The excavation of the tunnel for the new Sofia Metro line begins on April 6. The tunnel boring machine (TBM), alternatively known as a mole, will dig the section of the tunnel spanning from the Nadezdha interchange until the Patriarch Evtimii and Vitosha Boulevards junction.The mole will have to excavate a total of 3.8km of tunnel, and according to the Sofia municipality, the machine will process 9.4 metres of earth a day at an average depth of 15 metres under the surface. Authorities say that the machine will work around the clock.The new metro line tunnel must be excavated in full by July 2011.The Sofia Metro mole was constructed between February and December 2009 at a cost of 15 million euro by German factory Herrenknecht, in Baden-Wurttemberg province. It took two months for the mole to be transported to Bulgaria, and another two-and-a-half months for it to be assembled, Dnevnik daily reported.The moles are designed to bore through hard rock and sand and just about anything in between. Tunnel diameters can range from a metre (done with micro-TBMs) to almost 16 metres to date. The Bulgarian one is among the biggest produced, according to Stroitelstvo, or of the Earth Pressure Balance (EPB) type - specifically made for "softer soil with high water level" as are the predominant conditions under the capital.Public tender procedures for the third Sofia Metro line between the boroughs of Levski and Kniazhevo are currently ongoing.