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.
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.
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. Microtunneling 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 in Buenos Aires
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”.
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).
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.
"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." Alejandro Velez
Sika® Foam TBM 101 FB selected to optimize 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.