Survey of the design of back anchors for tunnel boring machines (TBM), used in the excavation of metro tunnels in Sofia

摘要

In the year 1974 the Minicipality of Sofia approved the city plan containing the intention to build a metro system in Sofia. The construction works commenced in the year 1982. The route for Sofia metro Line-1 Extension Project from Sveta Nedelya Square to Vassil Levski Stadium was financed with a loan from Japan International Cooperation Agency(JICA) and was carried out by the Japanese Contractor Taisei Corporation, under contract with the Sofia Minicipality.For this route, with length of 2,4 km, two metro stations, two ventilations shafts and one start shaft, for strengthening of this deep excavations was design and used more than 610 tons temporary steel structures to keep the R.C. wall and pilot in vertically position. Contemporary machine methods of the tunnel excavation in the soft ground in Sofia was used for this route. The machine excavation is executed as a full-face excavation, employing tunnel-boring machines (TBM), with diameter 5820 mm. The paper provides a review of the design approach for three types of temporary steel-frame structures, called "back anchors", used to support the TBM in the beginning of the drilling process, before the TMB enters the tunnel. The paper begins with the choice of structural configuration of the back anchors, used in the excavation of metro tunnels in Sofia. The construction process, the materials used, the static analysis methods and, the monitoring of the structure during the TBM start are also described.