For engineering achievements in particular: boundaries are motivation and never a limit.

Bigger, faster, deeper, further – and at the same time always safer. That’s the idea for mechanised tunnelling. The real engineering challenges await deep down in particular.

In geologies under extreme pressures and where ground, sea or river water penetrating through fissures and other anomalies affect tunnelling. The example of a water intake tunnel under Lake Mead shows how the geotechnologically difficult excavation of such tunnels can be accomplished.

Like a blue diamond, the largest reservoir in the United States lies about 50km southeast of Las Vegas, in the middle of the desert between Nevada and Arizona. Here the Hoover Dam, completed in 1935, dams the Colorado River: over a length of 170km and with a depth of up to 150m (the dam’s high water line is at 375m aod). The maximum storage capacity is an almost unimaginable 35 billion cubic meters of water – enough to supply a West European country’s private residences for over ten years.

But Lake Mead is no longer full to the brim. Since 1998 its level has constantly dropped – as a result of a previously unprecedented drought phase. Meanwhile its water level is at an historic low: only 332m above sea level. This means the water stands just a few meters above the two existing intakes – and threatens the water supply.

Third outlet to secure water supply
A new, third outlet must be built. Intake No. 3 is approximately 70m below the lake surface and about 3km from the shore. The intake structure extending 15m vertically from the lake bed was lowered from a floating barge into a previously excavated pit. The foundation was then poured with underwater tremie concrete, fixing the structure in place.

The actual Intake No. 3 consists of a 4.4km long, slightly ascending tunnel that the Herrenknecht TBM S-502 with an outer diameter of 7.2m excavated directly below the lake. The TBM was driven with an accuracy of 10mm into the specially constructed "soft eye" of the intake structure.

Before that, the specially adapted Multi-mode TBM had driven for about three years whilst it worked its way through complex geologies with shattered rock and clay partly filled with water from the lake.

New record
Due to the working depth – under water pressure rises one bar every 10m – over large parts of the tunnelling route an enormous water pressure of up to 15 bars acted on the machine. This was an absolute first for mechanized tunneling requiring significant innovation and development. Until then the record was 11 bar, set by a Herrenknecht TBM used to excavate the Hallandsas railway tunnel between Gothenburg and Malmo, completed in 2013.

The geological and hydrological conditions were very challenging to the construction team of Salini-Impregilo. On numerous occasions tunnelling had to be stopped and parts replaced. The abrasive rock under Lake Mead had destroyed the center disc cutters and parts of the cutterhead. Also the bearing seals were considerably affected by the high pressure and had to be replaced.

The search for the right solution
Everyone involved had asked themselves the right question long before the project began: how must a TBM be designed so it can constantly withstand such high, previously unmanageable pressures? On the one hand, by including more steel and making the walls thicker the TBM could be designed to withstand the 15 bar water pressure, which equates to 15 tonnes of water bearing down on every square metre of the shield – with a total length of 16m and a diameter of more than 7m that adds up to a huge load. Secondly, with seals that are robustly designed, such as on the main bearing and the tailskin. Furthermore, it must be ensured that even under the extreme pressure conditions both routine work such as cutter changes and unscheduled maintenance can be performed.

Based on the information gathered, Salini-Impregilo decided to use a Multi-mode TBM from Herrenknecht. In good, stable formations it worked in so-called open mode. Here the rock, broken into palm-sized chips by the cutterhead’s disc cutters, is mechanically removed from the working area. That happens quickly and is efficient. The S-502 makes "way": 40mm or 50mm per minute. At times it ate its way forward by more than 100m a week through rough terrain.

Not everything goes according to plan
Only about 40 per cent of the route could be driven at speed in open mode instead of the planned 70 per cent. The geology and the water inflow at the tunnel face made it necessary that the majority of the distance was completed in the time-consuming closed slurry mode. Here a liquid medium under pressure – usually a bentonite suspension – stabilises the ground at the tunnel face and balances water pressure in the fissures. Together with the suspension the excavated material is pumped out of the working chamber via a slurry circuit. In this way even fluctuating pressure conditions can be controlled very precisely.

The change from open to closed mode has to be quick because of potential high water flows at high pressure. The specification for the Lake Mead TBM says the machine must be able to be sealed within 120 seconds. To do this the main chamber is locked by closing the rear discharge gate of the screw conveyor.

Challenging chamber intervention
Closed, safe, easy going? No way. Because even if the TBM digs in the secured slurry mode, cutterhead and cutting tools require regular inspection and maintenance. Various monitoring systems collect important tunnelling parameters in real time via sensors and record them. This data serves as a basis for the machine operator to decide when chamber interventions are necessary. Data analysis is only the first step.

Actual replacement of the disc cutters, scrapers and buckets, however, is exhausting, time-consuming manual work.

Atmospheric cutter change
When tunnelling under high pressure the concept of accessible cutterhead arms has proven itself. The special design feature was first used successfully at 4.5 bar during construction of the 4th Elbe River Tunnel in Hamburg with a Mixshield in 1998. In TBMs with a diameter =10m the cutterhead arms can be formed as accessible hollow boxes. Under atmospheric pressure they are then accessible, worn or defective tools can be replaced relatively easily through the rear area of the cutterhead. Over the past two decades Herrenknecht has continuously developed this principle further and adapted it for significantly higher pressures.

Intuition needed during tunneling
Due to the cramped conditions, TBM diameters of less than 10m do not allow accessible cutterhead arms to be included in the design – as at Lake Mead, for example. Atmospheric chamber interventions are not possible. In this case cutter changes or maintenance work can only be carried out in socalled "safe havens". They allow safe access to the excavation chamber. Encountering such a natural, stable zone along a tunnel alignment, however, is a happy coincidence. It is not the rule. Here the experience and intuition of all project partners is called for: do you take the risk and continue tunnelling a certain distance further in the hope of reaching a safe zone soon? Or are the cutters so worn that you have to act immediately? Safe havens can also be created artificially, for example by means of pre-excavation ground improvement with drilling rigs on the TBM or from aboveground. This is very time- and cost-consuming, however, and not always possible.

In the worst case you turn to the fallback solution: you send divers into the pressure area of the TBM. First experiences with this method were also gained during construction of the 4th Elbe River Tunnel in Hamburg.

There the bucket supports needed to be rewelded and the buckets themselves replaced. The operation took six weeks – at pressures of up to 4.5 bar and thus in pressure ranges divers can only under exceptional circumstanced enter with "normal" compressed air. At depths such as under Lake Mead and a pressure of up to 15 bar, that no longer works.

Here you have to draw on experience from the "offshore sector". Saturation diving is the magic word. It makes use of the fact that under high pressure the gas intake of the human organism is eventually limited (saturated) – and hence decompression times have a natural, manageable limit.

Prepared for all eventualities
At the Lake Mead project, jobsite and machine were ideally prepared for saturation diving up to 15 bar. For this a seamless positive pressure transport route was designed and implemented.

This leads from the (pressurized) living chamber in the area of the launch shaft, in which the divers sometimes live for weeks, to the pressure lock in the front shield area of the TBM.

For a deployment the transfer shuttle must be transported through the entire back-up of the machine. Special design considerations are necessary for this so enough space for the shuttle remains open in the center.

Only in this way can the quick and above all completely safe entry of the professional saturation divers into the excavation chamber be enabled. In normal operation, on the other hand, these facilities must cause minimal interference to the tunnelling process. In the end, the complicated and time consuming use of saturation diving was fortunately not needed during tunnelling under Lake Mead. Nevertheless, in such difficult pioneering projects at the limits of technical feasibility, in addition to plan A you always need to have a plan B or even plan C in your pocket.

In future it is therefore very likely that all TBMs deep below the earth’s surface will be equipped with such technology. Tunnelling depths of 200m are no longer a fantasy. In Turkey, saturation divers are also the fallback solution: a 13.60m diameter Herrenknecht TBM is deployed to mine a tunnel directly under the Bosphorus and all possible facilities for chamber interventions are on board.

At its deepest point the 5.4km-long road tunnel of the "Istanbul Strait Road Tube Crossing Project" is about 100m below the water level.