Metro de Santiago: tunnelling under the Fray Andresito Bridge

The northern extension of the Line 2 of the Metro de Santiago, with approximately 1.6 km total length. Running tunnels consist of double-track single-tubes. At the southern route section, tunnel horizontal alignment bents with a 253 m radius to link to the existing Cal y Canto station. This constraint causes the tunnel to pass a few centimeters below the heavily loaded abutment foundation of the Fray Andresito bridge just before reaching the Mapocho River. To make possible the construction of this tunnel section, special construction and monitoring measures were required, as explained next. The bridge foundation was underpinned by six piles, constructed manually from within small-dimension pilot tunnels, excavated in advance of the main tunnel. Piles were provided with an active jacking and monitoring system to control load distribution and deformations during main tunnel excavation. Jacks were accommodated in the interior of a rectangular steel frame installed within the body of each pile and were accessible during the whole construction period. Main tunnel excavation was carried out with the use of mined tunneling methods (NATM). This comprised a sequential tunnel excavation, followed by the installation of a temporary reinforced shotcrete lining. The cross-section was subdivided in three independent partial headings. Side drifts were excavated in advance of the central heading. When deformations stabilized, the jacking system was removed for the installation of the permanent tunnel secondary lining. Structural and serviceability requirements were provided by the installation of a massive reinforced concrete cast in place secondary lining, with thickness ranging from 1 to up to 2 m. This lining was installed before underpinning piles were demolished. After the secondary lining was installed and achieved sufficient resistance, the exposed parts of the underpinning piles were demolished, transferring the total load of the ground and bridge foundation to the tunnel lining. Additionally to the typical control of ground and tunnel deformations, monitoring also included the screening and control of jack loads (pressure cells), total and relative deformations of piles and steel structures (transducer bars and a rotational laser system). At all times, monitoring data fed continuously an interpretation software designed specifically for this project, which compared registered values with pre-established trigger levels. Construction of this section was complete successfully with maximum registered deformations in the order of only 5 mm, as required by the client and foreseen by the designers. Results achieved by the use of the methods and additional measures carried out, showed the accomplishment of both design and construction purposes, allowing the construction of this metro line across a particularly complex section with a high level of concern on the impact of construction works on the surrounding urban environment. (A). "Reprinted with permission from Elsevier". For the covering abstract see ITRD E124500.

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  • Authors:
    • GOMES, A R
    • BOEFER, M
    • CRUZ, J H
  • Publication Date: 2004-7


  • English

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  • Accession Number: 01011530
  • Record Type: Publication
  • Source Agency: Transport Research Laboratory
  • Files: ITRD
  • Created Date: Dec 19 2005 3:15PM