Ever tightening safety standards for high speed rail, new concerns about tunnel fire and unexpectedly difficult hydrology and geology have combined to produce a near two year delay on Germany’s latest section of high speed railway line. The route between Nuremberg and Munich in Bavaria demands extensive tunnelling.

All the three contracts for the 89km long new build section of the line from Nuremberg to Ingolstadt have been hit by problems. These range from excessive groundwater in cut and cover tunnel sections to degrading rock, large-scale karstic voids in limestone and substantially thicker loose scree at portals than planned for. In the southern contract, soft ground has also revealed numerous unstable “fossil slides” which have demanded a re-think of work in cuttings, including lengths of cut and cover tunnel.

Problems have been particularly numerous on the Los Mitte or centre lot contract. Here main contractor Hochtief and its junior joint venture partner Alpine Bau Deutschland, have seen seventeen revisions to the project management team’s internal construction schedule. So far some months of extensions have been granted to the contractor by client Deutsche Bahn Gruppe. Claims for more have just been submitted.

Work on the southern of its two 7+km long tunnels is currently suspended, pending completion of special investigations of the geology. In addition solutions to tunnel drainage issues for the invert design and choice of a special high speed concrete track slab type, must be resolved. Hochtief is designer for the slab but it must be approved by the client.

Work on the inner lining for the northern tunnel, is just beginning and some of the invert excavation and construction is still in progress, with a delay of more than a year. Only a short intermediate tunnel, Schellenberg, is now seeing substantial progress on the concreting of the inner lining. But at just 650m this central tunnel is only a small portion of the works.

Hochtief’s overall contract is for an 18.3km long section of the project mostly in large 146m² cross-section double track tunnel, the northern Euerwang at 7700m, the southern Irlahüll at 7260m length and the small Schellenberg tunnel in the middle between two picturesque river valleys.

The contract, Hochtief’s largest ever German domestic award, is an unusual, partial design and build arrangement. The contractor is responsible for the design of the inner lining while the client’s engineer retains responsibility for the geological investigations and the design of the outer shotcrete tunnel “shell”.

Hochtief’s job includes some US$23M worth of embankment and three bridges, totalling 2.7km of outside trackwork.

It must also design and lay a state-of-the-art concrete slab track inside the tunnels, one subject of negotiations at present.

Apart from the big main tunnels the contract includes a total 14.5km of smaller emergency exit tunnels. “One of the early delays was caused by variation orders on these tunnels” says Nis Hansen, currently the tunnel project manager at Hochtief’s site offices close to the village of Kinding. Deutsche Bahn wanted to ensure that emergency exits were strictly accessible at 1000m intervals and that meant extending some of the construction adits after the initial $292M contract had been let in 1998. Adits will become maintenance and emergency access points in the permanent tunnel.

Not only were new bores needed but also existing access bores, pre-made before the contract, had to be widened. “Tunnels previously designed for pedestrian escape with cross sections between 13m² and 15m² were widened to 28-32m² cross section to allow use of vehicles” says Hansen.

Similar problems to the main tunnels were encountered. Geology of the contract area, well known as a beauty and holiday spot, is so-called “black Jura” giving way to first “brown Jura” and then “white Jura”. Sandstones and then limestones primarily, are forced up into relatively steep hills, enough to make the A9 north-south motorway struggle as its passes through. Even with tunnels the railway will face up to 2% gradients.

For the northern Euerwang, the ground to be excavated is therefore mainly Lias sandstone which has mixed clayey zones in a very heterogeneous formation. Heading south the tunnel meets an impermeable compact clay zone about 2km from the southern end. The remaining tunnel is in Oxford limestone and marl.

The same Oxford limestone is encountered in the short Schellenberg and then in the first part of the Irlahüll before giving way to Kimmeredge strata of limestone and marl interbedding. Beyond that is dolomite limestone in which the heavy karstification has been found, with voids often filled with tertiary clay material particularly towards the southern end of the tunnel.

However, the first construction problems were encountered at the northern portal of the Euerwang. It was known there was a loose rock layer to get through here, expected to be about 50m thick, it proved to be more than that.

Hochtief used jet grouted pile support at the portal and then a jet grouted forepoling “umbrella” to get through; the work took some six months longer than anticipated. “This in turn threw the logistics out” Hansen adds. On both long tunnels intermediate faces were being used, starting from halfway points already accessed by the construction adits. In the event, the northern drive in the Euerwang was obliged to come much further towards the portal than expected.

“We had other problems on the Euerwang” says Hansen “The ground was much wetter than we thought and this created difficulties with the mucking out, primarily in the Lias sandstone sector.”

The mixed sandstone and clay churned up badly in the wet and it was necessary to import substantial material to form a temporary road bed for trucks, he says. Kiruna and Zeppelin trucks were used in the main. “It was also difficult to identify the exact point of water ingress which was diffuse. We did install drains but it was a problem” he continues.

For both tunnels the contractor has been using a double eleven hour shift system, with a third reserve shift. Including side tunnels up to twelve faces were running at key moments.

Excavation has been done by tunnel excavators, in most cases using Liebherr machines, or drill and blast in the harder rocks using primarily Atlas Copco jumbos. Most drill and blast was in the southern Irlahüll tunnel, using Eurodyn 2000 slurry and Dynamit Nobel Supercord detonators. Up to 3m rounds were allowed for, though most were between 1.2m and 1.5m. A maximum performance was 5-6 rounds daily.

The big tunnels were worked with a three level excavation, arch, bench and base and support is via an initial shotcrete layer with mesh and steel arches followed by rockbolting and second shotcrete layer to create an overall 350mm outer shell. Tamrock drills did most of the bolting and Meyco machines most of the spray concrete.

Arch length was limited to a 300m advance on the bench but Hochtief requested and got permission to press on with the arch when the rock deflections were not too great. “The reasons were mainly logistical” says Hansen “because follow on benching and invert work could seriously disrupt the arch operations.” Early breakthrough also allowed a freer flow of muck out wagons and helped establish a natural ventilation flow, reducing the need for the 2.6m diameter vent ducts used, which filled a substantial portion of the arch drive.

The rock classes used tended partly to fall into heavier categories than expected, says Hansen. Apart from variations in support the tunnel profile also changed according to rock conditions.

Tunnel invert shape varies according to tunnel wall pressures with a flat base in lower pressure drained sections and three levels of curvature for the invert in more difficult areas, the maximum being 3.15m deep.

“The flat invert is used on drained lengths of the tunnel, we put in an umbrella membrane around the arch leading water to drainage in the tunnel floor.”

Where lateral loads are higher these drained sections also have a curved invert” says Hansen and at higher water pressures the tunnel must be watertight with a full membrane lining installed between the inner and outer concrete. “If the groundwater level is above 30m then the tunnel shell cannot deal with the pressures”

Negotiations are still continuing in the southern Euerwang tunnel over use of curved invert. Hochtief would like to see the remainder of the tunnel finished without any further flat base, believing the curved invert to provide greater security for the track bed.

The tunnel has seen some deterioration in the rock floor surface since excavation. It now looks as if another up to 500mm layer of rock will need to be removed in the unfinished sections of tunnel floor, and the void filled with sound material. But that will mean a deeper drainage filter layer, says Hansen, which will mean the side drains will need to go deeper. This may then not match the longitudinal drainage slope and water could remain in the drainage layer. “That would then lead to a danger of so called pumping as trains pass along the tunnel, with a potentially damaging pressure wave running under the track slab”

Hochtief must carry out slab and inner lining design and therefore is worried. It feels that Deutsche Bahn should allow changes due to rock conditions.

The issue is yet unresolved partly because the design for the concrete slab track, the so called Festbahn system, has not yet been fixed. There are about a dozen variants of the system and ongoing experiments on these continue.

But the discovery of far greater karstic voiding than expected is the major hold-up. Up to 40m high chimneys and chambers caused big problems during excavation, with the need for spiling and additional support and often the need for back filling. “We got through the biggest by closing with wire and shotcrete, backfilling with selected material, or closing with pumped concrete” says Hansen.

But the biggest problem is the voids beneath the track bed. Rock must be excavated and backfilled though speculation that even bridge structures might be needed is unfounded says Deutsche Bahn.

DB meanwhile has also become worried by the possibility of further hidden voids in tunnel floor, roof and walls which could create future dangers for the 300km/hr trains. On this contract and further south where another karstic tunnel has been encountered it has ordered a comprehensive geotechnical investigation which is ongoing.

Hochtief operations are currently suspended until the spring in three out of four of the tunnel’s working areas.

Meanwhile a big 145t rail mounted formwork has been waiting for more than a year at the southern entrance to the Euerwang tunnel. The purpose built rig from Austrian firm Östu Stettin is one of four to be used for making the arch lining to the tunnel, the last phase of main tunnel work after the invert is cast by the forward section of a concreting train. Hochtief designs the lining according to load parameters supplied by DB’s Austrian consultant ILF from Innsbruck.

Corrections for variations in the profiles of the rock face and for the subsequent shotcrete shell are precisely established with a new digital photogrammetric survey system used for the first time in Germany on this project. It gives a 10mm accuracy measurement of the tunnel wall surface allowing very precise design tuning.

Where underbreak is too great the tunnel is cut back to profile, and elsewhere additional shotcrete might be required. “We are allowed, exceptionally, a 700m thickness for the inner lining” says Hansen.

Much of this work remains however. Hochtief’s original schedule was to finish in March next year but will now extend to at least mid-2004. Trains will finally run in autumn 2005.

Cost of the section is yet unknown and the client and Hochtief are currently in discussion.

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