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What this covers
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Every segment ring in a tunnel passes through the tail of the shield on its way into place. For a short moment it sits in the most exposed spot on the machine: the gap between the steel tail shield and the concrete lining.
The tail seal is what closes that gap. If you specify, buy or maintain TBM components, it is worth understanding how the brush rows actually hold pressure, because the rating on a data sheet only holds when the whole system is set up correctly.
Where the Seal Sits and What It Faces
The seal closes the space at the rear of the shield, between the tail skin and the concrete segment lining. On the outside of that space is the ground, groundwater and the backfill grout that is pumped behind the rings to fill the annular space.
All three are trying to get into the machine. Groundwater pushes with the pressure of the water table. Grout pushes with injection pressure. Soil follows wherever water and grout find a path.
The seal has to stop all three while the machine moves forward, while rings are built, and while the shield sits still during stoppages.
Brush Rows and Grease Chambers
Tail seals are usually made of rows of wire brushes fixed inside the shield. The spaces between rows form chambers, and those chambers are filled with tail seal grease.
Each part has a separate job:
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Part |
Job |
What happens if it fails |
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Brush rows |
Keep contact with the lining as rings pass and hold the grease in place |
Gaps open and grease is lost |
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Grease in the chambers |
Fill the gaps between bristles and rows and carry the pressure |
Brushes carry pressure alone and leak |
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Filling system |
Keep the chambers full as grease is lost |
Chambers empty and pressure reaches the next row |
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Row spacing |
Set the size of each chamber |
Too much or too little grease per chamber |
The point to take away is that brushes do not replace grease. Brushes hold grease, and grease fills the gaps. A seal with good brushes and empty chambers is not a seal.
It also follows that a seal is only as good as the crew filling it. The filling system, the grease lines and the routine for checking chamber pressure deserve as much attention in the method statement as the brush specification. On many drives, the people filling the chambers are the ones who first notice a problem.
How Pressure Is Shared Between Rows
Think of the seal as a series of steps. The outer row faces the full pressure of water and grout. The grease in the first chamber takes part of that pressure. The next row and the next chamber take more. By the inner row, the pressure has been brought down to what the machine can tolerate.
That only works if every chamber stays full. If one chamber empties, the pressure skips a step and the next row faces more than it was meant to. If two chambers empty, the inner rows are doing the work of the whole seal.
This is why filling rate is as important as the brushes themselves, and why consumption needs to be watched closely in water-bearing ground.
Where Seals Usually Fail
Most tail seal leaks follow a small number of patterns. Knowing them makes the early signs easier to spot.
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Failure pattern |
Early sign |
Usual cause |
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Grout at the tail |
Grout traces inside the shield after a ring build |
Backfill grout pressure above what the seal can carry, or empty chambers |
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Water at the tail |
Damp or running water at the lower part of the tail |
Wash-out of grease in water-bearing ground |
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Rising grease use with no change in ground |
Consumption per ring creeping up week by week |
Brush wear opening larger gaps |
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Leak after a long stop |
Water or grout appears on restart |
Pressure kept working while nothing moved |
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Leak on a curve |
Problems on one side of the tail only |
Uneven gap between shield and lining |
Each of these has a different fix, which is why the record matters. A leak caused by empty chambers is solved by filling. A leak caused by worn brushes needs inspection and possibly replacement. A leak caused by grout pressure needs a conversation between the grouting crew and the seal crew about limits.
The good news is that almost all of them give a warning first. Consumption, line pressures and small traces at the tail usually appear days before a real leak. A crew that reads those signs every shift rarely gets surprised.
What a Depth Rating Really Means
Brush data sheets often state a depth or pressure the seal can withstand. One example is a brush rated to reliably withstand the sealing pressure required for construction at depths of 50 meters.
Read that rating carefully. For that product, the rating depends on several conditions being met: the proper filling rate of the tail seal grease, precise design of the spacing between brush rows, and adherence to the specific technical characteristics of the grease.
In other words, the rating belongs to the system. A brush rated with one grease, one row spacing and one filling rate cannot be assumed to hold the same pressure with a different grease or a different filling plan. If your drive will run near the rated depth, those conditions belong in the method statement.
Properties That Matter in a Brush
A tail seal brush works in one of the harshest spots on the machine. It is pressed against concrete, soaked in water and grout, and dragged along every ring.
The properties that matter follow from that:
- Compressive strength, so the brush holds its shape under pressure from the ground side.
- Wear resistance, so it survives being dragged along ring after ring in abrasive ground.
- Toughness and elasticity, so it springs back after being pressed and keeps contact as the gap changes.
- Corrosion resistance, so it lasts in wet, chemically active ground for the whole drive.
A brush that is strong but not elastic will lose contact on curves. A brush that is elastic but wears fast will open gaps halfway through the drive.
Matching Grease and Brush
The grease has its own list of properties. It has to resist wash-out, stick to the brushes, carry pressure and still pump through the lines.
Because the rating depends on both, it makes sense to choose the brush and the grease together. Ask the brush supplier which grease the rating was based on. Ask the grease supplier which brushes it has been used with. If the answers do not match, you are relying on a combination nobody has tested.
One matched example is a set of tail seal brushes for tunnel boring machines rated for the sealing pressure at 50 m depth, designed to be used with a fiber-blend tail seal grease that is described as highly anti-washout with strong adhesion to the brushes. Together they form a barrier against backfill grout, water and soil. Both come from Balsam Laboratory, a Vancouver tunneling materials company with its own laboratory.
A Short Checklist for the Spec
If you write the spec for a TBM drive, a short section on the tail seal can cover the points that cause most trouble.
- State the maximum depth and water pressure the seal must withstand along the alignment.
- Require the brush rating and the conditions it depends on, including the grease used.
- Require a grease filling plan, with the filling rate per ring and how it will be recorded.
- State the tightest curve radius on the alignment, so brush elasticity can be checked against it.
- Describe the ground, especially stretches that are permeable or water-bearing.
- Require a plan for inspecting and, if needed, replacing brushes during the drive.
None of those items is unusual. Together they make sure that the rating on the data sheet is actually achieved on site.
Inspection During the Drive
Brushes wear, and grease consumption tells you a lot about how they are doing.
Watch for a steady rise in grease per ring without a change in ground or depth. That often means gaps are opening as brushes wear. Watch for grout or water at the tail after stoppages, which can mean the inner rows are carrying too much. Use planned stoppages to inspect the brushes that can be reached, and keep a record of what you find.
Replacing brushes partway through a drive is possible on many machines, but it is far easier to plan for than to improvise. Agree in advance when it would be triggered and how it would be done.
The Short Version
A tail seal is a system: brush rows, grease chambers, a filling plan and the right row spacing. The brushes hold their shape and contact. The grease fills the gaps and carries the pressure.
A depth rating holds only when the filling rate, the row spacing and the grease all match what the rating assumed. Choose brushes and grease together, write the conditions into the spec, and watch grease consumption as your early warning for wear.
