Left, figure 10: New kaiser Wilhelm Tunnel - compressible homogeneous paste after soil conditioning
irregular foam supply due to the pressure gradient in the working chamber, which finally led to blocked foam lines. Modern machines use a rotary union that allows a proper and continuous one-to-one connection from the foam generator to injection point. Unfortunately, the rotary union design becomes more difficult with an increasing number of injection points on the cutterhead, but this investment pays back rather quickly. It is also important to spread the injection points well
over the whole cutterhead area. Unfortunately, still these days, we see machines with sections of more than 30% of the cutterhead not covered.
Foam System The design of the foam system itself is the other key part of the soil conditioning system. In the 1990s – and, unfortunately to a certain extent
still so nowadays – the TBMs were equipped with foam mixing tanks. The system is quite inflexible when it comes to changing foam recipes, which made the use of polymer a nightmare and were often the reason for numerous hassles, like overflowing foam or being a source of bacterial contamination. The industry shifted
to real-time in-line mixing systems, which are both highly efficient in use and quickly adoptable to the soil conditions. Also, the generator design itself is of great
importance. The two main criteria here are the creation of a stable foam with quite uniform foam bubbles as well as a minimum amount of free compressed air. As already indicated, the foam system should be easy
to use for the TBM driver. Most modern systems, instead, are rather difficult to operate and to be adaptable to the actual encountered soil and the necessary driving requirements of the TBM. Therefore, we developed a versatile foam system for Robbins some 10 years ago, which has in-line foaming solution regulation, including polymer mixing, and also has operations supported by an easy and quick to use touchscreen user surface as shown in Figure 12 (Langmaack 2016). It has water/ (polymerised) foam selectors for each cutterhead injection port and a quick rider system for easy general foam injection rate (FIR) adoption of all foam ports.
Special thanks to Lok Home giving me the opportunity to create that system.
Mean permeability coefficient k as a function of time
1,0E-03 1,0E-04 1,0E-05 1,0E-06 1,0E-07 1,0E-08
0 10
20 30 405060 70 80 Time t [min]
90 Above, figure 11: Permeability trials with SolidFoam® PHOTO CREDIT: RUHR UNIVERSITÄT BOCHUM, PROF. DR. THEWES November 2023 | 15 No conditioning
Bentonite slurry, SIR=20 vol.%, SFER=0 Limit value 1.0E-05 m/s
Foam, FIR=20 vol.%, FER=15
SolidFoam®, SFIR=20 vol.%, SFER=1.7
SolidFoam®, SFIR=10 vol.%, SFER=1.7
Permeability coefficient k [m/s]
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