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U-Bahn-Strecke A IV, Baulos 81

Frankfurts underground Metro Line A IV, which is to connect the Southern township Sachsenhausen to the city, required a tunnel crossing the river Main. In this area the underground consists mostly of miocene clay with embeddings of sand and silt and limestone rock layers as well. River bottom was practically waterproof and without connection to the ground water level. Further difficulties have been the close neighbourhood of Undermainbridge (very sensitive against settlements) and the permanent necessity to keep clear waterway and flood cross-section. Tunnel heading in two separated tubes was processed in NATM. Ground water level was lowered by gravity wells, which have been bored from a floating platform. Well heads became embedded in the river bottom. To protect the heading front against water inflow from the river, the soil around the heading crown was frozen. The freezing plant could supply 25 m³/h Calcium chloride with a temperature of - 40 °C. Tunnel excavation went forward in partitions of ca. 30 m length, at the end on a length of 8 m with extended cross-section. From this cavern 14 freezing tubes have been bored forward using a special support scaffold. Exact position of all borings have been controlled by special measuring equipment, intolerable deviations caused additional borings. Separately bored tubes with temperature sensors made freezing control possible. Partially additional grouting of layers with floating groundwater was necessary to get a continuous area of frozen soil. During heading process a 17 cm thick initial lining of shotcrete has been applicated. The final lining consists of 40 cm thick reinforced waterproof concrete.

 

  • Country: Germany
  • Region: Hessen
  • Tunnel utilization: Traffic
  • Type of utilization: Underground, urban and rapid transit
  • Client: Stadt Frankfurt, Stadtbahnbauamt
  • Main construction method: Trenchless
  • Type of excavation: Drill-and-blast
  • Lining: Shotcrete
  • No. of tubes: 2
  • Tunnel total length: 291 m
  • Cross-section: 38.5 m²
  • Contract Volume: 31.35 Mio. DM
  • Construction start/end: 15. 5. 1976 till 12. 4. 1979