Record rate on Prague metro27 June 2011
The TBMs requisitioned to excavate the Prague metro fifth extension, Line A, achieved a record advance rate on Wednesday, a spokesperson for Czech contractor Metrostav told T&T. The day shift advance of six 1.5m rings, combined with a seven-ring night time advance resulted in an advance rate of 18.5m over 24 hours, significantly higher than the average anticipated rate of 12m per day.
The northwest extension beyond the existing Line A extremity, Dejvicka Station, will contain four stations, Cerveny, Vrch, Veleslavin Petriny and Motol to be excavated by NATM, which will also be used to excavate the double-track running tunnels to Motol Station. The other tunnels, both of 4.05km, were designed to be single track to be driven by EPBM. Metrostav said this method was chosen as it has less impact, as well as being safer and quicker.
The 6.08m-diameter Herrenknecht full-face shield EPBMs, named Tonda and Adela by young patients of the Clinic of Paediatric Surgery at Motol, were equipped with 17 twin-disc cutters, as well as four single-disc gauge cutters that can be extended to increase the excavation diameter to 6.1m. The cutterhead was designed to allow the switching of cutter discs with soft ground rippers, which were equipped as T&T went to press. Three crews of eight were employed to operate the machines. Muck is transported via a screw conveyor to a belt conveyor behind the tunnel.
Excavation of the single-track running tunnels kicked off in April following machine assembly in prepared beds at the bottom of a 21m-diameter, 34m-deep shaft braced by secant piles. The shaft was located at the BRE1 construction site to the east of the Vypich intersection. Tonda began the drive on the left-hand tunnel tube from the drive direction. Adela will proceed, with a three-month lag time, in July.