2019 SA-NT Symposium - Tunnelling under Adelaide, 28 th Oct. 2019, Adelaide, South Australia Australian Geomechanics Society (AGS) and the Australian Tunnelling Society (ATS) 1 EXPERIENCES WITH THE USE OF THE CONVENTIONAL MINED EXCAVATION METHOD IN THE SANTIAGO METRO - CHILE Alexandre R. A. Gomes SMEC Australia Pty Ltd Edgardo González L. Universidad de Chile & Santiago Metro José Herrera A. Santiago Metro ABSTRACT The Santiago Metro is the largest rapid transit system in South America with 140 km of routes currently in operation. Most of the metro network was developed using the conventional mined excavation method, which was applied for the construction of running tunnels and stations often under shallow overburden conditions and densely built-up urban areas. This paper presents a brief snapshot of the historical evolution of the conventional mined tunnelling methods used for the Santiago Metro underground works, outlining relevant lessons learned along the way and how these cumulative experiences have enabled a continuous improvement and optimisation of design concepts and construction methodologies used for subsequent projects. 1 INTRODUCTION The construction of the Santiago Metro started in 1969 with sections being built predominantly along the axes of main avenues with the use of cut-&-cover (C&C) tunnel method and with some sections at grade. The use of these methodologies caused major disruptions of the urban surface, leading the government authority Santiago Metro to seek more efficient and less disruptive construction alternatives (Mercado et al, 2004). Following the first and successful mined tunnel experiment under a public park in 1994, the mined conventional excavation method became the method of choice for all the subsequent projects. To date, over 75 km of running tunnels and platform tunnels have been successfully built for the Santiago Metro with the use of this construction method. 2 GEOTECHNICAL CONDITIONS The Santiago metropolitan region is located in the so-called “Central Depression”, which is a basin surrounded by the Main and Coastal ranges of the Andes. It covers an area of approximately 80 km x 30 km and is characterized by the coalescence of three main alluvial cones that drain the Andean Cordillera, the cones of Colina, Mapocho and Maipo. These cones are filled with alluvial sediments composed of pebbles and gravels, which are mostly present in the Eastern and Southern part of the basin, clays/silts which are found mostly in the North, and a 40-m thick layer of stiff volcanic ash in the Western side. Clay/silts has also been found at the central area during the excavation of Line 6. A transition zone with various interbedded ground types runs across the centre of the valley. Figure 1 shows a simplified geotechnical map of Santiago (left) and a typical ground stratigraphy of a transition zone at the Line 6 of Santiago Metro (right). Figure 1: Simplified Geotechnical Map (left) and Example of Ground Stratigraphy at a Section of Line 6 (right)