Technical progress
for more efficient infrastructures



Pioneering underground.

Latest tunnelling technology
for groundbreaking missions.

Technical progress is driving tunnel construction forward. In partnership with clients, planners and construction companies, Herrenknecht is developing tunnelling technology to produce safe, reliably high quality and very long-lasting tunnel structures even in new, highly complex terrain. Here you get insights into high-profile pioneering projects and their technology that exemplify profound progress in tunnel construction.


Extreme tunnelling.

Maximum safety
under maximum pressure.



Extreme tunnelling.
Maximum safety
under maximum pressure.

________________ A large road tunnel deep beneath a strait, a railway tunnel through an enormously complex mountain range, a water tunnel under tremendous ambient pressures: mechanized tunnelling is penetrating into new terrain underground. True partnership with contractors and project owners leads to pioneering developments in machine technology. Integrating knowledge from professional offshore diving also contributes to significant progress in tunnel construction.

"That can't be done!" – This sentence is only true until the contrary is proved. 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 motto in mechanized tunnelling.

Tunnelling at the limit: in pioneering projects such as the Bosphorus crossing in Istanbul, safety for man and machine plays a crucial role.



Above: The Lake Mead reservoir near Las Vegas is fed by the Colorado River and is the main water source of the gambling capital.

Below: Mineral deposits on the shores document the historically low water level.

Pushing the boundaries of the possible

________________ 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 50 kilometers 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 170 kilometers and with a depth of up to 150 meters. The maximum storage capacity is an almost unimaginable 35 billion cubic meters of water – enough to supply Germany’s private households for about 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 332 meters 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. The planned "Intake No. 3" is approximately 70 meters below the lake surface and about 3 kilometers from the shore. The intake structure extending 15 meters 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 new Intake No. 3 is 3 kilometers from the shore and about 70 meters below the lake surface.

The actual "Intake No. 3" consists of a 4.4 kilometer long, slightly ascending tunnel that the Herrenknecht TBM S-502 with an outer diameter of 7.2 meters excavated directly below the lake. Precisely to the centimeter it penetrated the specially made entry into the “soft eye” of the intake structure. Before that, for about three years the specially adapted Multi-mode TBM had worked its way through complex geologies with shattered rock and clay partly filled with water from the lake. 



After around 3 years of tunnelling, in late 2014 the Herrenknecht Multi-mode TBM drove exactly through the entry construction of the new intake.

New record for water pressure

________________ Due to the working depth – under water pressure rises one bar every ten meters – over large parts of the tunnelling route an enormous water pressure of up to 15 bar weighed down on the machine: an absolute innovation in mechanized tunnelling. Until then the record was 11 bar in places, set by a Herrenknecht TBM used to excavate the Hallandsås railway tunnel between Gothenburg and Malmö, 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 had ground off the disc cutters and parts of the cutterhead. Also the bearing seals were considerably affected by the high pressure and had to be replaced. 



The abrasive geology in conjunction with the high water pressures took steel and seals to the absolute limit of their endurance.



Above: Due to the changing ground conditions along the tunnel route, the contractor opted for a Multi-mode machine from Herrenknecht.

Below: Depiction of the two tunnelling modes open (with horizontal screw conveyor) and closed (with slurry circuit).

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. After all, at 15 bar water pressure, 15 kilograms of pressure weigh down on every square centimeter of the shield – with its total length of 16 meters and a diameter of more than seven meters 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": four or five centimeters per minute. At times it ate its way forward by more than 100 meters a week through rough terrain.


Not everything goes according to plan

________________ Only about 40 percent of the route could be driven at speed in open mode, however – instead of the planned 70 percent. The geology and the water inflow at the tunnel face made it necessary that the majority of the distance was completed in time-consuming and wear-intensive closed slurry mode. Here a liquid medium under pressure – usually a bentonite suspension – stabilizes the ground at the tunnel face. 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 tunnel boring machine digs in secured slurry mode, cutterhead and cutting tools require regular inspection and maintenance. Various monitoring systems collect all 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. But data analysis is only the first step. The actual replacing of the disc cutters, scrapers and buckets, however, is exhausting, time-consuming manual work.

During assembly in Germany the further developed lock systems for the disc cutters were extensively tested.




Starting at a diameter of about 10 meters, the cutterhead arms can be made accessible for atmospheric cutter change.

Atmospheric cutter change with larger diameters

________________ 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 tunnel boring machines with a diameter ≥ ten meters 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.



Above: Maintenance work on the front of the cutterhead of the Lake Mead TBM in extremely cramped conditions in the shelter of a “safe haven”.

Below: With the help of drilling rigs, grouting can be used to create artificial maintenance zones along the tunnel alignment. This method cannot always be used, however.

Intuition needed during tunnelling

________________ Due to the cramped conditions, TBM diameters of less than ten meters 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 so-called “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.


Saturation diving as a last option

________________ 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 exceptionally still 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.


A look inside a transfer shuttle. Depending on the task, saturation divers spend up to several weeks at a time in positive pressure.



Saturation diving is an extremely complex process: from the living container (above) to the transfer shuttle to the TBM (below), everywhere project-specific special designs are necessary to maximize the safety of the divers. 

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 tunnel boring machines underway deep below the earth’s surface will be equipped with such technology reminiscent of space travel. Tunnelling depths of 200 meters are no longer a fantasy. On the dividing line between East and West, saturation divers are also the fallback solution: directly under the Bosphorus a 13.60 meter diameter Herrenknecht TBM is currently eating its way through the seabed between Europe and Asia – all possible facilities for chamber interventions are on board. At its deepest point the 5.4 kilometer long road tunnel of the "Istanbul Strait Road Tube Crossing Project" is about 100 meters below the water level.


Pioneer technology.

New solutions
for special fields of application.




HDD Downhole Tools

One step to the final diameter.
Innovative tools for creating
HDD boreholes.

AboveThe combination of Full Face Hole Opener and Down Hole Jet Pump gives drilling companies decisive advantages for increasing the efficiency of HDD projects.

















The Hole Opener is modular in design, so different cutter types can be used depending on the ground conditions.

The Hole Opener has already successfully demonstrated its advantages in several reference deployments in HDD projects in the USA.

Horizontal Directional Drilling is one of the most common methods for laying pipelines underground. The cutting tools used, known as downhole tools, play a key role in the success of the project. For years the market has been waiting for new ideas here from industry. Herrenknecht has therefore collaborated with recognized HDD specialists to develop innovative downhole tools. With these, crossings can be carried out significantly more quickly and economically.




In every HDD project a pilot hole is first created. Here, in particular with longer drilling distances, the typical process problem of frac-outs frequently occurs. Drilling fluid unintentionally escapes from the mud circulation into the formation or to the surface. This is due primarily to the high pressure level that prevails in the area of the drill bit in the borehole. With the Weeper Subs developed by Herrenknecht this risk is significantly reduced. They increase the volume flow in the borehole gradually. At the same time, the amount of drilling fluid required decreases toward the drill bit. This reduces the annulus pressure level and, in particular, the maximum pressure on the drill bit.

After completion of the pilot hole, this is usually enlarged to the target diameter in multiple passes before the pipeline is inserted. With the new Full Face Hole Opener from Herrenknecht this process can be carried out in a single step. The innovative tool is modular in design. Thanks to replaceable cutters it can be used in various ground conditions and quickly and economically refurbished or modified. The also recently developed Down Hole Jet Pump ideally complements it. It is installed directly behind the Full Face Hole Opener, cleans the borehole and removes the cuttings directly inside the drill string. The borehole is not used as a slurry line. As a result, the simplest drilling fluid can be used even with larger cuttings. Its only function remains the support and sealing of the borehole.

This technical approach is unique in the HDD business. With the combination of Hole Opener and Jet Pump an exactly round, clean hole is created and expensive bentonite can be saved. At the same time, the risk of frac-outs is reduced and crossings can be carried out faster, more safely and more economically. The new downhole tools from Herrenknecht thus offer drilling companies the potential to improve the efficiency of their HDD projects significantly.


Your contact person:

Tobias Gerhardt
Project Manager Research and Development | Utility Tunnelling
Tel. +49 7824 302 7678


Everything always in view.
Efficiently planned maintenance intervals thanks to optical monitoring.

Above: Camera and light sit directly behind the cutterhead and provide images of the excavation chamber. Power, compressed air and water are supplied from the rear.




The recordings of the monitoring camera are visualized directly in the control cabin for the machine operator.

On new machines camera and lights are integrated directly into the steel construction. The system can, however, also be retrofitted on existing machines.

In mechanized tunnelling, wear and situation monitoring plays a crucial role for rapid and safe project completion. This is especially true in Utility Tunnelling projects with diameters ≥ 1.2 meters. Here the limited space in the machine means the wear or maintenance-related entry of personnel into the excavation chamber is often difficult, thus involving costs and delays.

A new camera system recently developed by Herrenknecht provides additional permanent optical wear and situation monitoring to the driver. The special camera, mounted in the excavation chamber, transmits live images directly to the control cabin.


With hydraulic recognition systems the only information provided for individual tools is whether or not they are worn – more precise differentiation is not possible. With optical camera monitoring, on the other hand, the tools can be monitored continuously. The machine operator thus gets a complete picture of the state of the cutterhead and tools in real time. He can plan maintenance intervals and downtime precisely and efficiently. On longer advances of 500 meters and more in particular this is important for the efficiency of the project as a whole. Another advantage is the optical monitoring of the material flow inside the excavation chamber. Thus, for example, the flow rate of the slurry circuit or the various jet systems can be exactly matched to the current situation.

A positive side effect: if entry is required for tool changing or other maintenance the service personal and the tunnel face stability can be constantly monitored from the control cabin, significantly increasing work safety.

Herrenknecht worked closely with external partners to adapt the camera specifically for use underground, with the housing and glass robustly designed accordingly. An integrated cleaning system of compressed air and water jets and additional LED lights ensure a clear picture in the control cabin at all times. An optional video recorder captures the video signal for later evaluation.


Your contact person:

Boris Jung
Project Manager Research and Development | Utility Tunnelling
Tel. +49 7824 302 7919


Clever adaptation of tried-and-tested technology.
SDD rigs for the efficient exploration of unconventional fossil energy resources.

Above: With the newly developed SDD rig the entry angle can be precisely adapted to the specific project requirements. The rig is powered by a diesel-hydraulic power pack and operated from a control cabin as with HDD.





Herrenknecht SDD rigs have already proven their high flexibility in several project operations.

HDD rigs from Herrenknecht have successfully managed drilling lengths of up to 3,000 m in worldwide projects.

Herrenknecht has been manufacturing modern hydraulic HDD rigs for years. This machine technology has now been adapted for the newly-developed SDD method (Slant Directional Drilling). It represents a combination of HDD (Horizontal Directional Drilling) and classic vertical drilling. The new method can be used for exploring so-called unconventional raw materials, e.g. oil sands, or for degassing coal seams.

An important area of application for the innovative machines is the access to oil sands. Eight SDD rigs from Herrenknecht are currently working worldwide using the so-called SAGD method (Steam Assisted Gravity Drainage). For this, two boreholes one above the other are needed. Through the upper hole, steam is forced into the oil sands at pressure through filter pipes. The surrounding oil heats up, becomes more liquid and flows down into the lower pipe. From there, it is pumped away and processed further on the surface.

Another area of application is represented by so-called CSG projects (coal seam gas) where gas is present in deeper coal seams. With SDD rigs, you can quickly drill down to the appropriate depth and then deflect the borehole into the horizontal. You follow the soil layer you are looking for directly in the formation. With the installed rack & pinion drive and taking into consideration the permissible thrust forces, the drill string can be actively pushed forward. This means that considerably longer horizontal deflections can be achieved enabling effective purging of gas from the seams before the coal is extracted by mining.

With Herrenknecht SDD rigs, both methods can be carried out. Drilling companies investing in such a machine are flexible on the market.


Your contact person:

Andreas Steilen
Product Manager Pipeline | Utility Tunnelling
Tel. +49 7824 302 5220


Clever invert lining with
independent back-up system.

TBM advance in road tunnels decoupled from installation of the supply level.

Above: The independent, self-propelled back-up system follows the tunneling TBM at a minimum distance of about 50 m.





When the culvert segments are installed the ramp is temporarily closed for tunnel traffic.

Holding devices for cable ducts are preinstalled in the supply level.

In the mechanized excavation of road tunnels, secondary development of the carriageway level is often carried out immediately behind the TBM. To this end, with the help of culvert segments the preliminary road invert is installed on top of the tunnel lining. These generally rectangular elements form a kind of tunnel within the tunnel. Later e.g. supply lines for the operation of the tunnel or even a metro line underneath the carriageway run through it. Until now, large TBMs were equipped with an integrated back-up segment. From there the installation of the supply level was realized. Advance and invert lining were dependent on one another. If there is a problem with one of the two processes, the other may be affected as well - which can lead to delays.

For a twin-tube road tunnel being constructed in New Zealand for the NZ Transport Agency, Herrenknecht has developed the first self-propelled back-up system independent of the TBM (ISIG - Internal Structure Installation Gantry). With the help of the 95 meter long unit, parallel to the machine advance the miners line the tunnel invert with prefabricated culverts. No matter what happens several hundred meters further back: the EPB Shield can continue boring ahead. The two processes are decoupled.


The back-up system has an integrated ramp. The rubber-tired tunnel vehicles can thus drive directly over the installation area to the tunnel invert in the direction of the TBM and back, e.g. to deliver tunnel lining segments. For the installation of the prefabricated culvert segments, in the center of the back-up system a sliding platform is opened. The system cannot be driven on at this time. A special crane lifts a culvert from the tunnel vehicle and puts it into its final position. After the installation of 4 elements the back-up is hydraulically pushed forward. The entire system can be operated by only 5 people, in the pilot project up to 22 culvert segments were installed per shift.

There was another key advantage for the pilot project in Auckland. Before excavating the second tunnel the TBM had to be turned in a very narrow shaft. The detached back-up concept meant that the 14.4 meter EPB Shield could be designed and built comparatively short with a length of only 87 meters. This made the turnaround considerably easier. The construction company was able to save time and money. An interesting video of the TBM being turned can be found here.


Your contact person:

Christian Draeger
Area Sales Manager | Traffic Tunnelling
Tel. +49 7824 302 4670

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