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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. The 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.

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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 theMTBM. 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.