What is a microtunnel?
The term microtunneling is defined as a trenchless construction method for installing pipelines.
ASCE-Design and construction guidelines for microtunneling
The term microtunnelling is generally applied to small-diameter tunnels and pipelines installed by pipe jacking methods behind a remotely controlled tunnel boring machine.
BTS-Specification for tunneling
Originally, the term microtunneling was used to describe tunneling for tunnel diameters for which a person was not allowed to enter the tunnel (typically less than 0.9 m or 1.0 m). The term is now used also to describe the method (i.e. not requiring a person to enter) rather than a specific diameter range. Regional differences remain in the usage of the terminology
ISTT
From the technological point of view
A more detailed definition of this concept tells us that, for a tunnel to be considered as a microtunnel, regardless its size, it must keep to the following requirements (ASCE-Design and construction guidelines for microtunneling):
• Remote control
• Guidance
• Pipe jacking
• Continuous face excavation support
From the dimensional point of view
Another acceptable definition of microtunnel, with widespread use in Europe, is focused on the excavation dimensions, mainly the diameter, and the final uses for the pipelines, setting aside the methods and machinery employed.
This microtunnel concept is based on the fact that this is an specific underground linear infrastructure mainly applied on tunnels for water transport, sewage systems, oil&gas and that it excludes railway and road tunnels. The mentioned uses determine primarily the dimensions of tunnels because usually they have lower diameters than 4m so they can be considered as microtunnels.
Microtunnels
Usually the microtunnels comprise a launching or entrance shaft and an exit shaft. From the launching shaft, the tunnel and machine can be visited, if the tunnel size allows that, and there is where some of the main auxiliary equipment for the excavation is located. On the other hand, the exit shaft is the ending of the pipeline excavation and there the machine is recovered, lifting it by means of cranes after it gets disconnected from the tunnel excavation utilities.
The proper design of a microtunnel should bear in mind that the previous mentioned shafts have their own needs, as well as the following issues as described by the ASCE:
• Geotechnical and groundwater conditions
• Potential encountering objects along the alignment
• Environmental conditions
• Available work space inside and around the shafts
• Required and feasible drive lengths
• Pipe diameters
• Cost and construction schedule
• Alignment (straight and curved)
• Site access
• Depth
• Grade
• Tolerances
• Effects on surface activities;
• Location of existing, abandoned, or proposed utilities
• Rights-of-way
• Location of existing, abandoned, or proposed structures