The 1.6km long twin tube Weser road tunnel, under the Weser River in northern Germany, was officially opened by the country’s Transport Minister, Manfred Stolpe, on 20 January 2004. The project took a total of six years to construct and, with a two-year tunnel boring period through punishing ground, saw Mixshield tunnelling technology pushed to new limits.

The tunnel is the only crossing of the Weser River in the Lower Weser region, north of Bremen (Figure 1). With an expected traffic volume of 20,000 vehicles a day, it will provide a crucial improvement to the area’s transport infrastructure.

The excavation diameter of each tunnel is 11.67m. Both are lined with a 500mm thick reinforced 6 + 1 concrete segmental lining. The resulting 10.3m i.d. is required to accommodate the uni-directional twin-lane carriage width of 7.5m.

The twin tubes, which are connected by four cross passages (Figure 2), run at a longitudinal inclination of 3.5% – 5% under the 900m wide river. The tunnel invert’s deepest point is 40m below sea level (bsl), whilst at its shallowest tunnel cover is a mere 12m, or roughly one shield diameter. In addition to the twin tubes, the scheme included the construction of about 400m of cut and cover approaches on either side of the river.

The project was commissioned by the Federal Republic of Germany with the Highway Construction Administration of the State of Lower Saxony responsible for planning and design. Actual construction was the responsibility of the Highway Construction Office, at Oldenburg.

Following an open European tender process, the construction contract for the tunnel and approach sections was awarded in late November 1997 to the Weser Tunnel JV, with Hochtief Construction as sponsor, at a value of US$154M.

The client decided in favour of a privately pre-financed project model. This took place between the joint venture and a bank consortium. The client agreed to pay the construction cost and interest incurred during construction in 15 equal annual installments after final approval at fixed, contractually negotiated, conditions.

Geological and hydrological situation

The alignment’s mixed geology is dominated by a varied sequence of cohesive Holocene soils, Pleistocene sands and clays, as well as Tertiary sand – all typical of ground conditions beneath a river estuary (Figure 3).

The 15m-20m thick upper layers are described as the Northern German Coastal Holocene, consisting of inter-stratified clay some 10m-12m thick, and small to medium grain tidal flat sands at a thickness of a few centimetres to 2m. On the eastern side of the Weser River, there is also a layer of peat approximately 5m thick.

Pleistocene sands are found underneath the Coastal Holocene layers, with an average thickness of 10m-15m. In addition to the coarse grain sand and gravel present, this layer also contains a considerable amount of stones and boulders.

These sands sit on glacially condensed Lauenburg clays, made up of argillaceous silt, or silty clay, and small to medium grain sands with a low clay content. The thickness of this layer can be as much as 12m.

To the east of the river centre, a till marl, probably of Elsterian Ice Age origin, was also discovered consisting mainly of sand with little or no cohesion. The last, and deepest, layer was determined to be of Tertiary age, in this case Kaolin sand, made up primarily of fine, somewhat argillaceous, sand.

The mechanical soil properties of the formations described also displayed a significant lack of homogeneity. While the Holocene sands are loose or just slightly condensed, the Pleistocene layers are primarily dense or very dense. The argillaceous Lauenburg clay almost reaches the shear strength of rock, exceeding 400kN/m², whilst the Tertiary sand is dense to very dense.

In addition to the geological conditions, the area’s hydrological situation was crucial to tunnel boring. The impact of the tide was to prove of particular significance. This fluctuated in the tunnel area between +2m above sea level (asl) and -2m bsl twice daily and was calculated to vary in extreme conditions between +6m asl and -4m bsl, which meant variations of 10m, or a pressure difference of approximately 1 bar. These fluctuations were reflected in the groundwater, depending on location. In the centre of the river, the ground water fluctuations corresponded to the fluctuations of the river water level, while the groundwater levels in the riverbank areas were more or less proportional to their distance from the centre of the river.

Experiences with shield drives

The two tubes were driven using a slurry supported Herrenknecht Mixshield machine. The machine’s diameter was an impressive 11.67m with an installed nominal shield capacity of approximately 5MW. The six-arm cutterhead was equipped with scrapers, drag bits and disc cutters, whilst an independent active centre cutter was used to deal with the cohesive soil formations present in large areas of the tunnelling project.

Boring of the first, southern, tube, started at the end of 1999 and soon enough the geological and hydrological conditions presented two specific and significant problems.

Firstly, opportunities for machine inspection and maintenance work were extremely limited, being only possible in the excavation chamber, particularly regarding the excavation tools. The conventional practice of lowering the slurry suspension in the excavation chamber to the level of the shield axis was only possible at a few locations in the tunnel, due to the safety requirements for stabilising the working face to prevent blowouts.

In addition, the pressure variations that occur with conventional lowering were incompatible with the requirements at the working face. The submersion was limited to just 1/3 and was only feasible when enough of the existing soil pressure was absorbed by hydraulic work face supports (breast plates), with a pressure-constant exchange of suspension and pressurised air in the excavation chamber. Given the special conditions of the construction site, these requirements were extremely difficult to achieve.

Secondly, problems occurred in the area of the deep, cohesive soil formations. These were caused by the ground’s strong adhesive and cohesive properties in the excavation chamber, which restricted its extraction from the excavation chamber, resulting in increased wear and tear. In addition, the slurry pressure control was disrupted because of clogging in the opening of the immersed wall.

The upshot was a disappointing advance rate on the first tube of 4m per day. The southern tube finally brokethrough in November 2000.

As a result of the slow rates, the slurry Mixshield underwent further development before driving the northern tube. This led to the creation of a new slurry shield with bulkhead invert section (Figure 4).

In contrast to the conventional hydro shield, a complete bulkhead in the invert area, between the immersed wall and the pressure wall, always separates the function of the slurry pressure control from the function of the spoil extraction around the opening of the immersed wall.

Due to this bulkhead, the separated/fresh suspension is added directly into the excavation chamber. This takes place over different levels to ensure a closed flow circuit from the excavation chamber and the closed invert area to the intake of the loaded slurry. The slurry pressure control still follows the principle of connecting pipes through a cushion of compressed air, linked with a compressed air control system. However, the connection of the working chamber and the excavation chamber is established by two equalisation pipes that can be closed with valves.

In the case of partial submersion for inspection and maintenance, the control of the slurry pressure can be quickly and easily transferred to an externally suspended bentonite container.

Inspection, maintenance wear and tear

The main objective for developing the slurry shield with an isolated invert area was to have the option to prepare and conduct partial submersion above the half level of the drive area in a much shorter time. With the isolated invert the process only takes 36 hours, instead of two weeks as before.

Furthermore, the system allows for an effective use of endoscopic inspection with the help of a remote-controlled camera. This also led to a considerable reduction of the required compressed air work.

The wear and tear of the excavation tools and the cutterhead is designed to take place at specifically treated places. However, when adhesive and cohesive phenomena lead to sedimentation in the area of the excavation chamber, parts of the cutterhead are exposed to wear that they are not designed for. This unplanned wear can have severe consequences. With the principle of the bulkhead invert area and the resulting forced movement of the suspension and the excavation material, sedimentation in the excavation chamber is reduced significantly, which eliminates such unexpected wear. The machine’s performance recorded during the drive underlines the positive effect in the excavation chamber.

Boring of the north tube began in February 2001 and successfully broke through in November 2001. Assisted by a VMT guidance system, this was achieved perfectly on line. The energy consumption in the north tube amounted to some 50% of that for the south tube, even though the geology was mostly the same. The torque and ground pressure of the main cutterhead were significantly lower in the northern drive as well.

Thanks to the separation of the conveyor function and the function of slurry pressure control, the slurry pressure in the excavation chamber was controlled more accurately and with greater safety.

The comparison of the relevant and cost-related data on average driving speed over the entire course of the project (including maintenance) showed a 90% increase of up to 8m per day, and a maximum of 18m per day for driving the north tube. After adjustment for all idle time and subsequent installations, the total driving time for the north tube was only 64% of the time required for the same progress in the south tube.

Completing the tubes

To complete the tunnel’s road deck, invert fill was added following completion of the cross-passages. The 40,000m³ of sand came exclusively from excavation material.

High-impact walls have been suspended in front of the lining to protect it from the impact of vehicles, or fire. These high-impact walls were initially built with the help of two 15m mobile formwork systems. An early age high-strength concrete was used to allow the stripping of the formwork after only 24 hours.

Due to the high proportion of reinforcement, the concrete contained a liquifier to facilitate distribution and compaction in the formwork.

In the south tube a Red Max platform was used to fit fire protection boards, which helped dramatically with the speed of erection.

The project was completed on time thanks to the cooperative collaboration of all of the project participants involved, and their willingness to push technological boundaries to the full.

Related Files
Figure 2 – Cross section of the twin tubes at a typical cross passage location
Figure 3 – Longitudinal section of the Weser Tunnel illustrating its mixed geology
Figure 4 – Cross sections of the TBMs re-developed slurry system
Figure 1 – Location map of the Weser Tunnel