Delhi Metro Rail Corporation has been entrusted with the task of providing an MRTS network to the city of Delhi and National Capital Region. The network is partly at grade, elevated and underground. At present, 190 km of metro network is operational out of which 48 km is underground. In Phase III also, 45 km of section is planned to be underground. The underground network comprises stations built by cut and cover method, while tunnels are constructed by Cut & Cover, NATM (New Austrian Tunnelling Method) or earth pressure balance or slurry type shield machine depending on the geological strata of the project. Selection of TBM depends on ground conditions, surface features and dimension of the tunnel. DMRC used 14 tunnel boring machines in Phase-II to construct the tunnel of finished diameter of 5.8 meter.
Soil InvestigationThe geological strata of Delhi varies along the project alignment and consists of compacted alluvium soil which is also known as Delhi silt, this is generally a fine grained material consisting of different variation between clay and silt with variable fine sand content. The strata consist of coarse sand, gravel and kankar. Soil investigation was carried out throughout the tunnel alignment before the start of the project to finalise the type of machine to be deployed for the project. The cross section of the tunnel varied in level above and below the ground water table. The overburden was between 3.5 to 22 meter as water level varied from 4 metre to 18 metres.
Earth Pressure Balance (EPB) Tunnel Boring Machine (TBM)Depending on the geological parameter and construction programme tunnel boring machines of M/s Herenknecht, Mitsubishi, Kawasaki and OMC etc. were selected for the project. The time taken for the delivery of the machines ex-works to site arrival was around 10 to 12 month (manufacturing time being 8-10 months and shipping and road transportation of 2-3 months).
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