Microtunneling

Microtunneling is a digging process that uses a remotely controlled microtunnel boring machine (MTBM) combined with the pipe jack-and-bore method to directly install pipes underground in a single pass.

This process avoids the need to have a long stretch of open trench for pipe-laying.
Microtunneling includes these steps:

• Microtunnel ShaftExcavate launching and reception shafts at opposite ends of the tunneling drive.
• Hydraulic jacks in the launch shaft push a microtunnel boring machine (MTBM) into the earth.
• Pipes containing slurry water transport excavated spoils to the surface
• Retract the jacks and disconnect slurry lines and control cables.
• Lower a pipe or casing into the shaft and insert it between the jacking frame and the MTBM.
• Reconnect slurry lines and control cables and advance the MTBM another drive.
• Repeat the process until the MTBM reaches the reception shaft.
• Retrieve the MTBM and trailing equipment.

In addition to a hydraulic jacking system to advance the MTBM and pipe, most microtunneling operations require a lot of equipment on the construction site, including:

• A closed loop slurry system to transport excavated spoils
• A slurry cleaning system to remove spoils from the slurry water
• A lubrication system to lubricate the exterior of the pipe during installation
• A guidance system to provide line and grade control
• An electrical supply and distribution system to power equipment
• A crane to hoist pipe sections into the jacking shaft
• Trucks and loaders to transport spoils off site

MTBMs have a rotating cutting head to excavate the ground material, a crushing cone to crush larger particles into smaller sizes for transport through the slurry lines, a hydraulic or electric motor to turn the cutting head, a pressurized slurry mixing chamber behind the cutter head to maintain face stability, an articulated steering unit with steering jacks for steering corrections, various control valves, pressure gauges, flow meters, and a data acquisition system.

Additionally, the MTBM has inline cameras to relay information to the operator and a target system for guidance control. Precise control of line and grade is accomplished using the guidance system and steering jacks to locate and steer the MTBM during a microtunneling drive.

The guidance system usually consists of a reference laser mounted in the jacking shaft, which transmits a beam onto a target mounted inside the articulated section of the MTBM. This and other operational information is transmitted through wire cables to a control cabin located on the surface.