Tunneling in Karst: Your Questions Answered
On November 18, 2020 we held a webinar on a unique recent project, France’s Galerie des Janots. Crews utilized a 3.5 m diameter Robbins Main Beam TBM to bore a 2.8 km long tunnel through limestone with karstic features, with some surprises: Two uncharted caverns up to 8,000 cubic meters per size in the bore path of the TBM. To find out how the crew overcame the challenges, watch the video here:
We held a question and answer session during our event that was not recorded and wanted to share some selected Q & A with you below.
Question and Answer Session
Q: What type of karst was predominant along the alignment?
A: The karst was a kind of weak limestone formation. Very soft with a tendency to get pasty – like chalk in water.
Q: Could another type of TBM have been used in the geological conditions (limestone with karst)?
A: It was a rather small diameter tunnel and the less structure you have with the machine, the better. A shielded machine with segments at this diameter can work – but when in bad rock and you need to do something, the shield can become an obstacle to access the rock. We think the Main Beam machine was the best option and provided full access to the tunnel walls and strata.
Q: Do you think an EPB or Slurry machine could have been used in this tunnel?
A: I do not think so. When in rock, you will struggle with EPB or Slurry TBMs–producing conditioned muck or building up a filter cake and pressurizing the chamber will be very difficult to make happen in these conditions. However, our Crossover type machines do provide for operation in various modes and can run from rock into soft material.
Q: One of the typical challenges faced with karst is high water ingress. Can an open face machine deal with that?
A: Here we did not have water ingress, and did not have karstic aquifers either, so it was sufficient to employ normal water pumping. An open machine provides full access to the tunnel walls and the face. So, you can inject and fight the water ingress better compared to a shielded machine.
Q: In the Middle East, we had a project that, during pilot tunnel drilling in front of the cutterhead, resulted in sudden inrush of water ,which led to flooding inside the TBM. What types of mitigations measures do you recommend to overcome flooding risk?
A: A TBM can be furnished with doors and chutes at the relevant points to stop water ingress. A very appropriate tool is a so-called Guillotine Door, which is assembled to the muck chute above the TBM conveyor and allows for quick closing. Also, TBM conveyors can be furnished with a water-tight plug door, which can seal the machine when pulling back the conveyor. For sure, submersible pumps are needed, and to stop the water ingress, injection grouting should be used.
Q: What is BEAM Technology and was it able to successfully predict any real detected cavities before the TBM excavation?
A: BEAM Technology stands for Bore-tunneling Electrical Ahead Monitoring. BEAM is a ground prediction technique using focused electricity-induced polarization to detect anomalies ahead of the TBM. Yes, they were able to predict a cavity using BEAM on this TBM, see drawing below courtesy of Geo Exploration Technologies. The red spot spot in the colored band clearly indicates the location of the cavity, and the picture verifies that it was found at that location.
Q: What type of material did you use to fill the cavities?
A: It was mainly concrete reinforced with steel mesh and wood structures.
Q: How did they move past the first cavern and did ground support change during tunneling?
A: To cross the cavern space, crews erected a 4 m high wall of concrete so the TBM would have something to grip against (see picture below). The TBM was started up and was able to successfully navigate out of the cavern in eight strokes without significant downtime to the operation. Shotcrete was also employed – as long as necessary. Rock conditions changed often in this tunnel.
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