Tail Seal Performance in Varying Ground Conditions

Ask a site team how much tail seal grease they use and you will usually get a number per ring. Ask them why it changed last week and the answer is often a shrug.

Grease consumption is one of the most useful numbers on a TBM drive, and one of the least studied. If you plan, buy or manage consumables for a tunnel, it is worth understanding what moves it.

What the Grease Is Doing Back There

At the rear of the machine, a tail seal closes the space between the steel tail shield and the concrete segments. Most seals are several rows of brushes, and the chambers between those rows are packed with grease.

The seal keeps backfill grout, groundwater and soil from entering the machine. The brushes keep contact with the lining as each ring passes. The grease fills the gaps and carries the pressure.

As the machine advances, some grease is left behind on the outside of the lining and some is pushed out by the pressure behind the rings. New grease has to be pumped in to keep the chambers full. That replacement is what shows up as consumption.

The Main Drivers of Consumption

Consumption is not a fixed property of a grease. It is the result of the grease, the machine and the ground working together.

Two drives with the same machine and the same grease can show very different consumption, simply because the ground and depth are different. That is why a number from one project rarely transfers directly to another. If you must borrow a figure from another job, borrow it from a drive in similar ground at a similar depth, and treat it as a starting point to be replaced by your own numbers as soon as the drive is moving.

Why Low Consumption Is Not Always Good News

It is tempting to treat falling consumption as a saving. Sometimes it is. Sometimes it is a warning.

If the grease lines are partly blocked, consumption drops while the chambers empty. If the crew is underfilling to save grease, consumption drops while the brushes take more of the load. Both look like good numbers on a report right up to the moment grout or water appears at the tail.

So read consumption next to line pressures and the filling record. A drop that comes with steady line pressures and full chambers is a real saving. A drop that comes with rising line pressures or a gap in the filling record needs a look that shift.

There is also a quieter version of the same problem. A crew under pressure to keep costs down may fill to a target number rather than to full chambers. The report looks tidy, the number matches the budget, and the brushes slowly take more of the load than they were designed to carry. By the time the leak appears, the record shows nothing unusual. The fix is to set the filling rule by chamber condition, not by a target quantity, and to make that rule part of the method statement so nobody is tempted to trim it.

Premium and Economy Lines

Some suppliers offer more than one grease formulation for exactly this reason. A premium line is made for demanding conditions. An economy line uses more grease per ring but can suit stable, straightforward ground.

Choosing between them is a whole-drive decision, not a per-drum one. A grease that costs less per unit but is used at a higher rate may cost about the same, or more, over a full drive. A grease that costs more per unit may save money if it reduces consumption and the risk of a stoppage.

The way to compare is to estimate consumption per ring for the ground you expect, multiply by the number of rings in each stretch, and add an allowance for the risk of a seal problem. That turns two quotes into one comparison you can defend.

Tracking Consumption on Site

Good tracking needs very little: a few numbers recorded the same way every shift.

1. Grease pumped per ring, by line if possible.
2. Line pressures at the start and end of each ring.
3. The ground at the face and behind the shield.
4. Any stoppage over a set length, with its duration.
5. Backfill grout volume and pressure per ring.

Plot grease per ring against chainage, with the ground type marked. Most of the changes you see will line up with a change in ground, a stoppage or a curve. The ones that do not are the ones to investigate.

This record also pays off at the end of the drive. It gives you real consumption figures for the next project in similar ground, which is far better than a number borrowed from a brochure.

When Consumption Jumps Without a Reason

Sometimes consumption rises and nothing in the log explains it. The ground has not changed, the machine has not stopped, and the alignment is straight.

That is the moment to test the grease itself. Tail seal grease performance can be checked against the properties that matter: density, consistency, pumpability, malleability, adhesion, vertical wash-out and water sealing ability. A batch that has been stored badly, mixed with something it should not have been, or simply does not suit the ground will show up in those numbers.

Some suppliers will run that analysis for you and report back with steps to improve quality and reduce the chance of a serious incident in use. If your supplier offers tail seal grease performance analysis, use it early rather than after the seal has started to leak.

Ordering for the Whole Drive

Consumption planning is also supply planning. If the grease takes two months to reach the jobsite, the drive needs a stock that covers the time between orders, with a margin for the stretches of difficult ground.

Set a reorder point based on your consumption estimate, not on the first delivery. Recheck it after the first few hundred meters, when real consumption numbers replace the estimate. A seal that runs low on grease partway through a drive is a seal that starts to leak, and an emergency shipment rarely arrives as fast as the problem develops.

What to Ask a Grease Supplier

• What consumption have you seen in ground similar to ours, and at what depth?
• Which of the standard properties have you tested, and can we see the results?
• Do you offer more than one formulation, and where does each one fit?
• Is the grease approved by any TBM manufacturer?
• What is the lead time to our jobsite, and how do you handle repeat orders?
• Will you test samples from site if consumption changes without a clear cause?

A supplier with field experience in several countries and ground types will answer most of those quickly. One example is the tail seal grease for TBM drives supplied by Balsam Laboratory, which has been used on projects in Canada, Singapore, Hong Kong, Macau and mainland China, carries OEM approval from CREG, and is offered in a premium and an economy formulation.

Reading the Numbers at the End of a Drive

When the machine breaks through, the consumption record is easy to file and forget. It is worth an hour of review first.

Look at the stretches where consumption was highest and ask why. Ground, depth, stoppages and curves will explain most of it. Note the stretches where it was lowest and confirm the chambers stayed full. Then write down a consumption figure for each ground type you passed through.

Those few figures are the most valuable thing you can carry into the next bid. They turn grease from a guess into a planned quantity, and they make the premium versus economy question much easier to answer.

The Short Version

Tail seal grease consumption depends on ground, pressure, stoppages, curves and brush condition as much as on the grease itself. Low consumption can be a saving or a warning, so read it next to line pressures and the filling record.

Compare grease lines on cost per ring over the whole drive, not on cost per drum. Track a few numbers every shift, test the grease when consumption moves without a reason, and order early enough that the seal never runs short.

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