The 10.6km long Zurich western bypass, currently under construction, has been designed to ease the serious traffic congestion currently affecting Switzerland’s financial capital (figure 1). The bypass, that will divert traffic from the city by connecting the existing A3 and A4 highways, includes four tunnels totaling 8.4km (around 80% of the route). The US$746M, 4.4km long twin-tube Uetliberg tunnel is the longest of these and possibly the most challenging. A variety of driving techniques are being employed to tunnel through the rock and soil, the most interesting being the use of a TBM equiped with a 14.4m diameter reamer using the undercutting technique. Although well established techniques in their own right, combining a reamer with undercutting at the Uetliberg is thought to be a world first for the tunnelling industry.
Geology
The dominant geology along the alignment (figure 2) is through the 500m long Eichholz and 2.8km long Uetliberg molasse sections where the harder rock strata comprises flat-bedded strata of the upper fresh water molasse, an alternation of hard sandstone seams and soft marl strata.
The three other tunnel sections are through soft ground. Firstly, the 210m long Gjuch section (at the Wannenboden west portal) which cuts through a very heterogeneous end moraine called the Wettswil moraine complex consisting of a loamy, sandy gravel. The water table rises from the centre of the tunnel profile at the start to above the tunnel roof in an easterly direction. Secondly, is the 240m long Diebis section consisting of a base moraine overlaid with slope wash. The slope wash consists of moraine material and fine particles. At the start of the soft ground section, about half the tunnel cross-section lies in the slope wash, which then rises towards the east. After about 50m, the whole cross-section is in the moraine. In the Diebis soft ground the whole tunnel cross-section is in ground water.
Finally, is the 410m long Juchegg soft ground section comprising a base moraine composed initially of sandy-gravels and then of clay-like sands. Above this lies the Uetliberg loam, to the centre of the tunnel cross-section at the portal. After about 70m the whole cross-section is in the moraine.
The water table is below the tunnel cross-section at first, rising at the interface between the sandy-gravels and the clay-like sands of the moraine. At the interface between the soft ground and the molasse, the whole tunnel cross-section is in ground water.
Normal cross-sections
All of the soft ground tunnels and the Eichholz molasse section are being excavated as a horseshoe cross-section, 14.7m wide and about 12.7m high with an excavated area of 143m² to 148m².
The exception to this is the 2.8km long Uetliberg molasse cross-section that is 14.4m wide and 14.2m high with an excavated area of approximately 160m².
The tunnels are designed with a fully sealing double skin lining. The seal is pressure-maintaining in the Gjuch, Diebis und Juchegg soft ground and Eichholz molasse sections and drained (depressurised) in the Uetliberg molasse section.
The parallel tubes will be connected every 300m by cross passages for pedestrians and every 900m for vehicles. SOS niches will be situated every 150m.
There will be an underground central ventilation station in the Reppisch Valley (Landikon) located over the underground traffic interchange. The tunnel will normally be ventilated in both directions by the natural longitudinal ventilation/piston effect. A system of environmental ventilation is designed for the Basle tube which permits the air flowing out of the tunnel to be extracted before the Wannenboden portal. The air will be fed back through a network of ducts along the tunnel tubes located above the intermediate ceiling to the central ventilation station in the Reppisch Valley, where it is discharged outside via the exhaust air tunnel and Eichholz shaft.
Construction phases
The tunnel is being driven downwards (1.6º) from the Reppisch Valley excavation site through to the Zurich-South interchange in the Brunau area (figure 3).
Construction by sequential excavation started from the Reppisch Valley site in April 2001 on the two tubes in the Diebis soft ground section (SG-DIE = 2 x 240m). The section was completed in May 2002. A 5m diameter Wirth TB III 500 E TBM was installed in the Basel tunnel tube at the start of April 2002 and is currently boring a pilot tunnel through molasse section of the Uetliberg (MO-UET = 2 x 2,800m). From spring 2003, the 5m wide pilot tunnel will be widened to a final cross-section of 14.2m to 14.4m using the TBM with a reamer. The pilot TBM and the reamer will be partially pre-assembled before the portal in the Landikon excavation site and then transported to the starting cavern of the Basle tube where the assembly process will be completed. After driving the Basle tube, the TBM and the reamer will be dismantled in the dismantling chamber and transported back through the tunnel. They will then be re-assembled in the second specially prepared starting cavern for the purposes of driving the Chur tunnel.
The upward driving of the Juchegg soft ground section (SG-JUC = 2 x 410m) from the Gänziloo excavation site (Brunau side) started in February 2002. In April 2002 work also got under way on the downward drive from the Wannenboden excavation site under the Ettenberg towards the Reppisch Valley site. This section of the Uetliberg tunnel has to be driven firstly through the Gjuch soft ground section (SG-GJU = 2 x 210m) followed by the 500m long Eichholz molasse section (MO-EIC).
Driving lesson
All the soft ground sections are being driven by sequential excavation with support consisting of steel arches (HEM-180 girders spaced 1m apart) and 250mm thick steel fibre-reinforced shotcrete.
The 500m section under the Ettenberg (Eichholz molasse section), will be blasted, divided into the crown, bench and floor.
The Uetliberg molasse tunnel is being excavated by the 5m diameter Wirth TBM then reamed to its full cross-section of 14.2 to 14.4m diameter using the undercutting technique. The support here comprises cable bolts, Swellex bolts, mesh and shotcrete and is being installed directly behind the boring head. The final lining and the intermediate ceiling will be built in stages behind the machine.
Driving the Diebis soft ground section
As with the other soft ground sections, the Diebis drives have been divided into seven sub cross-sections: 2 x 17.35m² upper side-wall galleries on both sides; 2 x 22.55m² lower side-wall galleries on both sides; a 24.30m² crown; a 26.66m² core; and a 16.84m² base, giving the total 147.60m² area.
The upper side-wall galleries were excavated in metre steps by mini-excavators and immediately lined with a 5cm thick layer of shotcrete. Steel girders (HEM 180) were then installed at metre intervals to protect the seal (the seal layer is created with furnace-dried, prefabricated dry shotcrete due to its quick availability and increased safety). After a further excavation round, the installation girders were sprayed with steel fibre reinforced wet shotcrete. Rockbolts, mesh and drainage pipes were also installed in the roof due to the presence of water and to improve safety conditions.
In order to drive the crown with a span width of 8m, a 20m long pipe umbrella was created during the initial construction phases comprising 29 x 152.4mm diameter pipes. The first 50m of the Juchegg soft ground tunnel were also built this way. Following the pipe screen, the roof was secured with 30mm diameter, 4m long lances. The drilling face was then anchored with nine 15m long steel self-drilling bolts with a 3m overlap, with additional drainage pipes at the drilling face where there is water build-up.
The procedure for installing the support was the same used whilst driving the side-wall gallery. The additional measures were required for safety and for reducing deformation during roof excavation.
Base excavation level, ring seal
In order to reduce deformation across the entire cross-section, the floor ring closure had to follow 40m behind the crown face with the inner side-wall gallery walls being excavated 6m behind the crown face. The core was not excavated vertically, as originally planned, but connected to the base level with a ramp. As soon as the crown was excavated to 12m, the work was switched to the floor excavation. This was done in 12m steps from the face in the direction of the portal. The work progressed 3m-4m at a time in which waterproofing, steel support and steel fibre wet shotcrete were placed.
Where build-ups of water occur, measures had to be taken to ensure that the water was removed from the base area before the sealing layer was applied. In order to facilitate this, additional drainage measures were installed, that included customised membranes, seepage ditches and pump shafts.
Monitoring in the Diebis soft ground
Construction monitoring is carried out using a variety of equipment, such as piezometers, 3D convergence measurements, extensometers, distometers, cross-section surveys and steel extension sensors (strain gauges).
Deformations/movements are also monitored with optical 3D convergence measurements, distometer measurements and cross-section surveys. The convergence measurements showed that there were practically no measurable signs of deformation when driving the upper side-wall gallery. When the lower side-wall gallery was excavated, the upper side-wall gallery subsided by about 25mm. At the same time, the distometer measurements showed that there was a horizontal narrowing of the cross-section of the order of 10mm to 15mm. Deformation of up to 10mm was detected after excavating the crown. Deformation of about 30mm was recorded again in the ring seal, leading to total deformation of approximately 70mm. The cross-section surveys conducted at different times, i.e. before or after the ring seal, revealed values of the same scale. The deformation values calculated beforehand by the project coordinator were about 50mm – 100mm. The comparison with the effective deformation measurements and the total deformation confirmed the values calculated.
Progress report
Excavation of the Diebis soft ground section is complete as are the assembly caverns for the reamer in both tubes. The pilot tunnel has already been driven to 1500m in the Basle tube with current progress of approximately 25m per day reported. The hole-through of the ventilation tunnel, driven in the opposite direction, is on schedule for the end of 2002. Work got under way driving the Juchegg soft ground section in February 2002. The roof and the lower side-wall gallery have been excavated to a length of 50m in the Basle tube. The driving of the ventilation tunnel (upper right side-wall gallery) is currently at approximately 400m. The upper left side-wall gallery is at 180m. Construction started on both tubes of the Gjuch soft ground section in April 2002. To date, 140m progress has been made in the Basle tube and 20m in the Chur tube, both at full cross-section. The sinking of the exhaust air shaft in the soft ground (22.8m) is complete with blasting of the remaing 37.8m having begun in August. At the current rate of progress the project is well on schedule for the planned commisioning date in 2008.
Reaming with undercutting technology
The Wirth TBM fitted with the 14.2m diameter reamer, utilising undercutting technology, will be deployed in the Uetliberg molasse section in spring 2003. The boring head is currently being prepared by Wirth AG, whilst the Uetli JV is making arrangements for the back-up. The individual elements of this process, i.e. reaming and undercutting, have already been used with great success on many occasions in practice or have been tried out in extensive trials in Germany and Canada (undercutting technology). Although used in mining, it is believed they have never been used together on a civil engineering tunnelling project.
The reaming technique facilitates the mechanical driving of a wide area of the tunnel cross-section with direct rock securing, which can be adapted to suit the geological conditions met. Compared to conventional extending, it allows considerable savings both in terms of the rock securing resources used and the lining because of the less destructive nature of the excavation and the circular, statically favourable cross-section.
The reamer being used is essentially based on the driving installation already used successfully in the Tunnel de Paracuellos (Spain) and in the Tunnel de Sauges (Switzerland). As the reamer variant tendered by the contractor is mounted on the existing, tried and tested driving installation, the costs of supplying a complete TBM of this size were negated.
Liaising closely, the Uetli JV and Wirth have evaluated the technical capabilities for deploying the extender with undercutting. Undercutting has been acknowledged as an effective cutting principle ever since the early days of boring with a TBM. In this technique, the cutting rollers work against the rock’s tensile strength, which is considerable lower that the compression strength.
How undercutting with a reamer works
As before, the boring head of the reamer consists of a two-part boring head base and six boring arms. The boring head rotates on the inner kelly which is braced and bearing-mounted in the pilot tunnel and in the large tunnel cross-section. The cutting rollers are offset both axially and radially to the axis of the tunnel and arranged on axial moving slides on the boring arms. As the boring head rotates and the slides move in a radial action at the same time, each roller follows a spiral path around the axis of the tunnel. As the outer roller advances, this creates a stepped face, so that each of the cutting rollers can shear off the rock into a free space (undercutting principle). When boring starts on a so-called “shot” (axial excavation section per radial stroke of the slide), the inner cutting rollers start in the pilot bore, for example, and the outer rollers start at the last level bored by the inner rollers.
Inserts screwed into the cutting roller holders act as receptacles for the cutting rollers and can be exchanged quickly if they are damaged.
The length of the shot is limited to a maximum of the axial displacement of the cutting rollers on a slide (SA = 200mm). Smaller shots may be selected depending on the strength of the rock. As the six-armed boring head rotates, with its six cutting rollers on each arm, the 36 cutting rollers are moved on six spiral paths, 60° apart, from an inner boring diameter to an outer boring diameter (p x zA). Once the 14m boring diameter has been reached, the slides are retracted to the 4.5m diameter. The boring head is then moved axially by a shot (e.g. SA max. 200mm), and the next step starts.
As the shear forces of the blades are applied in a radial direction, compared to conventional cutting techniques the force components of the thrust action are neutralised by the diametrically opposite arrangement of the boring arms. The small number of cutting rollers (six per arm) with a contact pressure of approximately 100-120kN/cutter also reduces requisite torque at the boring head for releasing the rock. The loss of the large contact pressure forces (thrust) and the resulting high torque loads of the tunnel permit the enlargement of the boring head on an existing TBM. The reamer can extend tunnels with a pilot of 4.7m-5m diameter in stable, drillable rock, to a 14.4m diameter.
Reamer description
The base machine for the reamer, called the Tunnel Bore Extender (TBE) 450/1440 comprises the following modules:
Advantages of reaming with undercutting
The undercutting principle causes lower forces to be exerted on the TBM (main bearing, inner kelly). This meant that there was no reason why an existing machine could not be used/modified. Other advantages of the driving device selected by the Joint Venture:
Disadvantages compared to a shield TBM
Related Files
Fig 2 – Longitudianal section
Fig 4 – Finished cross section of the Uetliberg’s Molasse section constructed by 5m diameter pilot TBM and 14.4m diameter reamer
A typical reamer set up showing the pilot tunnel being used for bracing the main machine
Fig 3 – Driving directions and methods used during construction of the Uetliberg Tunnel
Fig 1 – Location map
