Söderströmtunnel: immersion in downtown Stockholm, Sweden

Transcription

1 Söderströmtunnel: immersion in downtown Stockholm, Sweden J. Glückert Dipl.-Ing, Züblin Ground Engineering, Stuttgart, Germany N.H.J. Vink MSe, M. Reijm BSc, P.T. van Westendorp BSc Strukton Immersion Projects, Utrecht, The Netherlands ABSTRACT: The Söderströmstunneln is one of the most challenging parts of the new Citybanan, a 6 km long railway tunnel crossing the ancient centre of Stockholm. Major parts of this underground link can be executed using the common drill and blast method. But this does not apply to the 450m long immersed Söderströmstunnel crossing the Söderström, connecting the islands Riddarholmen and Södermalm. Unique in the immersion operation is the use of external temporary jack supports on the piled and raft foundations and sliding plates underneath the tunnel elements, to support and slide the elements to their final position. Other aspects are the temporary underwater parking of the Riddarholmen element inside the joint house, to create space for the immersion operation of the centre element, and the usage of two different immersion pontoons, to deal with the different lengths of the elements and the reduced space around the abutments of the tunnel. 1 INTRODUCTION The immersed tunnel comprises of three prefabricated elements. The two straight tunnel elements TE1 and TE2 have a length of 107,5m and the slightly curved element TE3 has a length of 85m. Since the soft subsoil did not allow support directly on the seabed the tunnel was founded on four piled and one raft foundation. Once the construction was completed the tunnel was forming an underwater bridge with a free connection in the so called joint house in the North at Riddarholmen and a fixed restraint at the South side at Södermalm, formed by massive rock anchoring. underneath the tunnel elements, to support and slide the elements to their final position. Other aspects were the temporary underwater parking of TE1 in the Riddarholmen abutment partly inside the joint house, to create space for the immersion operation of the centre element and the usage of two different immersion pontoons, to deal with the different lengths of the elements and the reduced space around the abutments of the tunnel. 2 TUNNEL ELEMENTS AND IMMERSION TRENCH 2.1 Tunnel elements The tunnel cross section comprised of a track tunnel and a service tunnel. Figure 1.Section Söderströmstunneln Unique in the immersion operation was the use of external temporary jack supports on the piled and raft foundations and sliding plates 1 Figure 2. Cross section

2 The tunnel elements were constructed as self floating steel shells in Estonia and transported on a barge over the Baltic sea into lake Mälaren. There the steel shell have been launched into water by submerging the transport barge. Figure 3. Transport of steel hull The floating steel shells were then transformed into three concrete tunnel elements. The concrete base slab and part of the walls have been cast inside the steel shell on a construction area outside Stockholm. Then the half-finished tunnel elements have been brought to Riddarfjärden in central Stockholm were the concrete works have been finalized by casting the rest of the walls and the roof sections. Once ready, external and internal temporary works were mounted such as access towers, guide beams, ballast tanks and wire guides. 2.2 Immersion trench The trench in which the elements are placed slopes with about 3% from South down to North. Installation level of TE3 South is -13m to -23m at TE1 North, resulting in a general height difference of app. 10m. The trench is excavated with grab dredgers and the rock parts are removed using the blasting method. The trench is fitted with 4 pile groups (PG1 to 4) held together with cast in situ capping beams and 1 raft foundation (RF). The tunnel elements are immersed straight on the pile foundations, making the immersed tunnel look like a large under water bridge. The conditions for immersion were quite favorable, with almost no currents and a sheltered work area, in the historic city centre of Stockholm. 2 However the lack of space and the tight tolerances up to 25mm for landing the tunnel elements on the jack foundations were quite challenging. 2.3 Pile foundations Both temporary and permanent support was founded on top of the pile foundations which were constructed inside the Riddarfjärden lake bed. The piles were driven down to the bedrock soil, and held together by a concrete capping beam. The piles needed to cope with the loads from a flooded tunnel, but should also be flexible enough to compensate for elongation and shortening of the tunnel by shrinkage, creep and temperature differences. The final support was made by filling grout bags (installed before immersion) and removing the temporary hydraulic jacks. High level accuracy of the top of the capping beams was required to minimize the construction height of the jacks and grout bags. All jacks were successfully removed from underneath the immersed elements. Special foundations were located near the Riddarholmen abutment. Here special concrete catchers were constructed on top of the higher level bedrock. These transverse catchers were used during the immersion and moving of TE1 inside and outside the joint house. 2.4 Jacks and sliding plates All capping beam foundations support a temporary support system with 250 ton hydraulic jacks. Each tunnel element has been supported by 4 jacks. Figure 4. Detail hydraulic jack support

3 To prevent the jacks from suffering from horizontal loads, sliding bearings are applied (Teflon/stainless steel) and steel housings are applied around the jack to which the remaining loads are distributed. girders special winch frames were mounted to accommodate for the immersion winches. Figure 6. Immersion pontoon principle. Figure 5. Hydraulic jack + housing with Teflon plate The housings are positioned and bolted to the capping beams by divers. The hydraulic jacks were placed and connected just prior to immersion of an element. Two out of four jacks are interconnected (hydraulic coupling) in order to create a 3 point support. 3 IMMERSION 3.1 Transport system The tunnel elements were finalized right next to the immersion location. The elements were moved to the immersion trench using two longitudinal winches installed on the North and South abutment with steel wires connected to the tunnel element - and four transversal winches on the floaters of the immersion pontoons with wires connected to the anchor points around the immersion location. The strength and stability of all parts of the immersion pontoon was checked in different phases. The nominal immersion loads (20 tons per lifting lug), extreme loads on South end (50 tons per lug) combined with minimum 15 tons on North end. Main goal was to check whether the pontoon would remain above water, and what the deflection of the cross beams would become. The overall floating stability of the pontoon as a catamaran layout was also guaranteed due to the large length. An unwanted event as happened with the 17th century Vasa vessel (capsized on her maiden voyage) would not occur in this situation. 3.3 Immersion pontoon configurations For the immersion operations, two different immersion pontoon configurations were used. The immersion pontoon used for the first tunnel element TE3 was much smaller than the pontoon used for TE1 and TE2. Governing was available space around the tunnel elements during the immersion operations. 3.2 Design immersion pontoon The immersion pontoons were made of 2 cross beams mounted on support racks, built on two container pontoon based floaters. The cross beams were made up from two 42m HEB1000 girders with connections in 2 places (for transportation reasons). On top of the 3

4 Proceedings of the World Tunnel Congress 2014 Tunnels for a better Life. Foz do Iguaçu, Brazil. 3.4 Ballast tanks Especially with TE3 the space was very limited in the final stages of immersion. The quay line of Södermallarstrand ended up about 0,5m from the pontoon edge. Also the rock excavations left a narrow trench to move in the tunnel element. To stabilize and trim the element to the required floating situation, 3 ballast tanks including pumping / piping system were manufactured inside the elements. Due to the asymmetrical transverse layout of the element, two tanks were placed inside the track tunnel bore and one long tank was built inside the center of the service tunnel. Figure 7. Immersion pontoon for TE3. To deal with the circumstances, the floaters of the pontoon were made approx. 20m shorter than the TE1 and 2 pontoon. Also the immersion weight was reduced and the position of the lifting lugs and cross beams were positioned exocentric to be able to move in TE3 The floater was a rigid beam made from container pontoons. Figure 9: Ballast water layout The immersion pontoon of TE1 and 2 was much larger. Main reason was the longer length and weight of the tunnel elements, which resulted in a higher immersion weight. To prevent the longer floater sections from overloading (hogging moment), the middle of the container pontoon floaters was fitted with heavy duty hinge connections. The surface of each tank was approximately 200m2, which generated sufficient capacity for 3% overweight in the final immersion stage, where the element rested on the support jacks. The effect on ballasting sequence and influence on immersion loads was great due to this layout. Filling the tanks was done remote controlled by use of an interface system positioned on the deck of the immersion pontoon. In and out let of water was easily done by the same system at a capacity of app. 450 m3/hr. in total. All ballast steps were designed / calculated to high level by tailor made tools in order to be certain of the suspension lug loads and support jack loads in all stages of the process. 3.5 Immersion phasing Operation 1: TE3 TE3 was the first element to be immersed, with its final position against the Södermallar strand abutment. Figure 8: Immersion pontoon TE1 and 2 with hinge 4

5 After moving the element from the mooring location to the immersion trench (at app 60m horizontal distance from final position) water was taken into the ballast tanks to trim the element and accommodate for the required nominal immersion load of 15 tons per lug. Final ballasting was done to let the element go under the water line. During the lowering under water an inclination was achieved in 2 steps. First app. 1.5% inclination and after that inclination to 2.5%. After each step some ballast water was moved from North to South (due to ullage in tanks), to hold the immersion loads equal all around. Realignment with jacks between the outer walls of the joint was not necessary. Final leveling and ballasting followed. Operation 2: TE1 phase 1 After TE3 was connected to the South abutment, the preparations for TE1 phase 1 were done. These preparations consisted of placing the vertical jacks and rebuilding the immersion pontoon. TE1 phase 1 was designed as an operation in which the element was immersed and parked under water on the 4 vertical supports, and pulled app. 4m inside the joint house. For the pulling, 2 big stroke hydraulic push/pull cylinders were connected between roof and joint house in the maximum out stroke. Figure 11: Push/pull cylinders Figure 10: Immersion spread To prevent the back end from hitting the supports in this stage, the final inclination was not yet made. Small forward movements were the key to prevent the element from colliding with the cut trench (bedrock and concrete walls). Finally the element was positioned with the guide inside the catcher and on the 4 hydraulic jacks. It was leveled and connected with the initial compression system to the abutment. Compression of the Gina gasket was checked, before the immersion joint could be emptied. The joint was sufficiently emptied and the alignment of the element was within tolerances of +/- 25mm. 5 After lowering the element on the supports, the cylinders were connected. When the system was running, slow pulling of the jacks was enough to move TE1 inside the joint house. With only cm space all around the room inside the joint house was critical. Successfully positioning the element on the support jacks, intake of final ballast water and locking of all the jacks finished this operation. The big stroke jacks controlled and secured the element in place against currents and vessel movement for the 2 months when the element was parked under water. Operation 3: TE2 With TE1 safely inside the joint house, app. 2.4 meters of space was created between TE1 and TE2, which was narrow but sufficient for TE2 to be moved in place in between the two. The operation of TE2 was similar to TE3, despite the fact of 2 Gina gaskets being mounted to the element, one on each side. This so-called indifferent element could however be placed only in one direction.

6 After emptying the joint, the element showed to be in the required tolerances and final ballasting finished the operation. Ed. Züblin AG, design department, Germany MH Poly Consultants, Netherlands Operation 4: TE1 phase 2 The fourth and final operation was pushing TE1 against TE2. For this maneuver no pontoon was necessary, since all movement was done under water, by using the push/pull cylinders, sliding the element over the support jacks. Ballast was taken out, until the immersion weights were reinstated. The cylinders pushed the element app. 2.5m back against TE2. In the final 0.5m a horizontal catcher / guide on top of the roof slab made sure that the primary end of TE1 and secondary end of TE2 were aligned. The height was controlled by the vertical supports. Once the elements were touching (Gina gasket compressed) the immersion joint could be emptied. Alignment was controlled by the vertical catchers at the secondary end of element 1 near the joint house. No realignment was required. Immersion was completed on September 5th After this, the finishing works were started. 4 CONCLUSION Part of the new Citybanan metroline in Stockholm has been constructed using the immersed tunneling method. Unique circumstances, both technical and organizational resulted in a creative, and most of all low risk approach. High safety standards and innovative solutions for supporting, connecting and constructing these tunnel elements led to a successful immersion from April to September Colophon Client: Trafikverket, Sweden Client Consultant for the immersion process: Tunnel Engineering Consultants (TEC), The Netherlands Contractor: Züblin Ground Engineering, Germany, in cooperation with Züblin Scandinavia AB, Sweden Immersion specialist: Strukton Immersion Projects, Netherlands Designers: COWI, Denmark 6