The extensive rebuilding of central Berlin as the capital city of a united Germany includes significant repair, reinstatement and expansion of the city’s infrastructure network. Part of this process will see the realisation of a modified version of the pre-1939 rail and road plan for the city. A missing element of this original masterplan is the north-south rail link for regional and inter-city services, which forms the ‘stem’ of the city’s ‘mushroom’ concept of transport routes.
The entire link is budgeted to cost $2.1bn, funded by the city of Berlin, the federal government and the Deutsche Bahn. Planning was instigated in 1992. Work began on site in October 1995 and was split into a number of packages. Lot 3, which included the bored tunnels, was awarded as a design+build contract to a JV of Hochtief; Philipp Holzmann; Bilfinger+Berger; and Dyckerhoff+Widmann. A JV of Lahmeyer International and Emch+Berger was appointed construction supervisor. Client is the DB Projekt Knoten Berlin, a subsidiary of Deutsche Bahn,
The project comprises: a multi-level main station -Lehrter Bahnhof, north of the River Spree, which replaces that destroyed in WWII; a new underground interchange station at Potsdamer Platz, the traditional centre of the city; and a southern surface station at Papestrasse. The link is approximately 7km long, of which 3km is underground, passing under the government and parliamentary district, the Tiergarten and new developments around Potsdamer Platz, crossing under the River Spree and the Landwehr Canal en route. The four rail lines are carried in a combination of cut+cover, caisson and single-track bored tunnel sections, the choice determined by the depth of the tunnels and the sensitivities of the location in question.
Total bored tunnel length is almost 6km (4 x 1.5km), with the tubes 5m apart at stations and 10-12m apart along the drives. The alignment is more or less flat, featuring inclines and declines of 3m maximum, with a minimum radius of 1100m. It is designed to take trains at speeds of up to 120km/h. Because of the relatively short lengths and because freight will not be carried on the lines, no cross passages are included in the bored tunnel sections. Emergency escape routes are provided in the cut+cover structures at either portal.
The geology in Berlin is unusual, comprising mostly saturated ground consisting of sands with different strata of marls, glacial clays and silts, boulders and other deposits. All are overlain by a layer of fill. The water table is generally only 3m below surface level and the tunnels are located at depths of 18-25m . As a result, Herrenknecht slurry shield TBMs are being used to drive the bored sections of rail tunnel.
The influence of the high water table on construction cannot be overstated. Not only does tunnelling have to take account of water pressure and uplift, but tunnellers must be careful not to interfere with the surrounding water table. For example, a maximum of +/- 1m is allowed in drawdown or replacement to minimise damage to the parks and wooden piled foundations of many landmark buildings.
To maintain the water table during the massive reconstruction, 4 sq. km of the city have been installed with a complex groundwater management system comprising more than 90 monitoring stations and extraction and injection points linked by a network of pipes to the River Spree and the city’s canals. To avoid contaminating the groundwater, all water from the sites has to be filtered before discharge.
Another unusual aspect of construction in Berlin is the very real danger presented by unexploded munitions left over from the WWII. Any groundworks above 18m require investigation by geophysical scanning and removal of suspicious objects by specialists.
The logistics for such a major project in the centre of a city of 3.5m people would be bad enough at any time, but, with the entire city centre apparently under reconstruction, the potential for congestion is clear. Ironically, the damage caused to the city during the war has provided part of the solution. The contractors on the north-south link have been able to use the sites of destroyed stations and trackways, one in the north and one in the south, as logistics centres for delivery and storage of goods and materials. Here also, spoil handling and removal have been carried out away from the public.
The two sections of bored tunnel in Lot 3 run 710m beneath the Tiergarten from the Reichstag launch pit in the north to the reception pit adjacent to the Potsdamer Platz station box. The 580m southern tunnels are driven under the new developments of central Berlin and 7m below the Landwehr Canal to connect the Potsdamer Platz station box to the 200m long reception caisson at Gleisdreieck, also part of Lot 3
From the reception point at Gleisdreieck in the south, the rail lines continue in six huge concrete caissons, which provide a 3% ramp to grade. The use of caissons was a design option put forward by the contractor. They were considered attractive because they obviated the need for side and vertical anchors to overcome uplift in an area which was next to existing structures. In addition, because they were built on the surface and sunk into position, the effects of the high watertable during construction were avoided.
These structures, the largest of which is 60 x 40 x 12m high, were formed and placed within 1.5m of each other. The connections were achieved by cutting through the temporary steel bulwarks. The largest weighs 28 000 tonnes, has 1.5m thick walls and a 2m thick roof. They were placed within 50mm tolerance, far within the 150mm allowed; average sinking rate was 230mm/day.
It was initially intended to undertake the two sections of bored tunnel at the same time, with the two TBMs shuttling back and forth between launch and reception/turning pits to form the four tubes to the north and south of Potsdamer Platz. However, with the flooding of the southern launch caisson at Gleisdreieck in 1997, the plan had to be revised.
After the flood, it was decided to run the machines sequentially on the northern drives, with TBM1 driving while TBM 2 was being set up ready for the second heading to begin, and so on. Because of the high power demand of each machine, the practical limits of supplying lining segments, and the capacity of the 1200cu m/h Schauenberg bentonite separation plant, it was impossible to run two machines on one section at the same time.
According to the revised plan, once TBM 1 had completed its second northern drive (the third of the northern section), it was dismantled and transported to Gleisdreieck ready to start the first of the southern drives. TBM 2 will follow once the final northern drive is completed, by which time, TBM 1 will be on its way towards Potsdamer Platz.
One added complication, however, is that, because the reception point at Potsdamer Platz Station is now far further advanced than anticipated, it will be impossible to turn the machines around for the return drive. They will therefore be pulled back to the southern caisson and repositioned for the next drive.
This is similar to the procedure on the northern drives where, because only one machine could work at a time, instead of the TBMs being turned at the reception pit, they were removed and transported back to the Reichstag pit for the next drive while the second machine was driving. This solution, advanced by Herrenknecht, will see the cutterhead broken down into elements and removed and the shield sacrificed. A total of five shields will be sacrificed this way.
The two slurry shield TBMs have a diameter of 8.9m. Installed power is 2250kW, of which 1000kW is used for the cutterhead. To cope with granite boulders, up to 1.5m in diameter and 180MPa , each machine has cutter discs and a crusher. Boulders greater than 2m in diameter have been encountered on nearby sites, but, so far, the only problems have been with relatively small ones which have rolled around the cutting wheel until the TBM is shut down and they are removed.
The glacial clay becomes very plastic when disturbed, so an independent central cutterhead is also fitted into the main cutterhead to prevent material accumulating in the centre. The 8.95m diameter hydroshields are placing a single-pass segmental lining, claimed to the first for a German railway tunnel. The 1.5m wide seven segment plus key lining was manufactured by Imbau, the Dywidag subsidiary in east Berlin. Segments are manufactured to a tolerance of +/-0.2mm and require bolting only for the first 10-15m behind the face. Finished tunnel i.d. is 7.9m.
Because a Schöma diesel train is used to transport the lining segments to the erector, the contractor is providing ventilation using Korfmann fans and ducting.
According to Lahmeyer, the drives are going smoothly, with the TBMs coping well with the ground conditions. Each drive is taking about two months, with six weeks required for setting up and driving of the first few metres. Setting up difficulties are exacerbated by the congested nature of the launch pits, which are not large enough to assemble the full 55m back-up train before launch.
Initially, it was intended to employ 5 day, 24h working. However, to recoup some of the lost time, 7 day working is used, with a maximum of 22.5m achieved in one day. Progress is averaging 10-12 rings/day.
The quartz in the sand is very abrasive and cutters are checked every 150m and replaced at the end of each tunnel drive. The cutterhead can be drawn back 500mm to facilitate this. Depth to tunnel floor is 25m and the machine has to contend with 2.3 bar water pressure. Accuracy on the tunnel alignment has been stipulated as +/-100mm.
The start pit at Reichstag is a cut+cover structure, 50m x 65m and approximately 20m deep, formed by diaphragm walls; 20m long anchors to overcome flotation. This northern drive ends in a 45m deep cut+cover structure to the north of the Potsdamer Platz station box. Here, the machine is disassembled and turned into position for the return drive, using air cushions.
The first TBM drive was in September 1997 and the fourth was half completed at the time of T&T International’s visit at the end of August ’99. The northern drives were completed in early October. Meanwhile, to the south, TBM 1 was being set up in the once flooded caisson at Gleisdreieck, ready to begin the first of the southern drives to start in early 2000. TBM2 is expected to begin the second southern drive in April.
At the time of the visit, the final northern drive was at 350m, and close monitoring of groundwater fluctuations was being enforced because of the close proximity to the Reichstag building, whose oak piles could not be allowed contact with the air from lowering of the water table.
In preparation for breakthrough of the southern drives, pressurised reception bulkheads have been constructed at the tunnel ‘eyes’ at Potsdamer Platz to hold back the water pressure of 1.5 bar.
Each drive is expected to take three months, with three months’ turnaround time.
Ground movements are monitored closely, with a maximum of 20mm allowable. So far, only 5-10mm settlements have been recorded, except for very short sections at the beginning of the drives.
Because it could not be predicted exactly where the reception caisson at Gleisdreieck would come to rest, it was not possible to use conventional steel ring targets for the TBMs in the original plan. Instead, a 5m thick block of jet grouted columns was placed in front of the caisson wall from the surface so that the eye could be opened in the correct position from the inside, using an excavator- mounted hammer.
However, despite dry boring tests, eye No. 2 failed during opening in July 97 and water began flooding in. The contractor immediately flooded the caisson to balance the pressure and it remained in this condition until early 1999, when a recovery technique had been agreed. Meanwhile, 200cu m of sand had washed into the caisson, resulting in a serious surface slump, which fortunately was contained within the construction site. Although this was serious, further losses were avoided because TBM 1, scheduled to begin its first north bound drive, was on the surface waiting to be lowered into the launch area. If failure had occurred a few days later, the TBM might have been lost. The reasons why the grout block failed are still under investigation.
The drives from the south are expected to take about three months each, although the presence of more clay in this section right across the drive face is expected to slow things down over a length of 15-18m. The alignment also takes the machines close to the new buildings of Potsdamer Platz, so greater care will have to be taken.
The drives will be finished by the end of 2001.
Related Files
Geological profile
North-south rail link
