In our sequence of articles dedicated to the basic types of tunnel construction methods utilized in the Sofia Metro Lines, we have already discussed Open, Milan and New Austrian method. Besides those 3 types, there is another wide-spread construction technique, the so-called Shield Tunneling Method. This construction technique is suitable for central urban areas with heavy traffic, tunnels with considerable length, areas with archeological remnants, and where it is necessary to avoid large excavation works.
Description and Application
Shield Tunneling Method, also called Bored Tunneling is construction technique for mechanical boring of the soil and rock strata. For the particular geology conditions of this project, is used Shielded Tunnel Boring Machine, consisting of rotating cutter head, followed by main bearing, thrust system and trailing support mechanism. The machine excavates the soil and then erects concrete segments to support the tunnel walls behind. The machine moves forward with a system of hydraulic jacks, firmly attached to the ready construction of the tunnel. Due to the significant depths of the tunnels, this was the method of choice for the parts of the metro route from ‘st.Nedelya square’ to ‘Vasil Levski Stadium’, from ‘Nadezhda Road Junction’ to ‘str. Han Asparuh’, and the central section of line 3, from ‘Little City Theatre Off The Channel’ to ‘Krasno Selo District’.
Due to the deployment of the tunnels under the level of ground waters, it is envisaged for both of the sections to be used shields with hydraulic counter pressure in the face chamber, thus restricting the deformations of the soil and the negative effect on the buildings above. Besides the constant pressure control in shield of the face chamber, additional safety measures were applied in many places. Some of them are injecting cement or chemical compounds to strengthen the soils, increasing the pressure of the face chamber to compensate for additional loads over the tunnel caused by heavy duty equipment, reinforcement of entry and exit shafts of the Boring Machine at each station, etc.
Tunnel Boring Machine
It is curious fact that due to the extreme weight of the Tunnel Boring Machine – over 1400 tons, it was loaded on ships and transported from the German manufacturer to port ‘Belene’, then loaded heavy duty trucks to be delivered in Sofia.