The steep valley sides, ecological concerns and the ban on transporting materials in the Filstal were among the many reasons why the planning permission prohibited tunnelling operations in the valley. Therefore a 950 m long intermediate access tunnel connecting to the railway tunnel at km 44+500, was constructed at Umpfental to drive the two main tunnels.
Geological conditions
Starting from the northern portal at Aichelberg, the tunnel first cuts through brown Jurassic layers for about 6 km. Because the preliminary geological investigations had shown areas with very low rock strengths of only about 3 MPa (uniaxial compressive strength) and there being up to 280 m of deposits above the Boßler, the design had assumed that squeezing rock conditions and plastifying areas would be encountered at the excavation edge. Consequently, the use of conventional tunnel driving with a “flexible” construction (use of compressible elements) were envisaged in the critical areas instead of a TBM. On the remaining sections of tunnel, some karstification of the white Jurassic rock could be expected, mainly in the upper layers of Oxfordian 2 in the area of the southern portal at Buch. Therefore the tender called for the tunnels in these areas to be driven conventionally and lined with sprayed concrete (SCL).
In the brown Jurassic rock, water pressures of up to 4 bar (extraordinary load case 6 bar), in the area of the white Jurassic up to 6 bar (most unfavourable 9 bar) were assumed. To separate the lower aquifers from one another and to prevent seepage along the tunnel linings, six radial grout curtains were installed along each tube.
Commissioned design
When the design was commissioned in 2012, the proposal was that the tunnel would be mechanically driven with a TBM from the Aichelberg portal (km 39+720) to km 42+100 in single-shell construction with precast concrete tube segments, while the remaining 6.4 km up to the Buch portal would be constructed in SCL with an in situ concrete inner shell permanent support. The aim was to avoid driving the TBM through the critical areas (squeezing rock, Karst). Starting from the site compound near Gruibingen, work began on the 945 m temporary intermediate access tunnel at Umpfental, driving downhill at a gradient of 73 ‰ in SCL to connect to the two main tunnels at km 44+500 in an intersection gallery. After completion of the main tubes, the temporary tunnel was completely backfilled.
The tunnels were bored with a machine capable of operating in a range of driving modes. This allowed tunnelling to proceed in “open mode” without support to the excavation face whenever it was adequately stable and hydrological conditions were favourable. Compressed air support could be switched on temporarily or the machine switched to “closed mode” operation, during which a conditioned earth pulp provided support. Driving started under the protection of the 13 m long start-up gallery with a conventional fixed rigid steel structure. During driving, the 120 m long TBM with 26 double jacks including the back-up system pushes itself forward from the already installed segmental rings. After completion of the eastern drive, the machine was set up again to drive the western tube.
Updated design (extended TBM drive)
From the intersection gallery, work started using conventional construction (SCL) on the eastern tube with the first 47.70 m of the south drive (in the direction of Ulm) and for 731.20 m of the north drive (in the direction of Stuttgart). In order to be able to make the TBM drive as long and therefore as time- and cost-saving as possible, the contractor conducted an extensive exploration programme as work progressed with the objective of investigating the areas predicted to be squeezing rock and thus obtain an improved forecast of the expected rock conditions. This involved creating comprehensive measuring cross-sections in the main tunnel (downhill northern heading) in order to check the actual support and deformation behaviour as the machine approached the areas of low soil strength and obtain important parameters for the tunnel heading in accordance with the observation method. In addition, starting from the intersection gallery between the two future main tubes, an exploration shaft (8.0 m diameter, 49.50 m deep) and a 26 m long “flexible” exploration gallery (3.0 m diameter) with compressible support elements were constructed in order to explore the Aalenium 2 (al2) layers and more accurately determine in advance where the lowest strengths could be expected. The exploration shafts and galleries also provided a large number of field and laboratory test samples.
Using the results from these extensive exploratory investigations, it was possible to make a more favourable assessment of the rock mechanics model and the parameters of the al2. The exploration programme proved that the shield passage could be extended beyond km 42+100 – and provided robust data for further design and construction. It also served to find out whether the ground conditions in the 400 m final section of the tunnel in white Jurassic strata would permit the passage to continue up to the Buch portal, although the use of a TBM there had been initially excluded. The actual degree of karstification in the Oxfordium 2 was extensively explored based on additional investigations (including two 350 m long horizontal boreholes with core recovery, starting from the entry cut at Filstal) and from geophysical measurements. In the end, mechanical driving was also approved in this area and the TBM reached the prepared target gallery at Buch portal after 18 months. The TBM moved on to the second, western tunnel tube and began excavation in April 2017.
Single-skin tunnel segment construction
Under the protection of the shield, the single-skin, 45 cm thick precast concrete segment cladding is installed in the areas of standard tube construction (concrete class C 45/55, average segment length 2.0 m, left /right ring, small cam and plug interlock in the ring joint) in a 6+1 arrangement. The contact surface in the longitudinal joints is 1.90 ≈ 0.22 m.
The calculated 10.94 m tunnel O/D in unfavourable soil conditions (here: concrete class C 50/60 and an increased segment thickness of 65 cm) determines the 10.04 m I/D in the standard construction sections, which is greater than the railway authority minimum. To allow the change in segment thickness from 45 to 65 cm during driving (for the first time in Germany), a few issues had first to be addressed, e.g. the eccentric jacking forces and the constructional details of the transition.
The precast, reinforced concrete segments were manufactured on an on-site carousel production line near the Aichelberg portal. Special machines weld the bars into reinforcement ladders, which control the splitting forces in the concrete at the longitudinal and circumferential joints. The welded reinforcement cages are made using high-precision templates and lifted into the steel formwork.
When openings are made in the tubes later for the SCL crossovers, the loads are transferred from the incomplete rings into the neighbouring intact rings by special large shear dowels. Additional sensors (e.g. vibrating wire sensors) were installed to provide more accurate information about the actual loads and the stresses within the tunnel segments in the highly loaded areas near the crossovers and complete measurement data from the installation of the rings onwards.
Filling the annular gap
The 20 cm wide annulus between the precast tunnel segments and the excavated soil face is filled with grout in accordance with Guideline 853.4005 issued by DB Netze. Grouting is performed continuously through grouting ducts integrated into the shield tail as the TBM moves forward. The two-component grout provides an effective bedding and transfers the forces between the segments and the excavated soil face. The mortar component and the hardener do not mix in the ducts until immediately before grouting. Used here for the first time in Germany, this solution is increasingly popular internationally for mechanical excavation in solid rock and is advantageous because the grout hardens rapidly to keep the segments in place and the tunnel on course. It also reduces the tendency of the lining to float, accommodates the back-up system loads better and considerably reduces the risk of the grout flowing into the excavation face.
Intersection gallery heading
To achieve an even annular gap, precise alignment of the machine and a guarantee of adequate resistance in all operating circumstances (e.g. the required compression force necessary to prevent the sealing profile relaxing), the crown and bench were re-profiled with a layer of sprayed concrete (clear diameter ≤ 11.20 m). Two guide channels (S49 profile) were built into the rock in the invert and then the ground made up with 4.50 m of suitable fill (“rock fill”). Further measures were required at the crossovers and the intersection gallery area (backfilled ahead of the TBM). In March 2016, the eastern main tunnel breakthrough into the prepared cavern at Umpfental was made after 4,500 m of excavation and the installation of 2,227 tunnel segments.
Special problems, testing and investigations
The theoretical and numerical analyses were effectively augmented by extensive measurements and experimental investigations to verify the calculated results and to supply the required base data for excavation using with the observation method. Several other measures were taken, such as tests by the TUM to investigate load capacity and contact pressures in the longitudinal joints between the segments, the loading behaviour of the dowelled connections at the crossovers (with the UniBwM), the mechanical properties and durability of the annular gap grouting material and the fire behaviour of the tunnel lining. Fire tests were performed on individual test specimens and – for the first time – on the segment joints. The fire and displacement behaviour and the heat penetration at the joints were continuously recorded by fire testers MPFA Leipzig. The precast segments incorporated at least 1.2 kg/m3 polypropylene (PP) fibres.
The Boßler Tunnel is special because of the challenging geological and hydrological conditions, and the innovative solutions, diverse practical and technical optimisations made during its construction. The first trains are scheduled to pass through the tunnel in 2021.
