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.
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.