METRO SANTIAGO UNDERGROUND WORKS OF THE NEW LINE 5 TO MAIPU

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METRO SANTIAGO UNDERGROUND WORKS OF THE NEW LINE 5 TO MAIPU Alexandre R.A. Gomes 1, C. Mercado 2, M. Houska 1 & K. Maia 1 1 Geoconsult Latinoamerica Ltda., Santiago, Av. Andres Bello, 2777, of.

1 METRO SANTIAGO UNDERGROUND WORKS OF THE NEW LINE 5 TO MAIPU Alexandre R.A. Gomes 1, C. Mercado 2, M. Houska 1 & K. Maia 1 1 Geoconsult Latinoamerica Ltda., Santiago, Av. Andres Bello, 2777, of. 502, Las Condes, Chile 2 Metro S.A., Av. Libertador Bernardo O'Higgins,1414, Santiago, Chile KEYWORDS NATM, Shallow tunnels and caverns, Subway. INTRODUCTION The construction works of the new line 5 extension to Maipu started in March 2007 and were subdivided in 4 construction lots. The first two lots shall be ready for operation in December 2009, whereas the rest of the new extension shall be finished at the end of This new line extension shall connect the existing Metro network of the Santiago central area with the south-western city region, improving commuting services for over 1 million people. The present article describes the main challenges and the experience obtained during the construction of the NATM tunnels and caverns of the project, which were excavated in novel ground conditions and crossed underneath lakes, houses, buildings, canals and expressways with very low overburden. NEW LINE 5 EXTENSION TO MAIPU The line 5 extension to Maipu presents 3,8 km of elevated and 10,3 km of underground sections with an overall length of about 14,2 km (Gomes et al., 2008). The underground sections include single double-track NATM running tunnels (about 60 m 2 cross sectional area), 9 large shallow underground stations, galleries and caverns (with cross-sections up to 160 m 2 ) and several intermediate ventilation and emergency exit shafts and access tunnels. NEW-L5 EXTENSION For this extension, tunnel alignment was set higher than in previous lines as to reduce the stations depth, making tunnel cover minimum at some sectors. In fact, running tunnels overburden ranges in the order of 9-10 m with a minimum of 5m at the turn-out section. The crown of access and station tunnels in general is at 7-8 m depth, with a minimum cover of 6.5m. Underground tunnels and caverns are excavated following the principles of the NATM (New Austrian Tunnelling Method), with the use of the conventional cyclic and sequential excavation method. After each excavation step, a primary shotcrete lining (reinforced with lattice girders, wire mesh, steel bars or fibers) is immediately installed. The secondary lining - also of shotcrete - is

installed at a later stage to provide the final support and long-term stability to the tunnel section. These double-layer reinforced shotcrete linings are of permanent character.

22,5 kn/m 3 ; c = 20 35 kpa; = 45 ; E =200-250 MPa, which presents excellent geomechanical characteristics and is very suitable for tunnelling.

varying thickness (Gomes et al., 2008). Hydrological conditions are also favourable and the ground water level is far below the planned tunnel alignment at a depth of 70-80m below ground surface.

2 installed at a later stage to provide the final support and long-term stability to the tunnel section. These double-layer reinforced shotcrete linings are of permanent character. BEHAVIOUR OF THE NOVEL GROUND CONDITIONS The most typical Santiago s soil consists of the quaternary sediments of gravel from the Mapocho and Maipo river basins, the so called Ripio de Santiago, = 22,5 kn/m 3 ; c = kpa; = 45 ; E = MPa, which presents excellent geomechanical characteristics and is very suitable for tunnelling. Other typical ground conditions consist of sediments deposited by erosive streams from the Andean Cordillera, consisting mainly of clay and silt with low to moderate plasticity and sand lenses of varying thickness (Gomes et al., 2008). Hydrological conditions are also favourable and the ground water level is far below the planned tunnel alignment at a depth of 70-80m below ground surface. The occurrence of water infiltration is associated to existing water bearing layers, leaking tubes and surface water percolating through the permeable gravel layers. The underground excavation of the new line 5 extension has been carried out in different ground conditions, where there had been no previous tunnelling experience in Santiago. The tunnels cross through zones with deposits of ravelling gravel, fine silt-sands of volcanic origin with well to medium cementation, the so-called Pumicita and alluvium deposits of clayey silt and sand, which are encountered in layers or intercalated with gravel horizons with varying granulometry and fines content. (a) Gravel deposit, clayey-silt and sand lenses in bench (Running tunnel) (b) Clayey-silt (top heading) and pumicite (bench) (Access Tunnel) (c) Ravelling gravel (Access Tunnel) Figure 1 View of tunnel face ground conditions.

The gravel conglomerate found along the alignment presents mechanical and deformational properties similar to that of the typical Ripio de Santiago.

3 The gravel conglomerate found along the alignment presents mechanical and deformational properties similar to that of the typical Ripio de Santiago. Nevertheless, the percentage of fines that contribute to the cementation of this soil was found to be about 3% in some areas, which is lower than the typical 5% of fines found in other gravels of Santiago. This fact, in association with the ravelling ground behaviour observed in some exploratory pits, lead to the anticipation of potential unstable ground conditions during underground excavations. Therefore, for the more critical tunnel sections, special support measures such as forepoling with self drilling anchors or pipe-roof umbrella were applied. For the remaining sections, systematic forepoling was not applied during construction so that recurrent overbreaks occurred and had to be controlled by the reduction of round lengths, subdivision of top heading face and immediate installation of the shotcrete support. The volcanic origin soil (Pumicite) corresponds to a fine sand with approximately 35% of fines without any plasticity. This soil presents at tunnel springline the following properties: = 15 kn/m 3 ; c = 65 kpa; = 35 ; E = MPa (Gomes et al., 2008). During construction, the Pumicite presented an excellent behaviour due to its stiffness and well cemented particles with excellent face stability (up to 8 m high faces) and no tendency to overbreak. The installation of the shotcrete support had to be carried out carefully with moistening of the perimeter surface and application of a thin, initial shotcrete sealing of 1 to 2 cm thickness, as to provide the required bond for the application of the following shotcrete layers. (a) Pumicite with excellent face stability (Station Tunnel) (b) Gravel and clay layer (Station Tunnel) Figure 2 View of tunnel face ground conditions. In many sectors, the tunnel face presented mixed ground conditions composed by horizons of gravel, clayey silts, pumicite or sand lenses. In the contact between these strata, mainly when the gravel was located in the top heading zone, there has been some local instability which generated overbreak. For these sectors, excavation had to be carried out carefully, with short round length and a rapid installation of shotcrete. RUNNING TUNNELS The running tunnels present a cross-section of about 60 m 2 with a diameter of about 10 m and a typical horse-shoe shape with flat invert, which bears the secondary lining. A structural ring closure (arched invert) was applied only at some sections which presented special characteristics, such as, tunnel intersections, crossing of foundations from bridges and multi story condominiums or sectors where the soil showed unfavourable conditions or major deformation behaviour during top heading excavation.

The running tunnels excavation sequence was carried out either in stages (top heading, bench and eventually invert) or full face with the use of a massive supporting core (Figure 3).

At a later stage, a secondary lining was installed, consisting of a shotcrete arch reinforced with fibers or wire mesh and supported by a bottom slab with longitudinal beams or an invert.

Ground conditions had a major influence on excavation sequence and hence, deformation.

In some sectors, where the presence of ravelling gravel in the top heading generated overbreaks and the bench was of clayeysilt, the deformations reached a range of 9 to 12 mm.

4 The running tunnels excavation sequence was carried out either in stages (top heading, bench and eventually invert) or full face with the use of a massive supporting core (Figure 3). The round lengths were typically 1.0 to 1.2 m depending on the ground conditions. The ground support was guaranteed by a primary lining constituted by shotcrete reinforced with wire mesh. At a later stage, a secondary lining was installed, consisting of a shotcrete arch reinforced with fibers or wire mesh and supported by a bottom slab with longitudinal beams or an invert. In general, the double lining has an overall thickness of 35 cm and is designed as a support of permanent character. Ground conditions had a major influence on excavation sequence and hence, deformation. In general, sectors where the soil type was mainly gravel and pumicite, the range of surface deformations was between 5 and 7.5 mm and 2 and 5 mm, respectively. In some sectors, where the presence of ravelling gravel in the top heading generated overbreaks and the bench was of clayeysilt, the deformations reached a range of 9 to 12 mm. (a) Top heading excavation (b) Full face excavation Figure 3 - View of mined running tunnel excavation. TURN-OUT SECTION The construction of a new turn-out section required the demolition of an existing depot tunnel simultaneously to the excavation of the new enlarged tunnel with spans of approximately 19 m. Due to this enlargement, the final bifurcation presents a very low overburden (only 5 m). Part of this section is located below an artificial lake, which was dewatered prior to tunnel construction for safety reasons. (a) Excavation and demolition of existing tunnel shell (b) CD-Wall-construction of enlarged section Figure 4 Demolition of existing tunnel.

STATION TUNNELS In the new line 5 extension, station tunnels are typically constituted by one lateral shaft with 18 m diameter and an access tunnel perpendicular to the main station tunnel.

Figure 5 View of typical Station Lay-out The access tunnel presents a typical cross section up to 154 m 2 with a diameter of about 12 m and a length of more than 30 m.

provision of a temporary invert. In a second stage, the access tunnel heading was completed with bench and invert excavation with short ring closure.

5 STATION TUNNELS In the new line 5 extension, station tunnels are typically constituted by one lateral shaft with 18 m diameter and an access tunnel perpendicular to the main station tunnel. Exception is the Final Maipu Station, which counts with 2 access tunnels. Figure 5 View of typical Station Lay-out The access tunnel presents a typical cross section up to 154 m 2 with a diameter of about 12 m and a length of more than 30 m. Due to the low overburden and as to reduce surface settlement at the access tunnel and station tunnel crossing area, the access tunnel top heading was excavated below a pipe roof umbrella with the provision of a temporary invert. In a second stage, the access tunnel heading was completed with bench and invert excavation with short ring closure. The station platform tunnel presents 140 m length and a cross section of 160 m 2 with about 15 m diameter. As the rail level is usually at a depth of 17 to 18 m, the tunnel crown presents only 6-7 m overburden. Its excavation followed the access tunnel with the subdivision of the tunnel face in three separated headings (sidewall-drift-excavation) as to reduce deformations and control overbreaks. A forepoling umbrella protected the first excavation meters at the access and the station tunnel intersection. The temporary sidewalls were demolished by hydraulic crushers, as the central heading advanced. (a) Start of crown excavation of station (b) Station Sidewall drift tunnel central heading Figure 6 View of construction of station platform tunnels

INTERACTION WITH ADJACENT STRUCTURES The running tunnel underpasses several houses and buildings, such as a 14 storey condominium called Edificio Artec, where it was necessary to control the

6 Ground surface deformations were in acceptable ranges and within the predictions. For the Access and Station tunnels, ground surface settlements were in the range of 16 to 36mm and 16 to 28mm respectively. INTERACTION WITH ADJACENT STRUCTURES The running tunnel underpasses several houses and buildings, such as a 14 storey condominium called Edificio Artec, where it was necessary to control the magnitude of the ground deformations. As to achieve this, the mat foundations of the building was strengthened and tunnel excavation carried out below a row of pipe roof umbrella, extended foot-widening and short ring closure between top heading and invert. This crossing was carried out successfully with ground surface settlements lower than 8 mm and no visible cracks observed in the building. (a) Plan view of Edificio Artec (b) Deformation during excavation Figure 7 Underpass of 14 storey Condominium Artec The running tunnels also passed below expressway viaducts (General Velázquez and Teniente Cruz), Americo Vespucio and Autopista del Sol Expressways the Canal Zanjon de la Aguada Figure 8 Expressway General Velasquez: 4 -lane Bridge and 6- lane Expressway

7 In the case of the expressway viaducts, founded on lines of pillars, various strengthening beams were constructed as to uniform the deformation of the vertical single pile foundations, located just 3 m above tunnel crown. The tunnel was excavated beneath pipe roof umbrella protection and with short ring closure to reduce deformation and the impact on the adjacent structure. With these measures, maximum deformations of only 8-10 mm were observed at the centre piers of the overlying bridge at the General Velázquez Viaduct, below the specified acceptable limits. CONCLUSIONS The excavations of the first 2 lots of the new line extension was carried out successfully and finished in the end of February 2009 in accordance with the construction schedule and without major cost overruns. The client s efforts to pursue low disturbance, cost-effective and pragmatic technical solutions, together with a continuous engineering site supervision during construction has proved to be effective to guaranty a safe and rapid construction. The excavation works of remaining sections shall be finished until the end of ACKNOWLEDGEMENTS The authors would like to express their gratitude to Metro S.A. for the permission to publish this article. REFERENCES Gomes, A.R.A.; Pimentel, K.C.A. and Houska, M. (2008), Metro Santiago: Design and Construction of Shallow Tunnels, GEOMECHANIK UND TUNNELBAU, Vol. 1, No3, pp Gomes, A.R.A., Böfer M. and Muñoz, P (2007), Metro Santiago Building an Urban Tunneling Industry from the Ground Up, Proceedings of the ITA-AITES WORLD TUNNEL CONGRESS 2007 the 4th Dimension of Metropolises, Prague, Vol. 1, pp Gomes, A. & Böfer, M. (2005), Proyecto y Construccion de las Obras de Tuneleado de la Estacion Puente Alto, Linea 4 Metro de Santiago, Proceedings of the 1st International Symposium of Engineering Geology, Geotechnics, Geomechanics and Environment, CIP APIG, Lima. Gomes, A. & Böfer, M. (2004), Metro de Santiago: Tunnelling under the Fray Andresito Bridge, Proceedings of the 30th ITA-AITES WORLD TUNNEL CONGRESS Underground Space for Sustainable Urban Development, Singapore. Mercado, C.; Chamorro, G. and Egger, K. (2004), Santiago s Metro Expands, Proceedings of the North American Tunneling (NAT 2004) Conference, Atlanta. Gomes, A. and Böfer, M. (2002), Aspectos de Diseno y Construccion de las Obras de Tuneleado del Metro de Santiago, Proceedings of the 12 th COBRAMSEG / 1 st CLBG / 3 rd SBMR, ABMS; São Paulo, pp

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