TBM Launch Portal Sealing in Silty Sand and Artesian Ground

What this covers Why the Launch Is the Weak Point What Artesian Ground Adds The Usual Approach and Its Limits What a Flexible Seal Does Differently A Launch With the Results Recorded Planning the Portal Seal Where to Find a Reference Material After the Launch Sharp Curves After the Launch Who Owns Which Part of the Risk The Short Version

The first few meters of a tunnel drive often carry more risk than the next few kilometers. The machine leaves the shaft, the ground around the entrance ring is disturbed, and every leak path is short and direct.

If your launch shaft sits in permeable, water-bearing ground, the portal seal deserves as much planning as the machine itself.

Why the Launch Is the Weak Point

At launch, the TBM breaks through the shaft wall into ground that has not been supported by anything except the shaft and any ground treatment around it.

The gap between the shield and the opening in the wall is a direct path from the ground into the shaft. Behind the machine, the first rings of the lining are not yet surrounded by settled grout.

In stable, dry ground, that is manageable. In silty sand under artesian pressure, water and sand can follow the machine into the shaft within hours.

The consequences are not limited to the shaft. Every cubic meter of sand that flows in leaves a void somewhere above. Near the surface, that void becomes settlement. Under a road or a building, it becomes a claim. That is why a launch plan in this ground is as much about the surface as about the tunnel.

What Artesian Ground Adds

An artesian aquifer holds groundwater under pressure greater than the level of the ground surface would suggest. Open a path into it and water flows without needing to be pumped.

Silty sand adds a second problem. Disturbed by tunneling, it can lose strength and behave closer to a liquid. That raises the risk of sand inflow at the portal and surface settlement during the start of the drive.

Ground factor	What it does at the portal	What you need from the seal
Strong permeability	Water moves freely toward any opening	A low-permeability barrier around the entrance ring
Artesian pressure	Water flows into the shaft under its own pressure	A seal that holds pressure, not just fills space
Liquefaction-prone silt	Sand can follow the water in	A cohesive material that does not wash out
Deep overburden	Higher pressures at the opening	Pressure testing that matches the depth

The Usual Approach and Its Limits

Many launches seal the entrance ring with cement grout. Cement is familiar, cheap per unit and sets hard.

It also has weaknesses at a launch. It needs time to set, it can crack as the shield moves and the first rings settle, and once set it cannot follow movement. If a path opens after it cures, you are drilling and grouting again.

That is why it helps to think about the portal seal as two jobs: fill the gap, and keep the gap sealed while things are still moving.

Cost is part of the picture too, but not only the cost of the grout. A cement seal that cracks means more drilling, more grout and more time with the machine waiting at the wall. The material that looks cheaper per unit can end up costing more once the repairs are counted, which is worth remembering when the sealing method is chosen.

What a Flexible Seal Does Differently

A thixotropic grout gels at rest and flows when sheared. Placed around the entrance ring, it fills the gap and holds water pressure, but it does not crack when the shield or rings shift.

You can check whether it is working in a simple way. Open the reserved holes in the portal, one at a time. If the material is holding, it extrudes cleanly with no gas or liquid behind it.

You can also compare the extruded material with fresh material from the pump. If the two look the same, the grout has not been diluted by groundwater.

A Launch With the Results Recorded

The Shanghai Taopu Waste Water Treatment Plant project is a good reference for this case. It used a 4.91 m Komatsu slurry shield TBM, with 32.45 m of soil overburden. The entire tunnel section ran through silty sand and a strongly permeable artesian aquifer, in a layer prone to liquefaction from shield disturbance.

The project page lists the risks plainly: water and sand inflow at the portal, and surface settlement during tunnel initiation.

What was checked	What was recorded
Pressure at the tunnel entrance	Stable pressure established
Reserved holes in the portal, opened and closed in sequence	Grout fully extruded, no gas or liquid leakage
Observation port at the backfill grouting hole	Grout extruded, no gas or liquid leakage
Portal sealing material cost	85% lower than using cement grout to seal the entrance ring

The photo caption on that page notes that the extruded material looked like fresh material in the pump, which is the anti-dilution check described above.

Planning the Portal Seal

Before the machine arrives, settle these points with the contractor and the supplier:

1. Where are the reserved holes in the portal, and in what order will they be opened?
2. What material goes around the entrance ring, and how will it be mixed and pumped?
3. How will you confirm the seal is holding before the shield fully leaves the wall?
4. What is the fallback if a hole shows water?
5. Who reads the settlement monitoring during the first rings, and how often?

The answers belong in the method statement, not in a site conversation on launch day.

It also helps to agree what “holding” means before the launch starts. A clean extrusion from every reserved hole, stable pressure at the entrance and no movement on the settlement points is a clear, checkable standard. Write it down, so the decision to move the machine forward is based on agreed evidence rather than on the mood in the shaft. That standard also gives the night shift something firm to work to.

Where to Find a Reference Material

If you are writing the spec for a launch in this kind of ground, one option is a thixotropic annular grout used on TBM launches that is designed to stay stable in permeable, water-bearing sand, silt and clay, and that passed a 5-bar constant water pressure test held for 31 days. It is supplied by Balsam Laboratory, and you can read the Taopu portal sealing results for the full record of that launch, including site photos.

After the Launch

The portal seal does its main job in the first days, but it is worth watching for longer.

• Keep reading settlement points above the first rings until movement has stopped.
• Check the shaft wall around the ring for damp patches or fines.
• Recheck reserved holes if the machine stops for a long period early in the drive.
• Compare grout consumption at the portal with what was planned.

If the numbers stay flat for the first stretch of the drive, the launch has done its job.

Sharp Curves After the Launch

Some alignments do not give the machine a straight run after the shaft. A tight curve soon after launch adds a second source of leakage risk, this time at the tail of the shield.

On a curve, the gap between the shield and the lining is no longer even. One side opens, the other closes. Segment rings develop height differences, axis control gets harder, and segments wear at the tail. Uneven gaps between segments can leak, and the hinges of an articulated shield can leak too.

The Taopu alignment had four sharp curves, with radii of R65 m, R70 m, R75 m and R80 m. The project page reports that the grouting rate was increased during turning and no leakage was observed at the tail seal.

The lesson for planning is simple. If a curve follows the launch, treat the grouting plan for the curve as part of the launch plan. The same ground that threatens the portal will test the tail seal on the curve, and the same question applies: does the material hold pressure while things are moving?

Who Owns Which Part of the Risk

Launch failures often come from a gap between roles rather than a gap in the ground.

The designer sets the shaft, the alignment and the ground treatment. The contractor chooses the sealing method and runs the launch. The supplier knows how the material behaves and how to pump it. If those three never sit at the same table before launch, each assumes one of the others has the portal covered.

A single launch meeting, with the geotechnical report open, closes most of that gap. It also gives you a written record of who decided what, which matters if anything moves.

The Short Version

The launch portal is a short, direct path from the ground into the shaft. In silty sand under artesian pressure, it is often the riskiest point of the drive.

Seal the entrance ring with a material that holds pressure and does not crack as things move. Check it by opening the reserved holes, compare extruded and fresh material, and keep reading the settlement points until the ground has settled.