Curved Alignments in Tunnel Boring

A straight tunnel is kind to a TBM. The shield, the segments and the tail seal all work around an even gap, ring after ring.

A tight curve changes that. If your alignment includes sharp curves, especially in water-bearing ground, the tail seal deserves its own plan for the curved sections.

What a Curve Does to the Gap

On a straight drive, the gap between the tail shield and the segment lining is roughly the same all the way around. The brushes are pressed evenly and the grease chambers are evenly sized.

On a curve, the shield and the lining are no longer parallel. On the inside of the curve, the gap closes and the brushes are pressed hard against the segments. On the outside, the gap opens and the brushes have to stretch to keep contact.

That uneven gap is the root of most curve problems. The tight side wears. The open side leaks.

The Problems That Show up on Curves

Curved drives tend to produce the same set of difficulties. A real project list makes them concrete. On one slurry shield drive in silty sand with an artesian aquifer, the curved sections listed these issues:

Several of those land directly on the tail seal. Tight gaps wear the brushes. Open gaps need more grease. Uneven segments make it harder for the brushes to keep contact.

What the Tail Brushes Need on a Curve

A brush that works well on a straight drive may struggle on a curve. The demands are sharper.

On the tight side, the brush is pressed harder and dragged along a lining that may have steps between rings. That calls for wear resistance and compressive strength. On the open side, the brush has to reach further to keep contact, which calls for toughness and elasticity, so it springs back instead of taking a permanent set.

Corrosion resistance still matters. A curve does not change the water or the grout, and a brush that corrodes will lose its spring faster.

When you review a brush for a curved alignment, check the tightest radius on the drawings against what the supplier expects the brush to handle, and ask how the rating changes if the gap is uneven.

Grease on a Curve

The grease is under pressure on a curve too. On the open side, the chambers are larger and need more grease to stay full. On the tight side, grease can be squeezed out as the brushes are compressed.

Expect consumption to rise through a curve. Plan the filling rate for it rather than reacting after the fact, and watch line pressures closely, because an empty chamber on the open side is where a leak starts.

A grease with strong adhesion and good malleability helps here. It needs to stay on the brushes as the gap changes shape, and deform to fill a chamber that is not the same size from one ring to the next.

Inside and Outside of the Curve Compared

It helps to set the two sides of a curve next to each other, because they ask for opposite things.

Crews often focus on the inside of the curve, because that is where damage is visible. The outside is where water usually gets in. Both need a plan.

The same comparison is useful when you review a method statement. If it only talks about steering and segment build on the curve, ask what happens at the tail on each side, and who is responsible for watching it. A one-page curve plan that covers both sides is usually enough.

Backfill Grout on a Curve

The annular gap outside the lining is also uneven on a curve. On the outside of the curve, the overcut is larger and harder to fill and support. That is one reason settlement is harder to control on a curve.

One way crews deal with this is to raise the backfill grouting rate while turning, so the larger gap on the outside of the curve stays filled. That puts more pressure on the tail seal, which is exactly why the seal and the grouting plan for a curve should be written together.

On the drive described above, the project record notes that the grouting rate was increased during turning and no leakage was observed at the tail seal. The full account of a slurry shield drive with four sharp curves, with radii of R65 m, R70 m, R75 m and R80 m on a 4.91 m Komatsu slurry shield, is a useful reference for anyone planning a curved alignment in permeable ground.

Planning a Curved Section

Treat each tight curve as its own small project inside the drive.

• Mark the start and end of every curve on the drive schedule, with the radius.
• Check brush condition before entering the curve, and replace worn brushes if you can.
• Raise the grease filling rate through the curve and record it per ring.
• Agree the backfill grouting rate for the curve and the pressure limits.
• Increase settlement monitoring above the curve, especially on the outside.
• Inspect the tail and the lining joints after the curve before returning to normal rates.

The curve is also a bad place for a long stoppage. If you can, plan maintenance and deliveries so the machine does not sit still halfway through a tight radius.

A stoppage on a curve combines two problems. The uneven gap is already testing the seal, and the pressure from grout and water keeps working while nothing moves. If a stop on a curve cannot be avoided, raise the filling rate before the stop, check the tail more often while the machine is idle, and inspect before restarting. Those steps take little time and they catch most leaks while they are still small.

Choosing Brushes With Curves in Mind

If your alignment is mostly straight with one or two gentle curves, standard brushes and normal filling will often be enough. If it includes several tight curves, especially soon after launch or in water-bearing ground, choose the tail seal with the curves as the design case.

Ask the supplier for the brush properties that matter on curves and for the conditions any pressure rating depends on. A rating that assumes even gaps and a set filling rate may not hold on a curve unless the filling plan is adjusted. One option is a set of wear-resistant TBM tail brushes listed with compressive strength, wear resistance, toughness and elasticity, and corrosion resistance, rated for sealing pressure at 50 m depth under stated filling and spacing conditions. They are supplied by Balsam Laboratory together with a matching tail seal grease.

What to Record After Each Curve

Each curve is a test of the seal, and the results are worth keeping.

• Grease consumption per ring through the curve compared with the straight sections either side.
• Any grout or water seen at the tail.
• Brush condition at the next inspection.
• Lining joint condition and any leakage behind the rings.
• Settlement readings above the curve.

Those records make the next curve on the same drive easier, and they give you real numbers for the next project with a similar alignment. Share them with the supplier as well. A supplier who sees how its brushes and grease behaved on a real curve can give better advice next time.

The Short Version

On a tight curve the gap between shield and lining becomes uneven. The tight side wears the brushes and the segments. The open side needs more grease and more grout, and it is where leaks start.

Plan each curve as its own section: check the brushes before you enter, raise the grease and grouting rates, watch settlement, and inspect afterward. Choose tail brushes with wear resistance and elasticity in mind, and make sure any pressure rating still holds with the filling plan you will actually use on the curve.