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1 The Cross Rail project
The 9.6-km long new route for the Zurich Cross Rail leads from Zurich’s western rim to the new Löwenstrasse Station under Zurich Main Station and further on to Oerlikon.
2 Basic conditions and requirements
2.1 Route alignment and geometry
To complete this project, the 150-year-old South Wing of Zurich Main Station had to be underpassed between the Löwenstrasse Station and the Bahnhofquai. To allow expansion from two to four tracks, the tunnel widens from about 14 m at the eastern end of the South Wing shaft to roughly 30 m at the entrance to the Löwenstrasse Station (Fig. 2). In this part, route alignment increases by about 2.5 % (Fig. 3), causing the gap between upper rail edge and lower edge of the building foundation to drop from 15 to 13 m.
2.2 Geology and hydrogeology
Directly under the South Wing of the Main Station building, there is an approximately 5-m thick layer of filling underlain by a roughly 20- to 35-m thick layer of Limmat Valley gravel covering lacustrine deposits. The Limmat Valley gravel is characterised by high permeability k = 3 x 10-3 m/s, an average to high bearing capacity and hardly any sensitivity to settlement. The average groundwater level lies about 5 m beneath the surface. The lower end of the aquifer mainly contains lacustrine deposits of a mostly silty-sandy nature with a high density resulting from the glacial overconsolidation. If undisturbed, these lacustrine deposits can boast a relatively high bearing capacity.
2.3 South Wing building
As a listed building, the 150-year-old South Wing requires the utmost care. As far as possible structural damage to the building is to be avoided during tunnel construction. The South Wing is used intensively for transporting goods and passengers. Logistical services are provided to all users, including the Main Station, through the Supply Centre located within the project area. Access to this facility can only be limited to a minor extent.
3.1 Driving tunnels underground using the top-down method
According to this tunnelling method, a bracing cover is supported by diaphragm walls. As opposed to the standard top-down method, the partially prestressed concrete cover and the diaphragm walls have been executed underground via a network of main and transverse galleries (Fig. 6).
Supported by diaphragm walls, the bracing cover consists of 29 transverse galleries and bracing girders (Fig. 3 & 6). Once completed, the tunnel, including its main galleries and bracing cover, is completely sealed. Water pressure forces are transferred to the inner walls and lining through a single-layer plastic membrane. The raft absorbs the buoyancy forces and transfers them to the bracing cover via inner walls. Before driving galleries under the South Wing, the groundwater had to be lowered by about 10 m using small filter wells. The diaphragm walls were constructed after the two 110-m long main galleries had been pushed through. Transverse galleries were then driven in small steps, and girders for the bracing cover were put up. Once the bracing cover was completed, excavation took place in two stages from the South Wing shaft. Horizontal struts were laid between excavation stages for diaphragm walls to be temporarily supported until the raft was placed.
3.2 Tunnelling requirements
3.2.1 Main galleries
The size of the main galleries was to match the progress in diaphragm wall construction and advance operations in transverse galleries. The clearance profile of the barely 6-m high main galleries was large enough to deploy diaphragm wall machines with short booms and use grippers (Fig. 7).
3.2.2 Transverse galleries
The size of the transverse galleries was determined by tunnelling requirements and structural calculations for the interception beams to be installed. To meet the requirements, advance operations in galleries and adequate structural measures had to guarantee safe excavation with minimal settlements.
3.3 Tunnelling concepts
3.3.1 Main galleries
The tunnelling concept for the main galleries was designed according to the specified requirements. A double-pipe umbrella system was chosen as a pre-support to stabilise the excavation face (Fig. 8) was installed.
In addition to the pipe-umbrella system, face anchors of 10 metres’ length were put up every 5 m. The excavation was secured with two layers of netting, shotcrete and three-chord lattice girders. Full-face excavation with staggered face and rapid ring closure was applied; the individual advance rate was one metre. Ring closure was achieved on the latest 5 m behind the crown face; a support wedge was left in the crown as additional face support.
The following data about advance operations is worth mentioning:
• gallery length: 110 m
• face area: 32 to 47 m²
• height 6 m
• in total, 2 x 9 pipe umbrella every 10 m
• 37 umbrella pipes 139.7 x 10 mm, arranged in two rows in the roof
• length of pipe umbrella 14 m, overlap 4 m
• up to 26 self-drilling anchors grouted in the face, length 10 m, overlap 5 m
• immediate support with kiln-dried shotcrete (30 cm)
• reinforced shotcrete with two layers of netting (up to 14/14/150/150)
• a three-chord lattice girder every 1 m
3.3.2 Transverse galleries
Transverse galleries were driven and pre-secured with spile canopies and long face anchors ahead of the tunnel face (Fig. 9). The spile canopy was placed on lattice girders, which were all staggered by 1 m. Kiln-dried shotcrete was used for immediate support, just like in the main galleries. The driving of transverse galleries and concreting of partly prestressed interception beams took place according to the pilgrim-step procedure, i.e. in small steps. Excavation support for secondary transverse galleries rested on the concreted bracing girder in the primary transverse galleries.
The following data about advance operations is worth mentioning:
• a total of 29 transverse galleries of 10 to 25 metres in length
• face area roughly 18 m²
• 10 to 22 spiles (spile canopy) per driven metre
• up to 19 self-drilling anchors grouted in the face, length 10 m, overlap 5 m
• immediate support with kiln-dried shotcrete
• excavation support: a total of 20 cm of shotcrete
• reinforced shotcrete with two layers of netting (up to 10/10/150/150)
• three-chord lattice girder every 1 m
3.4 Structural computations
Main galleries were driven just below large loads stemming from the foundation of the South Wing. Consequently, the soil provided no distributing effect, which caused high stresses on the pipes. A total load of 250 kN/m² was calculated for dimensioning the pipe-umbrella system and the face anchors.
The dimensioning of ancillary constructive measures resulted in a double-pipe umbrella system being installed in the roof zone. The bearing load of the pipes in the thread section was tested at EMPA (Swiss Federal Laboratories for Materials Science and Technology) . Based on these tests, the necessary pipe length and the shape of the butts were established. The resistance taken into account in the calculations was also verified during the tests.
3.5 Predicting settlements
Maximum permissible damage such as cracks and spalling that do not endanger the bearing structure was agreed upon with the client for the South Wing building. Maximum permissible settlements and differences were determined to be 25 mm, i.e. 1/300 in total across all construction phases.
Settlement computations were carried out using a FE analysis in a 3D model , . This model also features transverse gallery drives and excavation work performed in stages beneath the bracing cover. As a result, anticipated settlements could be estimated for all construction phases. The settlement prediction could be verified using systematic monitoring on the South Wing.
The entire logistics of the construction activities was provided via the South Wing shaft. As there was very little space available, platforms were built for storage purposes and installations. All installation sites available around the South Wing had limited load-bearing capacity because of existing underground facilities .
The tower rotary crane (Liebherr 630 EC-H 40) was the critical piece of equipment for building the structure and handling all the material and machinery needed. The weight of the diaphragm wall excavators (40 t without counterweight and crawlers tracks) determined the type of crane to be used. The working area available in the shaft for underpassing the South Wing was only 10 x 20 m.
Excavated material and concrete waste were loaded into 7-m³ skips lifted by crane and transported on trucks to the tip. As the handling area was limited in space, all transports and deliveries were coordinated using an EDP tool.
The treatment plant, including the settling basin, was too heavy to carry for the platforms located on the building site. It was set up on a platform slightly remote from the building site, above the river Limmat.
4.2 Construction cycles, performance, installations
4.2.1 Main galleries
A two-metre-per-day rate of advance was laid down for the project to reach ring closure after two one-metre long lengths. The daily advance was limited to 2 m max. to keep pace with shotcrete curing. It implied that a pipe-umbrella stage of 10 m had to be excavated and secured in five working days (one week). This requirement could be met by selecting the best possible equipment and a three-shift operation. A tunnel heading machine with an integrated conveyor (ITC 312 H6) was used to extract and load the Limmat Valley gravel locally consolidated by pipe-umbrella grouting. In this way, there was no need to switch machines for mucking purposes. Intermediate transportation to the skips was ensured by deploying a compact tracked loader (Bobcat T300), which could turn in the only 3.7-m wide gallery.
While driving and supporting were executed in one main gallery, the pipe-umbrella system had to be installed in the other one within a week. Drilling took place using the AT casing system by means of a two-arm Atlas Copco 352 S drilling jumbo featuring an automatic system for laying pipe elements. On average, eight pipes of 14 m length each were laid per day during the early shift. The ground was grouted with double packers from the perforated pipe-umbrella pipes during the late and night shifts.
4.2.2 Diaphragm walls
Guide walls were erected underneath the raft of the main galleries while the excavation support was installed in one-metre stages. The overlying part was concreted in a conventional manner once advance operations were completed.
The diaphragm walls from the main galleries were produced using two crawler rope excavators (Liebherr HS841). The excavator’s dimensions defined the profile of the main galleries. Every day, a diaphragm wall panel (0.8 x 2.2 x 23 m) could be put up in each gallery in a two-shift operation. For logistical reasons, work was coordinated to avoid excavating and concreting taking place simultaneously in the two main galleries.
Compact tracked loaders transported the muck from the diaphragm walls to the shaft. This material was then loaded into skips and disposed of in the same manner as the excavated material.
4.2.3 Transverse galleries
The 29 transverse galleries were driven in three-shift operation, just like the main galleries. All these galleries were driven in small steps from the southern main gallery. The northern main gallery was used as an access to produce interception beams. After each round of advance, a 4-m long spile canopy was installed ahead of the face to secure the transverse galleries. Self-drilling anchors for the spile canopy were drilled with grout. As in main galleries, self-drilling anchors were placed every 5 m to protect the face.
4.2.4 Interception beams
A tunnel-sealing membrane was first laid in the roof zone of the excavated transverse gallery. Carbon-negative concrete and lost formwork were then incorporated into the raft. The reinforcement and the casing for prestressing were laid, the formwork for the control walkway placed in position and the faces shuttered. On average, it was possible to fill an interception beam with up to 380 m³ of self-compacting concrete (SCC) every eight days. From a bobbin placed in the shaft, the strands for prestressing were automatically fed into the casing through a guide pipe located in the main gallery. Once prestressing was completed, interception loads from the roof slab were actively transferred to both diaphragm walls using 140 jacks (diameter 750 mm / 6000kN).
5 Lessons learnt from construction
5.1 Advance operations
5.1.1 Main galleries
Driving the two main galleries was successfully completed by mid-November 2009. Activities could eventually be resumed according to schedule following various optimisations despite initial difficulties when drilling the pipes for the umbrella system. The double row of pipes in the roof and consolidation grouting between the pipes successfully prevented caves-in from the intrados.
Rapid ring closure and the use of fast-setting kiln-dried shotcrete in combination with adequate constructive measures made it possible to meet the high demands of the project.
5.1.2 Transverse galleries
The transverse galleries could be driven without any major incidents. However, in the primarily non-cohesive Limmat gravel, the spile canopy only proved effective to a certain extent. Although spiles were drilled and grouted with mortar, the soil between them could not be consolidated as much as desired. As a result, soft material sometimes trickled out from between the spiles. However, collapses could always be avoided by applying overlaps that were longer than usual and the arrangement of the spiles in multilayers that goes with them. Poor soil consolidation of the soil and low stiffness in the spile canopy, compared to that of the pipe-umbrella system, also impacted the face. Despite having a small cross-section, face stability had to be ensured by using partitioning.
5.2 Erecting diaphragm walls
Erecting reinforced diaphragm walls from the main gallery is a method which has proved its value. This innovative solution was improved upon and successfully implemented following further development by the responsible contractor when moving reinforcement elements further on. The stability of the open trench was always guaranteed, even though the lowered water table was only 2 m below the upper edge of the diaphragm wall. Setting a maximum width of 3 m for the elements and concreting the trench within 24 h were key measures for meeting safety requirements.
5.3 Settlements and damage to the building
Tunnelling operations in the main galleries caused settlements affecting the building of 10 mm on average. Minor cracks in the basement and partly loosened floor coverings were the results.
Settlements of approximately 20 mm were predicted for the transverse galleries. Settlements of about 10 to 15 mm were measured in large parts of the structure. In the west, where temporary diaphragm wall elements were erected as extra protection, settlements of about 25 mm had to be accepted. These settlements caused cracks in plaster walls in the South Wing and damaged them. However, structural damage was not observed.
To contain these settlements, the bracing cover was fully prestressed without bond and pressed on between the diaphragm wall head and the abutment by using jacks. By prestressing the interception beams and activating the jacks, the South Wing could be raised by 1 mm to 4 mm max.
After completion of the excavation under the South Wing (Fig. 14) deformations of approximately 2 mm were registered on the longest interception beams. No further settlements have occurred.
6 Concluding remarks
The adapted top-down construction method chosen for this project has fulfilled the expectations. Services at the station could be maintained despite occasional minor restrictions, and the high demands concerning maximum permissible damage were met.
The construction work required to implement the concept is time-consuming, cost-intensive and places high demands on quality assurance. The considerable effort involved can only be justified by unusual project conditions, like in the South Wing project.
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