Page 12 - TunnelTalk Annual Review 2017

Rock TBM progress against the odds
Lok Home, President, Desiree Willis, Technical Writer, and Robbins TBM Application Engineers
When boring a hard rock tunnel, homogeneous conditions are often the exception rather than the rule. While contractors can set goals such as meterage/month and performance incentives, it is also prudent to have a Plans B and C prepared for facing difficult conditions.
Plan A is to plan ahead, with the parties involved investing in enough geological testing for the risks to be reasonably and accurately assessed. Once the risks have beenacknowledged,theycanbemitigated, with a properly designed machine equipped with technology that can tackle fault zones, water inflows, squeezing ground, or whatever the predetermined risks may be.
The alternative - to modify the machine in the tunnel after bad ground is encountered - not only results in increased downtime, but also can expose your crew needlessly to additional risks. In the worst case, the entire operation grinds to a halt. In all but the most extreme of cases, this should not be happening in today’s tunneling world. We have the modern technology to prevent these situations.
Remove the guesswork
To plan for potential challenges adequately, the risks must be understood. It seems a simple statement, but all too often owners tend to skimp on geological surveys due to perceived unnecessary up-front costs versus benefits. The cost of having a stuck or damaged machine in the tunnel for months, or even years however is much higher than the initial cost of adequate geotechnical studies. If such surveys can be done, then the machine can be designed specifically to deal with any known features along the tunnel length to optimize excavation rates.
Known (and unknown) risks
For sure, meter-by-meter or even kilometer- by-kilometer geotechnical reports are not
possible or feasible economically. Pre- construction desk study investigations and surface mapping can also be limited.
In such cases appropriate contingencies are required to cover additional costs and delays if difficult conditions are encountered. TBMs today, whether open machines or shielded machines, including those capable of holding pressure, can bore successfully in difficult ground. TBMs can be fitted with a range of difficult ground solutions, including: • multi-speed gearboxes with optional
higher torque capabilities;
• rapid advance shield design using
stepped shields and shield lubrication
systems;
• water inflow control facilities, and
• other features that can mitigate higher
risks of high cover tunneling.
If a TBM is equipped with these features from the start, then costly downtime and in-tunnel machine modifications can be avoided. If a segmental lining is applied, shielded machines have the beneficial advantage of providing excavation support and safety within the machine itself.
In even the most extreme conditions, main beam, open-type gripper TBMs can be efficient and safe. Open access behind the cutterhead for ground support and consolidation, unrestricted probe drilling, and the absence of a shield are important attributes of open machines in extreme conditions.
In ground exhibiting squeezing- convergence and rock bursting, open-type machines often fare better than shielded machines, as they are less likely to become trapped. They can also utilize the McNally Support System, which can, in essence, be made to function like a shield. In swelling or slacking ground main beam TBMs also allow for immediate ground treatment behind or over the top of the cutterhead. Open-type machines are capable of operating in ground
with occasional to continuous water inflow - if mitigation strategies such as combining injection grouting to stem flows with sufficient pumps to remove the water are employed.
Squeezing ground
Over-boring is frequently recommended for squeezing or converging ground but in practice it is seldom used effectively. Over- boring is typically applied by pre-mounting extra gauge cutter housings in the periphery ofthecutterheadtoexcavateaslightlylarger diameter bore. In the over-bore zone, yielding type support structures should be erected if using an open-type machine. These elements can include yielding steel arches, steel arches in conjunction with yielding jacks, shotcrete structures with yielding rock anchors, or combinations of them all.
If using a shielded machine, a measuring system that comprises a hydraulic cylinder mounted on top of the machine and connected to the machine’s PLC can detect squeezing conditions. A machine with the shortest possible shield length, plus a stepped or tapered shield, shield lubrication and added emergency thrust can be immensely helpful to get a machine through a situation where it might otherwise become trapped.
Rock bursting
In rock bursting conditions, it is important to hold the rock in place to control and limit the disturbance of the rock to as great an extent as possible. With modern open- type TBMs, ground support such as the McNally Support System allows support of the crown to be extruded from the machine as it advances.
Probe drilling is as important on a shielded machine as it is on an open-type TBM. In all conditions, advance probe drilling is recommended 30m to 40m in advance of the face with a 10m overlap.
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