Five Critical Controls for a Long-Distance Microtunnel
A long-distance microtunnel does not succeed because of one oversized machine, a high-capacity jacking station or one conservative calculation.
Success depends on five connected systems:
- Guidance and alignment
- Jacking-force management
- Lubrication
- Slurry-system performance
- Intermediate Jacking Stations
There is no universal distance that automatically makes a drive “long-distance.” The difficulty depends on the relationship between drive length, pipe diameter, ground conditions, alignment, groundwater pressure, jacking capacity and slurry-system performance.
A 500-metre drive may be manageable in one set of conditions but extremely difficult in another.
01 — Guidance and Alignment
Small alignment errors can become serious over a long distance. The navigation system, survey control and steering strategy must therefore remain reliable throughout the entire drive.
The guidance system should continuously show:
- Horizontal and vertical position
- Deviation from the design alignment
- Machine pitch and roll
- Steering direction
- Steering-cylinder positions
- The machine’s response to each correction
Why does guidance become more difficult?
As the drive becomes longer, several problems may develop:
- Laser refraction caused by temperature differences
- Dust, humidity or vapour affecting the target
- Equipment blocking the laser beam
- Movement of the laser support or survey points
- Reduced accuracy over long distances
- Loss of direct visibility on curved drives
- Accumulation of small survey errors
Laser guidance is commonly used for straight drives. Long or curved drives may require gyro navigation, a hydrostatic water-level system or additional independent surveying.
Steering corrections
Corrections should be small, gradual and controlled. The operator must consider not only the current position of the machine but also the direction in which it is moving.
Large or late corrections can:
- Increase machine articulation
- Create additional overcut
- Increase pipe-joint deflection
- Raise jacking forces
- Create high contact pressure in curves
- Increase the risk of ground loss and settlement
The objective is not simply to remain within tolerance. The objective is to maintain a stable alignment with the minimum necessary steering.
02 — Jacking-Force Management
For a long drive, calculating the maximum jacking force before construction is not enough. Predicted and measured forces must be compared continuously during tunnelling.
The total jacking force mainly consists of:
[ F_{\text{total}} = F_{\text{face}} + F_{\text{friction}} ]
Where:
- (F_{\text{face}}) is the force required to excavate and advance the machine.
- (F_{\text{friction}}) is the resistance between the ground and the external surfaces of the machine and pipe string.
As the drive becomes longer, the external contact area increases. Pipe friction therefore becomes a major part of the total jacking force.
Factors affecting jacking force
- Pipe outside diameter
- Drive length
- Ground type and density
- Groundwater conditions
- Annular-gap condition
- Overcut
- Lubrication performance
- Straight or curved alignment
- Pipe-joint deflection
- Machine stoppages
- Ground closing around the pipe
- Machine steering
- Unexpected geology or obstructions
Why is trend monitoring important?
One high force reading does not always identify the problem. The pattern of force increase is more useful.
For every pipe, the team should record:
- Maximum jacking force
- Initial breakaway force
- Normal moving force
- Advance speed
- Lubrication pressure and volume
- Machine stoppage time
- Steering corrections
- Ground changes
- IJS forces and operating sequence
A sudden or unusual increase may indicate:
- Poor lubrication
- Blocked lubrication ports
- Collapse of the annular gap
- Pipe misalignment
- Excessive joint deflection
- High contact pressure in a curve
- Machine-body resistance
- Ground entering around the pipe
- Increased static friction after a long stoppage
Remaining below the theoretical pipe-crushing capacity is not enough. Joint rotation, eccentric loading, packing-ring behaviour and local edge stress must also be checked.
The objective is to identify abnormal resistance before it causes pipe damage or complete stoppage.
03 — Lubrication
Bentonite lubrication is not an optional support activity on a long drive. It is a major part of the system design.
Its main functions are to:
- Reduce friction between the pipe and ground
- Support the annular gap
- Limit ground closure around the pipe
- Maintain a continuous lubricating layer
- Reduce loads on the main station and IJS units
- Help restart the pipe string after stoppages
Lubrication is not the same as slurry
These systems have different purposes:
- The slurry circuit supports the excavation face and carries excavated material to the separation plant.
- The lubrication system injects bentonite into the annular space around the pipe string.
Both may contain bentonite, but their mix design, pressure, injection points and operating requirements are different.
Long-drive lubrication design
A suitable system should include:
- A ground-specific bentonite mix
- Proper mixing and hydration time
- Main and standby pumps
- Distribution lines along the pipe string
- Planned injection ports
- A defined injection sequence
- Pressure and flow monitoring
- Procedures for cleaning blocked lines
- Complete injection records
Injection only behind the MTBM may not be sufficient for a long drive. Re-lubrication must normally be carried out through selected ports along the pipe string.
Injection pressure
Pressure must be high enough to distribute bentonite through the annular gap. However, uncontrolled pressure can cause:
- Hydraulic fracturing
- Bentonite breakout at the surface
- Ground heave
- Leakage into nearby utilities
- Leakage through shaft eyes
The allowable pressure range should therefore be based on depth, groundwater pressure, soil permeability and surface sensitivity.
The quantity of bentonite used does not prove that lubrication is effective. Performance should be evaluated by comparing:
- Injection pressure and volume
- Bentonite use per metre
- Response of the jacking force
- Which ports are accepting material
- Changes after re-lubrication
- Breakaway force after stoppages
Good lubrication is measured by its effect on the drive—not simply by the total volume pumped.
04 — Slurry-System Performance
In a slurry-supported MTBM, the slurry circuit is both a spoil-transport system and part of the face-support system.
As the drive length increases, friction losses inside the slurry pipelines also increase. This affects pump performance, flow, return pressure, face-pressure control and separation capacity.
Main parameters to monitor
- Feed-line pressure
- Return-line pressure
- Excavation-chamber pressure
- Feed and return flow
- Slurry density
- Slurry viscosity
- Sand content
- Pump speed and load
- Tank levels
- Separation-plant performance
- Excavated-material volume
Hydraulic balance
Slurry pumps must be selected using the complete hydraulic calculation, including:
- Straight-pipe friction losses
- Vertical level difference
- Bends, valves and connections
- Changes in pipe diameter
- Slurry density and behaviour
- Solid concentration
- Maximum particle size
- Pump wear and efficiency loss
- Possible requirement for booster pumps
If slurry velocity is too low, particles may settle and block the line. If it is too high, it can increase:
- Pipe and bend wear
- Energy consumption
- Pump loading
- Pressure losses
- Separation-plant loading
The target is not simply the highest possible flow. The target is stable transport without settlement, blockage or unnecessary wear.
Separation capacity
The separation plant must be designed for the actual ground and the maximum planned production rate.
An overloaded or unsuitable plant can cause:
- Increasing slurry density
- Higher pump loads
- Excessive wear
- Reduced transport capacity
- Unstable face-pressure control
- Tank overflow
- Production stoppages
Material balance
The relationship between feed slurry, return slurry and separated material should be continuously checked.
Less return material than expected may indicate:
- Material accumulation inside the chamber
- Partial blockage in the slurry line
- Reduced excavation efficiency
More material than expected may indicate:
- Excessive excavation
- Uncontrolled ground loss
- Face instability
- Increased settlement risk
The purpose of slurry control is not only to remove excavated material. It must maintain face stability, hydraulic transport and separation performance at the same time.
05 — Intermediate Jacking Stations
Intermediate Jacking Stations, or IJS units, divide a long pipe string into shorter jacking sections.
Instead of moving the entire pipe string only with the main jacking station, each IJS advances a specific pipe section in a controlled sequence.
Why are IJS units required?
They are used to:
- Reduce the load on the main jacking station
- Limit the maximum force transferred through the pipes
- Divide accumulated friction into manageable sections
- Protect pipes and joints from excessive compression
- Make longer drives technically possible
IJS location
IJS units should not simply be installed at equal distances. Their locations should be based on:
- Expected face resistance
- Predicted pipe friction
- Pipe and joint capacity
- Main-station capacity
- IJS cylinder capacity and stroke
- Curves along the alignment
- Changes in geology
- Expected lubrication performance
- Required safety margins
The first IJS may require special consideration because it may need to overcome both machine resistance and the friction of the front pipe section.
Activation criteria
An IJS should not be activated only after the main station reaches its maximum capacity. Activation limits should be defined before tunnelling starts.
Possible criteria include:
- Main force approaching a planned limit
- Pipe load reaching a defined percentage of capacity
- Abnormal force increase
- Reaching a planned drive distance
- No reduction after re-lubrication
- Restarting after a long stoppage
- Entering a curve or difficult ground
Late activation may expose the pipe string to unnecessary loading.
Operating sequence
IJS units must operate in a planned sequence:
- The selected IJS opens.
- The front pipe section advances.
- Other IJS units operate in sequence.
- The main jacking station advances the rear section.
- The system is prepared for the next excavation cycle.
An incorrect sequence may cause:
- Pipe-joint opening
- Uncontrolled gaps
- Loading of cables and slurry lines
- Pipe-string locking
- Unbalanced forces in curves
Stroke, pressure, force and operating sequence should be recorded for every IJS.
The Five Systems Are Connected
These controls cannot be managed separately:
- Poor alignment increases friction and jacking force.
- Poor lubrication raises loads on the main station and IJS units.
- High jacking forces increase the risk of pipe and joint damage.
- Low slurry flow reduces excavation performance and face control.
- Long stoppages increase breakaway force.
- Poor separation increases slurry density and pump load.
- Late IJS activation exposes the pipe string to excessive force.
For this reason, all major operating data should be reviewed together.
Minimum Drive Records
For every pipe or jacking cycle, the team should record:
- Advance distance and time
- Machine stoppage time
- Horizontal and vertical deviation
- Pitch and roll
- Cutterhead torque and speed
- Main jacking force
- IJS forces and strokes
- Lubrication pressure and volume
- Slurry feed and return pressures
- Slurry flow and density
- Sand content
- Face pressure
- Separated-material volume
- Ground observations
- Alarms and corrective actions
Conclusion
The success of a long-distance microtunnel is not determined by one piece of equipment.
It depends on the coordinated control of:
- Reliable guidance and alignment
- Continuous jacking-force monitoring
- Effective lubrication along the pipe string
- Balanced slurry and separation performance
- Correctly positioned and timely operated IJS units
Long-distance microtunnelling is more than using a powerful machine. It is the controlled management of several connected systems throughout the entire drive.
Oğuzhan Ataşalar Microtunnelling & NDRC Project Manager · Dubai