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What this covers
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On a short pipe jacking drive, nobody thinks much about thrust. The jacks push, the pipe moves, and the numbers on the gauge stay well inside the limit. On a long drive, thrust is the number that decides whether the job finishes on schedule or stops halfway.
If you design or run long drives, it pays to understand where that force comes from, because most of it is not where people expect.
Most of the Load Is Friction, Not the Face
Jacking force has two parts. One part is the resistance at the cutting face. The other is skin friction between the outside of the pipe string and the ground around it.
Face resistance stays fairly steady along a drive. Skin friction does not. Every pipe you add puts more surface area in contact with the soil, so the friction load grows with distance.
On a long drive, skin friction becomes the larger share of the total. That is why lubrication is one of the biggest levers you have on jacking force. You cannot shorten the drive, but you can change how the pipe and the soil meet.
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Part of the load |
How it behaves along the drive |
What changes it |
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Face resistance |
Stays roughly level |
Ground conditions, machine settings, face pressure |
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Skin friction |
Grows with every pipe added |
Lubricant, overcut, ground contact, stoppages |
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Restart load after a stop |
Peaks after downtime |
How well the lubricant holds its place while idle |
What a Lubricant Actually Has to Do
A pipe jacking lubricant only works while it sits in the overcut as a continuous layer between pipe and soil. If that layer breaks up, the pipe touches the ground directly and friction climbs.
That gives you a simple test for any lubricant. The question is not how slippery it is in a bucket. The question is whether it stays in the annular gap for the whole drive.
Three jobs follow from that:
- Lubrication. A fluid layer lowers the friction between pipe and soil.
- Support. A filled, pressurized annular gap keeps the overcut from closing onto the pipe.
- Water control. A low-permeability layer slows groundwater moving along the outside of the pipe.
A lubricant that does the first job but not the other two will lose its film in the ground where you need it most.
Why Friction Climbs in Permeable Ground
In fine, low-permeability soil, a bentonite slurry builds a filter cake against the ground and stays put. In coarse, permeable, water-bearing sand, that filter cake can fail. The slurry dilutes, bleeds into the ground, or washes away.
When that happens, crews usually respond by pumping more. More volume goes into the annulus and the gauge still climbs, because the slurry is leaving as fast as it arrives.
The sharper way to see it is this: a lubricant that stays in place beats a lubricant pumped in larger volume. Volume is a symptom. Retention is the cause.
Stoppages Are Where Thrust Peaks
Every drive stops. Shift changes, pipe deliveries, equipment repairs and weekends all leave the string sitting still in the ground.
Friction builds during a stoppage. The ground relaxes onto the pipe, a thin slurry drains away, and the restart after a long stop is often the highest load of the day. On a short drive, you have margin to absorb that. On a long drive, the restart after a stop can be the moment you reach the limit of the main jacks.
That is the case for a lubricant that keeps its structure when it is not moving. A thixotropic material gels at rest and flows again when it is sheared. It holds the gap while the string sits still, and it lets the pipe move again when the jacks push.
Intermediate Jacking Stations Are a Fallback, Not a Plan
Intermediate jacking stations split a long string into sections, so each section only carries part of the friction. They work, and long drives often plan for them.
They also cost time. Every station has to be installed, operated in sequence and later removed. A drive that can be completed on the main jacks alone is simpler to run.
So when you plan a long drive, treat the station count as an outcome of your friction control, not a fixed input. Better lubrication does not guarantee you will skip the stations, but it changes how often you need them.
A Long Drive With the Numbers Recorded
The Ligang Water Intake drive is a useful reference because the friction numbers were recorded on both sides of a change.
It was a single drive of 3,214.96 m in DN 2600 steel pipe, more than 40 m deep under the Yangtze River, in water-saturated sand. A Guinness World Records certificate for the longest pipe jacking distance is shown on the project page. Replenishment stations were set every 1 km along the line.
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Stage |
Lubricant |
Injection rate |
Jacking force |
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Before section 77 |
Traditional bentonite |
300% |
8,000 kN 
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After the switch at section 77 |
Thixotropic grout |
150% |
6,778 kN, about 15% lower |
Two things stand out. The injection rate halved, and the force still fell. That is the retention point in practice: less material went in, and more of it stayed where it was needed.
The station record is worth reading closely too. On the left line, no intermediate jacking station was used. On the right line, a station was used once, after a long stoppage. That matches the pattern above: the stop is where friction peaks.
Overcut, Steering and the Pipe String
Lubrication is not the only thing that sets skin friction. The overcut and the line of the pipe string matter too.
The overcut is the small ring of space left by a cutting head slightly larger than the pipe. That ring is where the lubricant lives. If it is too small, the ground closes onto the pipe quickly. If it is left unsupported, it closes anyway, because nothing is holding it open. A pressurized, filled annulus is what keeps the overcut doing its job.
Steering matters for a similar reason. Every correction pushes one side of the pipe harder against the ground. A string that wanders up, down and sideways along the drive carries more contact, and more contact means more friction. Smooth steering and a stable, supported annulus work together.
Buoyancy is a third factor people forget. A dense mix around the pipe adds buoyancy, which helps the string hold line and level instead of dragging along the invert. That is one more reason the material in the annulus should keep its body rather than thin out.
So when friction climbs, look at three things together before blaming the ground: the state of the lubricant, the steering record and the pressure in the annulus.
Questions to Settle Before the Drive Starts
You can avoid a lot of trouble by answering a few questions at the planning stage rather than at pipe 200.
- What is the ground along the whole alignment, and where does it turn permeable or water-bearing?
- How much margin do the main jacks have over the predicted friction load?
- Where are the likely stoppages, and how long might each one last?
- How will the lubricant be replenished along the string on a long drive?
- What does the jacking force log need to record so you can see a trend early?
The last one matters more than it looks. A force log read daily shows friction creeping up before it becomes a problem. A force log read only after a stall shows you what already happened.
Where the Lubricant Choice Fits in the Spec
On many projects, the lubricant is left to the contractor and chosen out of habit. On a long drive, that is a risk the designer carries too, because thrust drives pipe wall design, jack capacity and the station plan.
If you write the spec, it is reasonable to ask for evidence that the lubricant holds its place in the ground you expect. That can mean pressure test results, a record from a comparable drive, and the mix and pumping method the supplier recommends. One example is a lubricating annular grout for pipe jacking built to stay stable in permeable, water-bearing sand, silt and clay, supplied by Balsam Laboratory, a Vancouver tunneling materials company. Its record on the 3,214.96 m Ligang single drive is set out with the force readings described above.
What to Watch Once the Drive Is Moving
Once the string is moving, the job is to catch friction trends early.
- Plot force against distance, not just time. A rising slope per pipe tells you the film is breaking down.
- Compare restarts. If the restart load after each stop is creeping up, the lubricant is draining while idle.
- Track injection against loss. If you are pumping more and the force still rises, you are feeding a leak, not a film.
- Check the ground log. A change in friction often lines up with a change in soil.
None of this needs special equipment. It needs someone reading the numbers every shift and asking why they moved.
The Short Version
Jacking force on a long drive is mostly skin friction, and skin friction grows with every pipe. The lubricant only helps while it stays in the overcut, so retention matters more than volume.
Plan for stoppages, because that is where friction peaks. Treat intermediate stations as a fallback. And ask for evidence, measured in ground like yours, that the lubricant stays where you put it.