Tunnel is a place where size matters. As a tunnel gets bigger in diameter comes a raft of challenges, some construction related, some design related. This blogpost takes an overview of these scale issues.
Scale to the power of 1 or less
This means the issue has an approximately linear relationship to the tunnel diameter, or less than that.
Ground Cover. Tunnels need to be buried sufficiently deep in order to utilise the arching effect of the soil for stability. As a rule of thumb the ground cover should be 1 times tunnel diameter as a minimum requirement. Larger diameter tunnels need to be buried deeper, proportionate to its diameter. The cover being less than tunnel diameter can lead to issues such as excessive joint rotation, insufficient bending moment resistance of tunnel lining, buoyancy floatation and risk of face blow-out.
Lining thickness. Also following this relationship quite well is the tunnel lining thickness. For a TBM-built tunnel, the tunnel lining thickness is usually approximately 3% to 4% of the tunnel diameter. For large diameter tunnels, tunnel segments can be very thick – 500mm to 600mm are not uncommon.
Ring segmentation. The weight capacities and dimensional limitation of the logistics, such as the cranes and the on-road transports, are gonna be stretched to the limit. It’s quite common, and perhaps necessary, to break down the ring further to more segments so that each is lighter and smaller, but this lengthens ring building time. A balance needs to be struck at the sweet spot incorporating considerations of ease of handling and speed of ring assembly. The ring break-down is slightly less than being proportional to the tunnel diameter, and it is usually step changes rather than a linear relationship.
Scale to the power of 2
Materials. Any material related, both into and out of the TBM, is proportional to the area of the tunnel face, i.e. square of the tunnel diameter. This includes concrete, annulus grout, spoil removal. The amount of materials in and out of the tunnel bore is gigantic and therefore the challenges associated with logistics rises exponentially higher.
Buoyancy force. A scenario particularly important to large tunnels is buoyancy uplift. This usually isn’t a problem for tunnels, but can be a major problem for large diameter tunnels with shallow cover, as the uplift force is a function of the square of the tunnel diameter.
Surface settlement. The volume loss of the tunnelling needs to be tighter controlled for large diameter tunnels, as surface settlement correlates to the tunnel diameter by a power of 2. The conventional 1% volume loss assumption may result in a ridiculous amount of peak settlement, such as 100mm-200mm. Well-spec’ed TBMs have demonstrated the ability to control settlement within 0.5% maximum, and normally 0.2-0.3%.
Scale to the power of 3 or above
The first is the sheer torque required to spin the big cutterhead. Torque is the torsional moment required to spin the TBM cutterhead. This requires a huge amount of power for large diameter tunnels, especially for EPB machines because they are more grindy – more frictions. Slurry based TBMs use slurry as a medium of lubrication and has less friction. The total thrust of the TBM correlates to several factors including face area, the circumference, friction angle of the ground and the depth of the tunnel, and can go as high as the power of 4 in relation to the tunnel diameter. The power requirement directly corresponds to the thrust.
Others
And there are face stability issues. Give that the diameter is big, you are more likely to run into varying ground conditions across the face. The big tunnel usually needs to be buried deep and very likely the groundwater pressure will be big as well. there is usually a hydrostatic pressure gradient at the face balancing pressure. And all the above said present challenge to face pressure control. Because of the high groundwater pressure, face intervention needs to be planned very well, and use safe havens for free air intervention where possible, because there is a safe pressure limit to hyperbaric intervention.