Text: Alejandro Velez

Most of us are familiarized with the word “TBM” (tunnel boring machine) linked to the mechanized excavation of huge and very long tunnels linking distant regions  underground, extending metro lines in crowded cities or usually in projects crossing below famous mountains.

TBMs excavate our worldwide tunnels

Twenty one EPB TBMs excavated the brand new Metro system in Doha in world record time, the Grand Paris Metro Extension just starting with several EPB TBMs, the hard rock shield TBMs making excavation of important hydropower projects in the Andes Mountains in different countries in south America or just the strong  gripper TBMs used to excavate the Gotthard Tunnel in Switzerland, which few years ago became the world's longest rail tunnel, are clear examples of the  important of the TBM technology.

The micro-tunneling technology for smaller TBMs

But these big machines, commonly with diameters from 6 up to almost 17 meters,  have also smaller sisters which are silent excavating the required infrastructure of the Megacities worldwide. Micro-tunneling technology uses smaller TBMs with diameters starting from few centimeter up to 4-5 meters. Micro TBM machines excavate tunnel facilities used for the transport of water, sewage or even electrical lines without disturbing the daily activities on the surface and reducing the  excavation of big trenches. Sometimes these “mini” TBMs and the jobsite facilities are so small optimized that we do not even notice that they are digging on the  street, most probably in front of your house or office, right now.

Sewage water tunnel improves life of 30.000

The city of Buenos Aires in Argentina took advantage of this top technology and  selected the local contractor Coarco SA for the construction of the project “Traza Colector Primarias Ezeiza, Tramo Esteban Echeverria”. The project consists of the installation of bigger pipes and the increase of capacity of the treatment plant in  order to serve and improve the life quality of around 30.000 more people. This  tunnel with a length of over 4km will be used to transport sewage water collected from the houses in the south west of the city to the next treatment plant named “Jagüel”.

Sewage is what we produce in households

Sewage is a type of wastewater that is produced from a community of people. It is characterized by volume or rate of flow, physical condition, chemical and toxic  constituents, and its bacteriologic status. It comes mostly from sinks, tubs,  showers, dishwashers, and clothes washers, flush toilets, soaps and detergents.

The cutting wheel is specially designed for the geology of the project

This Micro TBM, called “Regina Agnese” in honor of the mother of the owner of the construction company and with a diameter of “only” 1.9m uses basically the same technology as the bigger TBMs (in EPB machines the addition of foaming agents in front of the machine is required in order to allow smooth excavation of the  ground). The machine cuts the ground with a cutting wheel specially designed for the geology of the project then the material is being removed by a small screw  conveyor into wagons which are then finally pulled out of the tunnel by a winch. The tunnel itself is been built by connecting single 3m long concrete pipes with a final inner diameter of 1.5m (this is probably the smallest diameter to allow a pilot inside).

Pre-casted concrete segments are installed at the starting shaft

In order to allow the advance of the TBM in the front, and due to the lack of space for additional installation inside, the pre-casted concrete segments are installed at the starting shaft and are pushed from outside at the begin of the  tunnel (and not from inside as in the bigger TBMs where more space is available). Additional facilities usually installed on the back-up of bigger TBMs remain in Micro TBMs outside: push rams, pumps, tanks and other main facilities remain in the  starting shaft or in separated containers in the surface.

Foaming agent Sika® Foam TBM 101 FB for optimizing the TBM process

With an excellent local technical support, Sika Argentina managed to supply the  foaming agent Sika® Foam TBM 101 FB (foaming agent for ground with high to low permeability, polymer modified) optimizing the consumption compared to other competitors, making a better conditioning of the excavated ground and allowing a smooth and constant excavation with TBM speeds over 100mm/min, reducing the amount of wagons needed to remove the material from the front of the tunnel and moreover assuring performance of around 30-40m excavated per day.

For more information on TBM technologies and products, please visit our website.