The International Society for Trenchless Technology

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Pipe Jacking & Microtunneling
Microtunneling is a trenchless construction method used to install pipelines beneath highways, railroads, runways, harbors, rivers, and environmentally sensitive areas.  Microtunneling is defined as a remotely-controlled, guided, pipe-jacking operation that provides continuous support to the excavation face by applying mechanical or fluid pressure to balance groundwater and earth pressures. Support at the excavation face is a key feature of microtunneling, distinguishing it from traditional open-shield pipe-jacking. 

Microtunneling requires jacking and reception shafts at the opposite ends of each drive. TPipe Jaking & Microtunnelinghe microtunneling process is a cyclic pipe jacking operation.  A microtunnel boring machine (MTBM) is pushed into the earth by hydraulic jacks mounted and aligned in the jacking shaft.  The jacks are then retracted and the slurry lines and control cables are disconnected.  A product pipe or casing is lowered into the shaft and inserted between the jacking frame and the MTBM or previously jacked pipe.  Slurry lines and power and control cable connections are made, and the pipe and MTBM are advanced another drive stroke.  This process is repeated until the MTBM reaches the reception shaft.  Upon drive completion, the MTBM and trailing equipment are retrieved and all equipment removed from the pipeline.

Most microtunneling operations include a hydraulic jacking system to advance the MTBM and pipe string, a closed loop slurry system to transport the excavated spoils, a slurry cleaning system to remove the spoil from the slurry water, a lubrication system to lubricate the exterior of the pipe string 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, and various trucks and loaders to transport spoil 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 in-line cameras to relay information to the operator and a target system for guidance control.

Pipe Jaking & Microtunneling

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. 

Microtunneling machines are capable of independently counter-balancing earth and hydrostatic pressures.  Earth pressure is counter-balanced by careful control of advance rates and excavation rates of spoil materials.  Groundwater pressure is counter-balanced by using pressurized slurry in the soil-mixing chamber of the MTBM.

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by pavoneggi