Tunnel boring machines (TBMs) are commonly found with diameters from five to 17 meters. There are also microtunnel boring machines which are smaller, with an excavation diameter up to four meters. Today, they are extensively used to construct many kilometers of underground infrastructure worldwide. Microtunneling is a fast alternative to conventional pipe installation systems, since it is a fully automated method of boring and pipe installation without the need for disturbing the surface.
Micro TBMs have diameters ranging from a few centimeters up to four meters across. They are typically used for excavation of many vital utility facilities such as fresh water, wastewater, oil, gas, communications and electricity.
Projects with micro TBMs do not require trench excavation work. It is a technique especially suited for minimizing disruption in densely populated areas, crossing difficult geological zones, penetrating through ground with high water levels, under rivers or generally overcoming obstacles where room for construction is restricted. The use of microtunnel boring machines is increasing, especially because it has less environmental impact compared to traditional excavation methods.
Microtunneling is a fast method of continuous excavation with minimal interruptions.
Microtunneling Project Considerations
Main parameters to be considered when planning a microtunneling project include:
- Size of the tunnel
- Length
- Alignment
- Location and depth of shafts
- Entry and exit points
- Groundwater
- Geology (even the most complex geologies can be excavated over long distances, depending on the selected technology)
There are primarily two different types of microtunneling machines - slurry machines and earth pressure balance (EPB) machines. It is possible with both to excavate through most ground conditions including silt, clay, sand, gravel and even hard rock.
Slurry machines inject a bentonite suspension into the face to reduce settlements on the surface. In slurry machines, the excavated ground is mixed with the suspension and pumped out of the tunnel to a separation plant. This type of machine is mainly used for silt and cohesive soils and can also be used in gravels and rocks.
EPB machines do not need the injection of bentonite during excavation but require the conditioning of the ground, using foaming agents and sometimes polymers, before removal with a screw conveyor. In EPB machines the excavated ground is collected in a chamber and transported out of the tunnel mainly in wagons pulled by a winch, short trains or pumped with a thick matter pump. This micro tunnel boring machine is mainly used in cohesive soils.
1. Pipes and Concrete Segments
Concrete pipes and segments for tunnels must be made to a high quality with no surface imperfections. The concrete is often required to have high early strength for fast production and increased durability to extend the service life.
Sika® ViscoCrete® is a superplasticizer which improves the concrete workability and increases the concrete strength, while SikaRapid® is used to control the hardening and setting process.
Sika® Separol® release agents are used to ensure easy demolding and improving the concrete surface.
In specific cases, where there are aggressive agents in the ground that could attack concrete, Sikagard® coating systems provide a higher level of concrete protection.
4. Injections
Before excavation work starts, it may be necessary to improve the condition of the ground in order to avoid groundwater entering the shaft, as the machine penetrates the sealing ring. In specific cases, it may be necessary to stabilize the ground to avoid surface settlements.
Within the Sika® Injection and SikaFix® portfolio, there are specific products for ground treatment.
5. Sealants
Instead of installing pipes for large diameters, concrete lining segments are fixed inside the tunnel to form the rings.
Sika® tail sealants are pumped between the rows of brushes in the gap between the tail-skin and the lining segments to prevent water, soil or backfilling grout from entering the machine.
6. Foams
Not all ground is ideal for excavation with a micro EPB TBM. Foaming agents are sometimes necessary to condition the ground in front of the machine to achieve better advance rates.
Sika® foaming agents are specially developed to fulfill the different requirements during excavation, making it easier for micro EPB machines to excavate through the ground.
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Author
Alejandro Velez
Corporate Product Engineer
TBM / Infrastructure / Mining
Target Market Concrete
Sika Services AG