Structural design and construction of the Carouge-Bachet underground railway station in Switzerland

Transcription

1 Structural design and construction of the Carouge-Bachet underground railway station in Switzerland Franco ROJAS Civil Engineer BG Consulting Engineers Lausanne, CH Franco Rojas was born in His main area of interest is related to underground structures and bridges. Michel CAPRON Civil Engineer BG Consulting Engineers Lausanne, CH Michel Capron was born in His main area of interest is related to underground structures and bridges. Summary This paper presents the design and construction of the Carouge-Bachet underground railway station in Switzerland. This design was largely determined by the local environmental constraints regarding the existing buildings and highways in the surroundings. The balance between the different construction techniques at each construction phase lead to progressive structural configurations affecting the behavior of the elements in time. The 33x32x21 m cube-shape station is built from top to bottom by using reinforced concrete diaphragm walls connected to different floors in successive stages that include the use of a pre-cambered steel-concrete beam (Preflex, normally used in bridge engineering) to take the lateral earth pressure and the use of vertical and horizontal temporary supports to liberate the space needed for the machines excavating the Pinchat Tunnel that starts from the station. This paper discusses the background of the design of the underground station. Keywords: Underground station, railway, reinforced concrete, diaphragm walls, preflex beam. 1. Introduction 1.1 Explosion of mobility needs The franco-valdo-genevoise region is home to 900'000 inhabitants from two Swiss cantons (Geneva and Vaud) and from two French regions (Haute-Savoie and Ain) [1]. This region employs 400'000 people and by the year 2030, 100'000 extra jobs will be created with the increase of 200'000 inhabitants. In this context, the study of engineering solutions according to the future needs in terms of mobility will guarantee his correct development. Fig. 1: Cross border flows in the franco-valdo-genevoise region.

2 1.2 The CEVA project Fig. 2: Underground link between Geneva and Annemasse (CEVA) More than 910'000 people now live in a radius of 40 km around Geneva. By 2020 this number will have risen to 1'100'000. The authorities, aware of this challenge, will satisfy the mobility needs by the construction of a major underground link between the cities of Geneva and Annemasse (CEVA) connecting the Swiss rail network (CFF) and French rail network (SNCF) [2]. This project offers the opportunity to design, plan, develop and create living spaces acting as the trigger for high quality urban renaissance. As an expected tool for transforming lifestyles, the new railway link is planned to enter into operation on December 2019 with a global cost estimated at 1,567 billions of Swiss Francs. This two track railway will be complete by the construction of five new stations along the 16 km route: Lancy-Pont Rouge; Carouge-Bachet; Champel-Hôpital; Genève-Eaux-Vives and Chêne-Bourg. Fig. 3: Global view of the railway link indicating the civil works

3 2. Carouge-Bachet underground railway station This station acts as a gateway to the city of Geneva, with all that this implies in terms of intersecting road infrastructure (highway, rail, national road) and as a public transport exchange hub (tram, bus, bicycle and pedestrian) [3]. This complexity is increased by the construction of this new station, one Park + Rail facility and by the planned future urban development in the surroundings. Fig. 4: Aerial view of the multimodal exchange 2.1 Architectural concept The underground station is structured vertically over three different levels: roof, mezzanine and foundation. The foundation level supports the railway, equipments and passenger platforms. From this point, the passengers flow reaches the mezzanine and roof levels by the stairs and escalators to take one of two exits at each side of the station. Fig. 5: Longitudinal section In the upper side of the station, an inclined transparent steel roof is planned to protect the passengers from rain and snow. The flow of natural light towards the inside of the structure will perform a vital role in the dayto-day use of the station, directly affecting the comfort of the passengers. For this purpose, all the electrical, sanitary, heating, cooling and ventilation installations have been placed inside the concrete slabs at different levels, in segments according to each construction phase. Fig. 6: Future view of the station at mezzanine level

4 2.2 Structural concept This cube-shape underground station has consecutive slabs and walls that are connected in successive stages using the top-down construction technique chosen in order to reduce as much as possible influence of the works in the surroundings. 4,7 m Roof Mezzanine 9,8 m 21,4 m Foundation Fig. 7: Elevation Simply supported or fixed elements were built at every level to ensure an appropriate structural behavior resisting vertical and horizontal loads during each construction phase. Moreover, the evolution of the structure during construction need the introduction of temporary lateral, vertical and diagonal steel supports demanding a careful study of each element individually and the general structural stability at the same time. 31,7 m 17,6 m 32,8 m Fig. 8: Plan view

5 2.3 Ground conditions During the construction phase, the station will be supported on reinforced concrete piles. By contrast, for the final structure, a foundation slab will be cast in place. This foundation will be founded on the silty clay soil layer. Fig. 9: Ground conditions for the Carouge-Bachet Station and the Pinchat Tunnel 3. Structural model and construction 3.1 Main structural elements The roof slab has a variable thickness from 0,59 m to 0,71 m. The mezzanine slab has a constant thickness of 0,80 m and is first partially concreted in order to liberate the space needed for the site plant during the excavation of the Pinchat Tunnel. The foundation slab has a constant thickness of 0,80 m. The temporary steel supports are composed of double tee sections and are temporarily connected to the diaphragm walls. The diaphragm walls have a constant thickness of 1,00 m. The temporary concrete piles have a diameter of 0,80 m. Fig. 10: Temporary steel supports arrangement to take the lateral earth pressure during construction

6 3.2 Pre-cambered steel-concrete beam (Preflex) The Preflex technology was introduced early in the 1950 s by Belgian engineer Abraham Lipski and Professor Louis Baes. This technology is currently applied in bridge engineering, allowing the maximum optimization of a cross section when a long span needs to be covered by a slender beam. Even if this concept has been in use for more than 60 years, the application of this technology for taking lateral loads in underground construction by rotating the section horizontally could represent the first application to successfully use a Preflex in buildings. Fig. 11: The Preflex beam has a span of 14,81 m The Preflex system consists of a steel beam encased in concrete in different stages to generate a composite beam action, increasing its flexural capacity and stiffness for the ultimate limit state while controlling the deformation and cracks in the serviceability state [4]. For the particular case of the Carouge-Bachet station, the limited space required by the architectural drawings makes impossible the use of a prestressed concrete beam or other similar solution to take the important lateral pressure of 1766 kn/m'. During the fabrication of the Preflex, a vertical force of 2x2000 kn was introduced by two jacks at the factory to prestress the lower flange of the beam. Fig. 12: Load history for the Preflex fabrication Losses inherent to a prestressed section were computed with time-dependent properties and reduced by limiting the time between the fabrication (Bad Homburg-Germany) and the connection with the diaphragm walls on site (Geneva-Switzerland). After arrival on site by special transport, each 25 t Preflex beam was turned horizontally by a special crane and then moved on rollers towards its construction place where it was connected to the diaphragm walls. Subsequently, the final concrete section was cast in place, leading to a composite tee section increasing the moment of inertia.

7 3,5 m 0,8 m The final section could be described as composed of four different materials: the diaphragm wall with a low strength concrete, the main beam in structural steel, the bottom prestressed flange of the beam with high strength concrete and the intermediate zone between the bottom flange and the diaphragm wall cast in place. All these materials work as a composite element. Regarding the structural model, a complex finite element model was used which took into account the vertical concrete joints for the adjacent diaphragm wall panels and two coupled cages at the end of the beam to take the negative flexural moments in the interface Preflex/Mezzanine slab. In addition, a rigid element was used to connect the linear elements (Preflex) with the plane elements (diaphragm walls) assuring a global composite behavior. 1,0 m 1,1 m Fig. 13: Complete cross section (Preflex+ diaphragm wall). 3.3 Construction sequence The simplified construction sequence of the station box is presented below: Phase 1: Concreting of diaphragm walls (yellow) and temporary concrete piles (blue). Phase 2: Concreting of roof slab (green). Temporary horizontal supports on roof level (blue). Phase 3: Excavation down to mezzanine level. Connection Preflex/Diaphragm walls (red). Phase 4: Partially concreting of the mezzanine slab (green). Excavation. Fig. 14: Schematic construction sequence Phase 5: Temporary steel supports (blue). Excavation down to foundation level. Phase 6: Foundation slab concreted (green) and temporary steel supports retired. Partial demolition of diaphragm walls to begin tunnel excavation.

8 3.4 Finite element model Two main global models were used: Model n 1 (construction) Roof and mezzanine slab supported vertically on temporary concrete piles; Preflex coupled with diaphragm walls to take lateral load at mezzanine level; Mezzanine slab partially concreted to liberate space for tunnel machines; Temporary lateral steel supports between the mezzanine and foundation slab; Foundation slab not yet concreted. Fig. 15: Finite element model of intermediate construction stage (transparent render) Model n 2 (final) Roof and mezzanine slab supported vertically on precast concrete columns; Mezzanine slab completely concreted; Diaphragm walls demolished on front and back sides of the station for tunnel excavation; Temporary concrete piles demolished; Foundation slab already concreted; Steel roof supported on roof and mezzanine slab. Fig. 16: Final finite element model of final construction stage (solid render) 4. Conclusion This paper explains the main design challenges overcome for the construction of the Carouge- Bachet underground railway station. The analysis of the different structural elements and the use of the Preflex beam in this building were described in function of the construction phases. Tunneling is presently in progress and the station is planned to enter into operation on December References [1] [2] [3] [4] PORTELA G., BARAJAS U and ALBARRAN-GARCIA J., Analysis and load rating of Pre-flex composite beams, US Army Corps of Engineers, Geotechnical and Structural Laboratory, 2011.