Shani Wallis, TunnelTalk

Last week (early January 2013), TunnelTalk published an obituary in memory of Siegmund Babendererde who was instrumental in the advance of slurry TBM tunnelling and the development of the innovative extruded concrete concept of primary lining behind a TBM as used on the Lyon Metro project in France in the mid-1980s. In the text we published - incorrectly - that failure of the extruded lining required modification of the TBM and conversion to a segmental lining to complete the twin tube running tunnel under the Rhône and Saône Rivers. In fact, and for the record, the extruded lining and the Hydroshield TBM system did complete the project although many severe situations were encountered and overcome for both. These were described by Babendererde at a presentation he gave to the British Tunnelling Society (BTS) in London in 1986. The report of the presentation (as published in the BTS magazine T&TI) is here summarised and the main highlights of the experience plus details of the extruded lining concept are detailed.

Twin-tube metro alignment in Lyon

The Société D'Economie Mixte du Métropolitain de l'Agglomération Lyonnaise (SEMALY) awarded a 265 million French Franc contract in the early 1980s to the joint venture of Dragages/Campenon Bernard/Hochtief/Colas to drive twin 1,250m long x 6.5m o.d. running tunnels under the Rhône and Saône Rivers for the Lyon Metro Line D. To complete the work, the JV adopted the Hydroshield and extruded concrete primary lining concept, two techniques promoted and developed by JV partner Hochtief of Germany.

The geological conditions on the alignment, with reaches of 350m and 100m under the Rhône and Saône respectively, comprised mostly loosely-compacted quaternary deposits of coarse gravel and large stones with outcrops of decomposed granite. A stone crusher in the slurry Hydroshield reduced stones of up to 400mm to a maximum particle size of 120mm, with larger boulders requiring removal by man-entry into the compressed-air excavation chamber. Excavated material was transported in the bentonite slurry medium at 1,000m³/hr in the 300mm diameter pipework to the surface separation plant, which removed solids to 40µm before recycling back to the excavation chamber.

As well as application of the relatively new Hydroshield technology, SEMALY, which had left decisions about the proposed line, level and construction method of the new metro extension to the contract tenderers, further permitted application of the extruded lining concept by the selected JV.

Extruded concrete lining concept

The lining innovation was first used and proven on a contract for the Frankfurt Metro system through Frankfurt's clay deposits. In design in Lyon, the initial support of the tunnel comprised a 300mm thick extrusion of steel-fibre reinforced concrete pumped via six injection points through the annular stop end of the 6.5m o.d. tail shield and into a set of 12 x 1.2m long full round telescopic shutters that were struck and moved forward on a traveller as the tunnel heading progressed. Hydraulic jacks maintained pressure on the stop end and provided thrust to propel the TBM forward.

After completing the TBM drives, supported with their initial primary extruded lining, a final lining with a plastic waterproofing membrane and a 300mm thickness of plain in-situ concrete was cast.

Fibre-reinforced concrete
The specification for the B35 concrete mix comprised 420kg/m³ of cement, 120kg/m³ of flyash and 50kg/m³ of steel fibre reinforcement. The Dramix fibres, supplied by Bekaert, were 0.6mm diameter and 40mm long. The concrete was mixed on the trailing back up and pumped 50-60m into the shutter. The concrete was required to achieve a strength of 12N/mm² within 12hr to allow for a strike and reset of the last shutter. Only rounded aggregate, a high content of fines and admixtures to improve flow and delay the set were essential for the success of the lining method. Excavation and extruded concreting could be either concurrent or independent of each, the shield's telescoping section permitting a 1.4m relative movement between the forward shield can and tailskin sections.

Taking the place of annular grouting and rings of precast concrete segments, the extruded lining was said to provide excellent control of settlement at the tail shield end of the machine. Settlements of less than 1mm were said to have been recorded under the land sections of the Lyon drives. Heave above the extruded lining operation was also a potential result and was controlled at a 3mm maximum.

The extruded lining is placed under excess pressure and is bedded in the surrounding ground. Subsequent loading from soil and water pressure produces slight deformation. The result of crown bedding in a comparison with conventional lining is shown with the bedding modulus method.

Some circular shrinkage cracks were reported in the primary extruded lining and while the membrane and secondary inner lining provided watertightness on the Lyon project, it was noted that a single extruded lining was sufficient on the Frankfurt prototype project.

The Hydroshield started at a launch shaft close to the right bank of the longer 350m drive under the Rhône River and experienced serious difficulties in the learning curve phase. At the face, the TBM ran into buried timber piles of an ancient bridge. Some had metal shoes and were being removed via man-entry into the pressurized excavation chamber until it was realised that the Hydroshield could cut through them.

The heading then hit a pocket of large boulders and suffered four blowouts and complete loss of the bentonite slurry and face support. Sand, gravel and flyash was dumped from barges into the scour holes. This was then grouted to give the made-ground a nominal strength into which excavation could resume.

Air loss through the loose gravels during man-entry interventions under the rivers was also high. Air loss of 105m³/min at a maximum air pressure of 2.5 bar was reported. This was countered by vibro-compaction of the riverbed soils. Shutters on the openings of the relatively open cutterhead provided additional protection for man-entries. Five air compressors with a total rated capacity of 160m³/min were mounted on the trailing backup to control air-bubble of the Hydroshield and create the compressed air chamber for man-entry interventions.

At the tail end of the shield, there was trouble with the extruded lining into the loose water-saturated sand and gravel deposits. Striking the shutters was pulling away concrete and revealing bare patches through which volumes of groundwater flowed. Ground freezing was applied to stabilise conditions until crews mastered the operation and the routines were established.

Cover above the shield, under the land and riverbed reaches, was said to be in places less than a tunnel diameter and as small as 2.5m at a minimum. Such risks would not be contemplated or accepted in today's contracting and construction environment. Taking the drives deeper under the rivers and beneath the urban fabric of the city was not possible. Gradients of 3.7% maximum were optimised to tie into open cut running tunnels either side. It was these and the maximum gradients for metro train operations that dictated the depth of the bored tunnels.

Extruded concrete applications to the mid-1980s

While the slurry TBM concept - as originally developed by experiments in the UK, early work in Japan, and by the contractor Wayss & Freytag in Germany and machine manufacturer Bade & Theelen - went on to find world-wide application, particularly after further development and refinement as the Mixshield technology by Herrenknecht, the extruded lining concept has been left behind. The original patents filed by Hochtief will have long since lapsed and while there was positive interest shown by the Japanese through a technical licensing agreement between Hochtief and Teken in Japan, the number of extruded lining applications remains limited to a few project examples.

According to Ulrich Wadepohl, current head of tunnelling with Hochtief, the extruded lining application in Lyon remains successful. "As far as I know, there are no reported issues of water leaks or structural issues with the Lyon metro tunnels under the rivers," he said.

Seigmund Babendererde was Wadepohl's boss during his time as Director of Tunnelling with Hochtief in the 1980s and told TunnelTalk that he knew well Babendererde's interest in mechanised tunnelling, the slurry TBM method in particular, and the extruded lining development. "The issue for extruded lining is that it cannot be applied with certainty of quality as a single shell lining and the production, design and ring build of precast segments has became so advanced. High specification concrete and improved sealing gaskets now produce watertight one pass segmental linings of exceptional standard and these are less expensive than the primary and secondary passes that would be required for an extruded lining operation."

Another issue, explained Wadapohl, is making watertight the joints in the primary extruded lining before the secondary lining is cast. "Inevitable holdups in the tunnelling and extruded lining operations will lead to cold joints between lining pours and it is very difficult to seal these. Again the one pass segmental lining overcomes these issues and delivers a high quality product."

Original expertise has also been lost and revival of the concept would have to start from scratch and be based on using developments in modern concrete technology to make advances. That would be expensive in the R&D phase before convincing a project client to provide an opportunity to apply a prototype. As always, the industry today benefits from the innovations, opportunities and major risks taken by those in earlier times.

References

Remembering Seigmund Babendererde - TunnelTalk, January 2013

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