Microtunneling is a trenchless construction method used to install small underground pipes without digging a long open trench. A remote-controlled boring machine cuts through soil while pipes are pushed in behind it from a launch shaft. This matters in cities because roads, sidewalks, utilities, and traffic can stay mostly undisturbed.
It is commonly used for sewer, water, storm drain, and utility tunnels.
Understanding Construction Machines: Microtunneling
Work begins with a ground investigation, not with the cutting machine. Engineers need to know the soil layers, groundwater level, buried cables, old foundations, and nearby building loads. Clay, sand, gravel, and rock behave very differently under pressure.
Loose wet sand can move into the tunnel face if pressure is too low. Excess pressure can lift the ground above the tunnel. The machine must keep the soil at its front stable while it moves.
Engineers choose the pipe size, cutting tools, face pressure, and route from this information. They also plan the launch and reception work areas carefully because these locations carry large loads from the pipe jacks.
Guidance is one of the most important parts of the job. A laser in the launch area can shine toward a target inside the machine. The operator sees whether the machine is above, below, or to one side of the planned line.
Steering cylinders make small corrections by changing the direction of the cutting head. These corrections need to be gentle. A large correction can create a curved tunnel or make the pipe joints work unevenly.
This is especially important for gravity sewers. Their slope must remain very close to the design level so wastewater flows by gravity rather than collecting in low spots.
Grade error means actual elevation minus design elevation. Even a small error can matter over a long pipe run.
Many microtunneling machines use slurry to move excavated material away from the cutting face. Water mixed with fine particles forms a flowing slurry. Pumps send this mixture through pipes to the surface.
At a separation unit, screens and other equipment remove soil particles so much of the fluid can be used again. The slurry system helps control pressure at the tunnel face. It must be watched closely for changes in flow, density, and pressure.
A blockage can stop excavation. A leak can send fluid into surrounding soil.
In difficult ground, workers may inject a slippery material around the outside of the pipe. This reduces friction between the pipe and soil.
The jacking system must overcome that friction while keeping the pipe safe. Each new pipe section adds length and surface contact, so the needed pushing force often rises as the drive continues. Stress equals force divided by area.
Engineers use this idea to check that pipes, joints, and jacking equipment can carry the load. They may add intermediate jacking stations on longer drives to spread the force. Students should pay attention to the link between measurements and decisions.
Advance rate equals tunnel length divided by boring time, but a fast rate is not automatically good. A slower drive may be necessary when ground conditions change, guidance needs correction, or slurry pressure becomes unstable. Safe work depends on accurate monitoring, patient adjustments, and clear communication between the operator, survey team, and surface crew.
Key Facts
- Microtunneling uses a remote-controlled microtunnel boring machine, often called an MTBM, to excavate soil from a launch shaft to a reception shaft.
- Jacking force pushes pipe segments forward from the launch shaft as the cutting head advances.
- Average advance rate = tunnel length / boring time.
- Grade error = actual elevation - design elevation, and small errors can affect gravity-flow pipes.
- Pipe stress can be estimated with stress = force / area, or σ = F / A.
- Slurry systems carry excavated soil away by mixing it with fluid and pumping it back to the surface for separation.
Vocabulary
- Microtunneling
- Microtunneling is a trenchless method for building small underground pipe tunnels using a remote-controlled boring machine.
- MTBM
- An MTBM, or microtunnel boring machine, is the cutting and steering machine that excavates the tunnel face underground.
- Launch shaft
- A launch shaft is the starting pit where the boring machine and pipe-jacking equipment are placed.
- Pipe jacking
- Pipe jacking is the process of pushing pipe sections into the ground behind the boring machine using hydraulic force.
- Slurry
- Slurry is a mixture of water and excavated soil that can be pumped out of the tunnel during boring.
Common Mistakes to Avoid
- Thinking microtunneling is the same as digging a trench, which is wrong because the pipe is installed underground between shafts with little surface excavation.
- Ignoring steering and grade control, which is wrong because even a small vertical error can prevent sewer or drainage pipes from flowing correctly.
- Assuming the boring machine pulls the pipe, which is wrong because hydraulic jacks usually push the pipe segments forward from the launch shaft.
- Forgetting soil removal, which is wrong because the cut soil must be carried away by slurry, augers, or another removal system to keep the tunnel advancing.
Practice Questions
- 1 A microtunneling crew installs 96 m of pipe in 12 hours of boring time. What is the average advance rate in m/h?
- 2 A hydraulic jacking system applies a force of 1,200,000 N to a pipe with a contact area of 3.0 m². What is the average stress on the contact area using σ = F / A?
- 3 Explain why microtunneling is useful under a busy road compared with open-trench construction. Include at least two effects on the surface.