Microtunnelling,
explained with evidence.

Microtunnelling is a remotely controlled and guided pipe jacking operation that continuously supports the excavation face. A microtunnel boring machine excavates while hydraulic jacks in the launch shaft push the machine and the jacking pipe string forward. Routine personnel access to the excavation face is not required.

NDRC means Non-Disruptive Road Crossing. In project usage, it describes crossing works that install a utility beneath a road without excavating a continuous open trench across the carriageway. NDRC is an umbrella project term rather than one excavation technology: the approved method may be microtunnelling, pipe jacking, auger boring, HDD or another trenchless system, depending on the design and authority requirements.

Not exactly. Pipe jacking describes the installation principle: purpose-designed pipes are pushed through the ground by hydraulic jacks as excavation proceeds at the face. Microtunnelling is a specialised form of pipe jacking defined by remote operation, active guidance and continuous face support. Therefore, microtunnelling normally uses pipe jacking, but not every pipe jacking operation is microtunnelling.

No universal modern definition is based on one fixed diameter. Terminology has varied by country and over time; FHWA notes that North American usage applies microtunnelling to remote-controlled pipe jacking across diameters. The more reliable technical distinction is the operating system—remote control, guidance, steering and face support—not diameter alone.

It is commonly considered where a pipeline must pass beneath busy roads, railways, buildings, waterways or congested utilities; where deep excavation would create major disruption; or where a gravity pipeline requires accurate line and level. It reduces continuous surface excavation, but it still requires suitable launch and reception shafts, work areas and a project-specific engineering assessment.

Selection should be based on a project-specific ground investigation and an interpreted geotechnical model. Relevant information includes soil and rock strata, strength and abrasivity, particle-size distribution, permeability, groundwater level and pressure, obstructions, cobbles or boulders, mixed-face transitions, contamination and nearby assets. Boreholes alone are not enough if their spacing or testing does not represent the tunnel horizon and shafts.

Yes. Closed-face slurry machines are widely used in water-bearing and unstable ground. In a slurry system, controlled pressure in the excavation chamber is used to balance groundwater and ground pressure while excavated material is transported to the separation plant. Successful operation still depends on suitable face-pressure limits, shaft sealing, slurry hydraulics, separation capacity and contingency planning.

Settlement risk is managed by maintaining face stability and limiting ground loss. The principal controls include correct face pressure, controlled excavation and spoil volume, suitable overcut, stable guidance and steering, effective annular lubrication, appropriate advance rate and continuous comparison of operating data with surface and subsurface monitoring. No single reading proves that settlement is under control; the system and trends must be assessed together.

The launch shaft must safely accommodate the jacking frame, reaction system, pipe handling and machine launch; the reception shaft must permit controlled breakthrough and recovery. Shaft design depends on depth, plan dimensions, ground and groundwater, imposed loads, stability, watertightness, access, lifting operations and whether the shaft becomes part of the permanent works. The shaft, tunnel and eye-sealing arrangements must be designed as one system.

Total jacking force is governed mainly by resistance at the machine face and friction along the machine and pipe string. It is influenced by pipe diameter and drive length, ground behaviour, overcut, lubrication, groundwater, alignment, joint deflection, stoppages and ground closure around the pipe. Design must check both the available jacking capacity and the allowable load of pipes, joints, packers and the shaft reaction structure.

Lubricant is injected into the annular space around the jacking pipes to reduce ground-to-pipe friction and help support the excavated annulus. It is separate from the slurry circuit used for face support and spoil transport, even when both systems contain bentonite. Mix design, hydration, injection locations, pressure, volume and re-lubrication strategy must be matched to the ground and drive.

An IJS, also called an interjack, is installed within the pipe string to divide a long drive into shorter jacking sections. When activated in a controlled sequence, it reduces the force that must be transmitted through the complete pipe string from the main jacking station. Its location, capacity, activation criteria and operating sequence must be designed in advance; adding an IJS does not compensate for poor lubrication or unsuitable pipe design.

There is no single maximum length that is valid for every project. Practical drive length depends on diameter, ground and groundwater, alignment, pipe and joint capacity, predicted friction, lubrication, main-jack and IJS capacity, slurry-pump hydraulics, separation performance, guidance range, shaft logistics and intervention or recovery strategy. The allowable length must therefore be demonstrated by integrated project-specific calculations and risk assessment.

Yes, suitable systems can construct designed curves. Feasibility depends on machine articulation, pipe length, joint angular capacity, pipe outside diameter, minimum curve radius, guidance technology, ground response and the effect of curve contact forces on jacking loads. Curves must be designed into the drive; they should not be treated as a method for correcting an unsuitable alignment during construction.

The exact list depends on the machine, but a controlled drive normally records position and deviation, pitch and roll, steering-cylinder status, advance rate, cutterhead torque and speed, main and intermediate jacking forces, face or chamber pressure, slurry feed and return pressures and flows, slurry properties, lubrication pressure and volume, spoil balance, stoppages and settlement instrumentation. Trends should be compared with the design control limits, not reviewed as isolated numbers.

The response depends on the obstruction, ground stability, groundwater pressure, machine access provisions and the approved contingency plan. Options may include controlled crushing, tool intervention, ground treatment, a local recovery shaft or redesign; none is universally suitable. The correct approach is to stop uncontrolled advance, stabilise the system, review verified data and implement an engineered method that protects people, the ground and nearby assets.

No. Remote excavation removes the need for routine personnel at the face, but underground construction still includes significant hazards: shafts and lifting, stored hydraulic energy, pressurised slurry, electrical systems, confined spaces, groundwater, flooding, ground instability, plant movement and possible intervention work. Safety must be designed into the method, supported by competent supervision, monitoring, emergency planning and the applicable local regulations.