusted according to them. Different failure modes observed in rigid and soft rock are analyzed together with the solutions provided during construction. The tunnels excavated in stiff rocks were less demanding than those provided for in the design. Instead, the tunnels in soft rocks required more supports than those stipulated in the design.

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