Self-building immersed tube concept

NEW PRODUCTS and INNOVATIONS
Self-building immersed tube concept Nov 2009

Shani Wallis, TunnelTalk

Combining immersed tube tunnelling with soft ground TBM technology has created a concept that is more exceptional than the sum of the two parts. TunnelTalk interviewed the engineers involved to explore the potential of this promising new idea.

Mechanised construction of immersed tube tunnels across busy waterways - immediately the obvious advantages spring to mind. No floating elements; no interruption of shipping; no dry-dock; no divers; no delays due to bad weather. Why didn't anyone think of it before?

The answer for the engineers involved is that there has been a natural progression. Immersed tubes, the long established alternative to bridges across shipping lanes, have fallen out of favour of late, principally for environmental, ecological and social reasons. As well as costly disruption of commercial shipping, there is large-scale dredging of river or estuary beds, with the risk of disturbing in some cases potentially contaminated material, and the need for major earthworks on either bank as well as construction of a dry dock in which to cast the elements.

To overcome these drawbacks, bored TBM tunnels have become increasingly the preferred tunnelled alternative. But bored tunnels have their drawbacks as well. The tunnels are necessarily deeper and longer and are consequently more expensive with portals set well back from the banks.

With the pros and cons of both tunnel methods now well documented, the time was ripe for invention of a method that draws together the best of both concepts, offers all the advantages, none of the disadvantages and promises substantial time and cost savings as well.

Graphics of the TIMBY tunnelling concept in operation

TIMBY

The new concept, dubbed TIMBY (Tunnel IMmersed by BouYgues), is the brainchild of Pierre Longchamp, a leading design engineer with Bouygues Traveaux Publics, the giant construction company of France, and in collaboration with Herrenknecht of Germany for development of the mechanical systems.

"We began thinking about the idea about three years ago," said Longchamp. "We had no particular project in mind but we were well aware of the concerns clients have with the two existing tunnel-method alternatives for waterway crossings." Since then Bouygyes and Herrenknecht in partnership have developed a concept that is protected by an international patent and have started its promotion.

"There is nothing exceptionally new about the concept", explained Longchamp. "It uses systems that already exist but combines them in a way that is different."

In short, the concept is based on using a pressurised TBM to build a segmentally lined tunnel across the bed or a river or waterway. The TBM prepares the trench ahead of the alignment and erects the tunnel segments in the tailskin for release into an environment that is almost entirely water. "Compensating for the absence of external forces, overcoming significant buoyancy forces, and maintaining the integrity of the precast segmental lining until the finished tunnel is backfilled and protected are the main design criteria that needed to be solved", said Werner Burger, the project's lead design engineer with Herrenknecht.

Graphics of the TIMBY tunnelling concept in operation

Prototype design

The patent prototype design is for construction of a four-lane highway tunnel and adopts the Double-O-Tube (DOT) cross section. This optimises the space needed for two traffic lanes in each direction but the concept can be as easily applied to circular or elliptical tunnel sections.

The TBM system comprises a machine of about 10m long with a forward excavating shield; a middle forward thrust shield; and a rear tail shield in which the segment rings are built. The segmental lining design is based on existing soft ground principals but adapting them to this particular application. The rings are about 1.5m wide x 90cm thick and the segments have a second sealing gasket at the extrados to prevent water circulating along the joint faces. The tail shield is fitted with a series of conventional wire brush seals and a pressure bulkhead separates the exterior excavation chamber from the free air interior of the tunnel and working area of the machine.

The prototype concept is fitted with two roadheader- or dredger-type cutter booms that are activated by hydraulic cylinders to move forwards and backwards as well as side to side. Although it could become available as a future development, the current concept is that the TBM does not excavate the trench. "It is a development that many have suggested", said Longchamp, "and that maybe possible in future when working in soft mud materials, but it was considered easier and more economical at this stage to excavate the tunnel trench using conventional dredging equipment. In addition, the amount of dredging, when using TIMBY, is less than that required for a conventional rectangular immersed tube tunnel."

To prepare the bed of the pre-dredged trench, the twin cutting booms move excess material to the sides or it is moved if necessary via a short open circuit suction line and deposited back to the riverbed to the side of the machine near the tailskin area. "It is envisaged that there will be no extraction of material through the tunnel and to the surface for onward disposal, which is another major advantage", said Burger.

At the leading edge of the TBM is a partial wall that will support a compressed air bubble in the upper section of the excavation chamber. This can be used to buoy the front of the machine to correct nose-dive and also allow for man-entry via a man-lock attached to the bulkhead. The cutter booms are raised into the compressed air bubble for maintenance and tool changes while behind the bulkhead, the machine's working area and the advancing tunnel remains in free-air conditions. Trailing gantries carry all logistic supplies and ancillary equipment as well as the machine operators' cabin, much the same as for a conventional TBM operation.

Graphics of the TIMBY tunnelling concept in operation

Construction cycle

A TIMBY operation begins and ends in the approach ramp excavations on either bank of the waterway. The machine is assembled in the cofferdam of an approach ramp and launched by breeching and flooding the cofferdam.

In principle, operation is the same as current soft ground TBM advance. The cutter booms level the pre-dredged trench ahead of the machine, thrust rams pushing off the segmental lining to move the machine forward. At the end of the stroke, excavation stops and the next ring of segments is installed. For the DOT prototype cross section ring build starts with the invert Y segment and uses a twin set of erectors to install the ring segments, finishing with the upper Y piece and finally the centre wall segment. Services are then extended, the back-up is pulled forward, and the cycle
beings again.

Exceptions to soft ground TBM tunnelling centre on the need to compensate for the fact that there are few counteracting external forces and substantial uplift buoyancy forces. When fully submerged, the machine and the concrete segments weigh just 10% of what they do in air. Overcoming these buoyancy forces is managed by several interrelated systems again drawing on systems and techniques that are already well-established civil engineering practice.

At the start, when the machine and lining system remain only semi-submerged and the water pressures and forward thrust counter forces are low, the machine must be tied back to the tunnel walls to ensure the minimum thrust force needed for steering and ring build stability. This is achieved explained by Longchamp "by means of tensioned cables and jacks diverted from 'heavy lifting system' between the machine and the anchors in the side walls".

Within the shield there are large ballast tanks to counteract uplift forces and can be operated individually to effectively shift the centre of gravity and assist machine steering and alignment control. The machine also has a ballast wagon that sits on top of the last three rings that leave the tailskin and stabilises the three-ring set until the permanent counterweight provided by infilling the invert is installed.

Invert infill also progresses on a three-ring set. As the last ring of the set is completed, a precast element is installed to create the end wall of the fill section. Fill material is transported in from the working shaft and placed in layers by a lateral conveyor, each layer being compacted as it is placed. As each three-ring invert section is filled, the support rails are installed, the backup is pulled forward, and the next three-ring cycle is ready to begin. Permanent drainage pipes and other utility conduits can be installed as part of the infill process.

Graphics of the TIMBY tunnelling concept in operation

Pre-stressed lining

Integrity of the segmental lining in its fluid environment relies on a system of pre-stressing in both the longitudinal and the circumferential directions.

Longitudinally, the rings are linked using consecutive steel bars. Each bar extends fully through the ring length and is connected to the temporary anchoring of the last bar recessed in the segment of the last ring erected. Once the bars of the new ring are inserted and attached they are tensioned using a normal tensioning jack.

In the circumferential direction, joints are secured using hydraulically tensioned steel cables. After completing a ring build and the longitudinal steel bars are in place, the four circumferential steel cables (two per ring for the two tubes of the DOT section) are threaded into place from the invert Y to the crown Y using an auxiliary tow cable. The steel cables (prepared for each ring on the TBM's gantries) form an open buckle between the invert and crown Y segments and are tensioned using hydraulic cylinders. As the completed ring leaves the tailskin and the wire brush seal, the annulus at the invert is backfilled via tailskin grout lines to provide a sound structural foundation.

Graphics of the TIMBY tunnelling concept in operation

Self contained system

Apart from the initial dredging (which is much reduced for a TIMBY operation), and the final backfilling of the structure, there is no further need for external work. The tunnel is built completely from within the machine and the extending tunnel. By launching and receiving the tunnelling system from within the cofferdams created to complete the tunnel landfalls, disturbance of the river or waterway banks is minimised. There is no interruption of waterway traffic. For a crossing of busy international shipping lanes this is a major commercial consideration.

Once the TIMBY tunnel is completed and the structure is covered with backfill and protection layers, the facility behaves as a conventional immersed tube with the prospect of a better maintenance record for the gasketed joints of the segmental lining compared to the large construction joints and the gasketed joints between sunken elements. The elimination of a dry-dock for casting immersed tube elements (frequently in more than one casting operation) is a major cost and environmental saving.

The self-contained nature of the TIMBY concept, using familiar soft ground shield tunnelling processes and eliminating the need for divers, tugs and steel guy ropes as part of the construction phase, provides significant safety advantages and removes some of the greatest risk hazards of conventional immersed tube construction. Only under extreme weather conditions would work be suspended for safety reasons. "This is also often the case for large sub-aqueous TBM drives under relatively shallow cover to the river bed and was the case for the 4th Elbe Road Tunnel in Hamburg", said Burger.

Plan of a conventional immersed tube tunnel crossing and a TIMBY alternative.

Cost comparison

With the first TIMBY tunnel yet to be built, cost comparisons are theoretical but still nonetheless telling. Data provided in promotional material and explained by Longchamp compares TIMBY with the conventional immersed tube tunnel recently built by a Bouygues-led JV at Rostock in northern Germany.

"Had the TIMBY concept been available at the time, it would have been a viable alternative for the four lane traffic crossing", said Longchamp. "The Warnow River at the crossing is about 700m wide and is about 12m deep at the deepest point. The comparison is quite easy."

Under a design-build contract, the immersed tube tunnel of two lanes in each direction comprises six elements of some 180m² in external cross section to span the middle 780m between of the open cut and cut-and-cover ramp sections built within a total 46,400m² of slurry walls. The project also required construction of a 220,000m³ temporary dry-dock in one of the banks that was used twice to cast three elements in each phase.

An alternative TIMBY application would have reduced trench and earthworks excavation by at least 65% (no dry-dock and significantly shorter open cut approach ramps). The 201m² external dimension of the DOT cross-section of the four-lane segmentally lined TIMBY tunnel extends for a shallower 1025m across the riverbed between smaller landfall cofferdams.

Given the various time and cost savings (excluding an estimate for the social and environmental cost advantages, which could be considerable) "a TIMBY alternative for the Rostock Tunnel would have saved 12-13% on construction cost and the benefit of an anticipated project opening of at least six months earlier", said Longchamp, over the conventional €135 million, 40-month design-build immersed tube tunnel option. "And as a bonus, at the end of the project, the TIMBY machine is available for application on a similar TIMBY project."

In recent months, Longchamp and his Herrenknecht colleague Werner Burger have promoted the concept at international conferences and have given private seminars to construction companies and to owners of potentially viable TIMBY projects. These projects have included the Port of Miami traffic tunnel in Florida where Bouygyes is leading the PPP concession selected to build the project and the Tyne Tunnel in the UK. For the Miami project TIMBY is too new. Progress is already "far into the design and construction process of the selected TBM bored tunnel option". The Tyne Tunnel, at about 400m long, "is a bit too short for application of a TIMBY operation. The trench for the proposed immersed tube option will be four to five times the volume of the tunnel".

While these projects have missed being the initial application of this innovative method, the future is very encouraging for TIMBY. Reports of a prototype application as the selected preferred alternative to an immersed tube or a sub-aqueous TBM bored tunnel cannot be far off

References

Miami Port Tunnel finally a project! - TunnelTalk, Oct 2009
Tyne immersed tube highway tunnel, UK - TunnelCast, Nov 2009

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