Engineers working on the 4km-long, USD 308M Phase 4A of Cairo Metro Line 3 made an unusual change to their TBM prior to the final drive. The looming presence of sandy clays and silty sands prompted Vinci Construction Grand Projets to convert their 9.5m-diameter NFM EPBM to make use of Herrenknecht’s Variable Density slurry technology.

The third metro line for the Egyptian capital has been under construction since 2007. It stretches for some 30km, mainly underground and has made use of both EPB and slurry machines, depending on the geological conditions.

The outside diameter of the tunnel is 9.15m and it has an inside finished diameter of 8.35m to accommodate two trains per tube. The tunnel has a precast segmental lining made up of straight segments in a 7+key arrangement, the thickness is 400mm and the length is 1.5m. The pinch is ± 45mm. Stations have a standard design of 150m with a 20m width and 25m deep, although Heliopolis Station was revised to a 2x223m double ramp cross shape with connection ramp instead of the standard 150m station. This was to provide a connection between two lines, and is effectively a double station. Phase 4a divides from the earlier Phase 2 at Haroun El Rachid Station. It was here that the NFM EPB that had completed the predominantly clay drives for Phase 2 and was parked for almost three years in preparation for the sandier geology of of Phase 4A.

Florent Detraux, tunnel construction manager at Vinci Construction Grands Projets told Tunnels and Tunnelling that instead of opting for two TBMs of different types, the Egyptian-French joint venture, along with the Egyptian National Authority for Tunnels, made the decision to procure a machine that could be converted from EPB to slurry mode.

“The modification was forecast from the early design of the machine, which has been finally turned in 2015, inside one station under construction, into a hybrid Variable Density machine, while avoiding heavy dismantling operations. Extraction processes, transportation and treatment systems for muck, automation and the control cabin were all successfully modified, while reusing most of the existing equipment,” Detraux explains.


The conversion to slurry was planned due to the highly permeable geological conditions to come, the presence of highpressure water and the need to avoid settlement in the urban environment. However, there was an initial section to contend with that comprised more clay than the remainder, which would have been problematic for slurry. This presented a conundrum.

“We had two choices,” says Detraux. “The first was to convert the TBM immediately at Haroun Station, but it would have been difficult to run a slurry TBM in the clay geology; the other was to proceed to the next station, Heliopolis, before converting it. This would have been difficult in terms of schedule and also very risky, requiring us to run in EPB mode through high sand permeability.”

The compromise was to halt at Rachid Station to incorporate Variable Density slurry technology, on the recommendation of Herrenknecht. “Of course in both cases the process takes time and the cost is very expensive as we needed to install pumps and pipes in the TBM. We also needed to have a slurry treatment plant,” adds Detraux.

The standard modifications included:

  • Removal of the screw conveyor in order to place the slurry pump, suction line and grid;

Installation of several feed and flushing lines through the shield, connected to the bubble chamber (which was already in place as the conversion was expected) and the excavation chamber

The major differences compared to the EPB setup being:

  • The creation of a deep opening to add a rock crusher;
  • Modify the shield wall to create a submerged gate wall;
  • To add refreshing pipes at the bottom of the suction grid;
  • To change rotary cutting at the centre to place a slurry type on it.

Detraux points out that this was a lot of work to do on the TBM and they did not consider it achievable inside the station.

“It would take a couple of months to dismantle the shield and to put it on the surface to perform the modification. In the Herrenknecht solution, the only heavy modification expected on the shield was to install communication pipes equipped with motorized valves and cleaning devices.

“The key of the system is to place the crusher and a slurrifier box at the back of the conveyor to replace the transportation of excavated material from a rubber belt conveyor with a slurry circuit. We also modified the screw conveyor to add this box and then we uninstalled the whole conveyor belt system and replaced it by big piping circuits.”

The TBM and especially the screw conveyor were not designed for the slurry box and crusher. Heavy loads and space would be needed. This meant heavy reinforcement of the screw and heavy modifications to the structure of the gantries were needed.

Substituting out the belt conveyor and adding the slurry pumps called for a great number of components, and to supply them extra distribution was needed from the electrical cabinets.

All of the equipment required power and so the team needed to do a complete re-arrangement of the electrical distribution on the TBM.

“The idea was not having an additional transformer or to replace the two existing, again we lacked the access and space,” Detraux says. “So we removed three motors on the main drive and twelve at the beginning, to get free power for the crusher and additional slurry pumps.

“Our EPBM had enough torque initially to allow to remove three motors and remain more powerful in this new configuration than a normal slurry TBM.

In this configuration our new EPB TBM offers both advantages of slurry TBM and EPB TBM, which are able to manage plastic clay with high torque and screw conveyor to empty the excavation chamber without clogging the suction side of the slurry pump. In addition to this, there is the capacity of running in sandy and high permeability geology.”

Herrenknecht provided design services, equipment and personnel to assist with the conversion, but a number of items were manufactured in Egypt, for example the last gantry to extend the rail track and slurry pipes.

The installation of pipes and pumps presented a positional challenge, as the TBM was equipped with a gantry deck to accommodate two locomotives. This left very little space for the new equipment and routing of the pipes. The contractor looked at removing one of the tracks, but this was not considered feasible for segment supply reasons.

In terms of timing, the conversion took roughly eight months due to a change of customs release regulation, after that it took three months to install and commission everything.


The project has been slightly amended because at the end of the route the client changed from underground to overground routing for external reasons.

Detraux explains that the Variable Density TBM has achieved very high rates, especially in the clay. “By not facing any clogging effects, we have low consumption of tools, low consumption of bentonite so we made a gained a lot of economy compared to the worst cases we expected,” he says. “We were also able to run in sand or clay up to 70mm per minute, which was amazing.

“The best rate is 19 rings per day for 1.5m in 12 hours shifts. We are able to run typically 12-14 rings per day. This including difficult clay conditions area where a slurry machine installs only 1 or 2 rings a day due to clogging or overload waste bentonite disposal.”