No abstract

The bi-national pipeline Loma de la Lata (Argentina)-Talcahuano (Chile) belonging to Gas del Pacifico, crosses the Andes at Latitude 37.1° South (Buta Mallin pass), following the Lileo river valley. In the region, there are large ancient landslides within an area of about 50 km2, which have been attributed to Holocene glaciations and seismic activity. In the winter of 2005, when snow limited the access to the area, it was found a pressure loss, that later was attributed to a landslide in a sector of the south bank of the valley. The adiabatic expansion generated a considerable volume of frozen soil around the pipe. The following summer it was studied the characteristics of the sliding and analyzed different solutions of the affected section. The geotechnical study showed details of the slipped area and its relationships with ancient landslides. It was found by comparative analysis of aerial photographs that an old slide about 1 km3 was not fully reactivated. The general morphology has remained unchanged at least in the last 50 years, when the oldest aerial photography was taken. As additional verification, it was found that a small set of cascading ponds located in the slipped mass, has remained stable at that time, bearing the influence of the great 1960 Mw = 9.6 Valdivia earthquake. It was identified tension cracks delimiting the slipped area that was a modest portion of the historical landslide. Geotechnical parameters were estimated by back analysis of the land involved and it could establish a model for sliding mass process. A general analysis of long-term stability took into account the influence of distant earthquakes such as the subduction zone, which has a recurrence of about 100 years and other local seismic sources. Prior to define the most appropriate solution, a 250 meters long trench was dug preventively releasing the pipeline from the terrain to avoid new deformations. Among the solutions considered were the construction of an absorption system with movement monitoring, or the relocation of the trace on the opposite bank of the river. It was decided to adopt the latter solution due to the difficulty of ensuring the stability of the terrain and the inaccessibility during the winter. It implied an additional river crossing and consequently, the need to monitor the stability of the channel to the river erosion.

RESUMO: Las presas de enrocado con pantalla de hormigón (CFRD) son muy frecuentes en la actualidad. Sin embargo, algunas variables; por ejemplo la topografía, el clima, las filtraciones, o la posibilidad de uso de distintos materiales, hace que se estudien otros tipos de presa. Un tipo es la presa de enrocado con núcleo de asfalto (ACRD). La geometría de los dos tipos de presa sigue recomendaciones derivadas de la práctica. Los parámetros geométrico que se emplean, son aquellos que han mostrado un buen resultado. Los conceptos de modelación tenso-deformacional de los perfiles transversales se han empezado a utilizar recientemente. En ambos casos, el elemento de "impermeable" es una barrera de dimensiones transversales reducidas. Se presenta un análisis comparativo del comportamiento de estos dos tipos de presas. Se han utilizado secciones típicas, junto con un modelo de elementos finitos de uso convencional en la solución de estos problemas geotécnicos. Se ha evaluado el comportamiento de la presa en puntos característicos. El enrocado ha sido representado según modelos constitutivos de rigidización, para una mejor interpretación del comportamiento. Se tomaron en cuenta dos condiciones de evaluación (fin de construcción de la presa y estado de régimen). Las modelaciones se efectuaron para tres alturas (30m, 60m y 100m).