Ground Freezing Combined Method for Urban Tunnel Excavation-Paper

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The Powiśle station project in Warsaw required three connecting tunnels to be excavated below the water table beneath the operating Wislostrada road tunnel. An earlier excavation attempt using jet grouting and forepoling failed, and further grouting caused uplift, creating unacceptable risks to the road tunnel.<br /><br />Artificial ground freezing was selected as temporary support and groundwater control. The work crossed water-bearing sands and silty sands over stiff clay; freezing was extended into the upper part of the clay. A combined system was used: brine formed a vertical frozen wall from within the road tunnel, while liquid nitrogen rapidly froze the tunnel shells from the shafts. Brine then maintained the frozen ground during excavation. The target was a frozen wall at least 1.8 m thick, with temperatures at or below −6°C.<br /><br />Accurate drilling, laser measurement of pipe deviations, hourly temperature readings, and continuous structural monitoring were essential. Despite disturbed ground in one tunnel, the frozen shells were achieved and remained stable during excavation and lining. All three tunnels were completed without water leakage or disruption to road traffic. Settlements measured after completion ranged from 0 to 20 mm. The works took 9.5 months overall.<br /><br />The project demonstrates that combining rapid liquid-nitrogen freezing with economical brine maintenance can provide safe, timely, and environmentally low-impact support for complex urban excavation beneath existing infrastructure.

Keywords

Powiśle station

Warsaw tunneling

artificial ground freezing

liquid nitrogen

brine freezing

underground excavation

water table

road tunnel protection

groundwater control

urban infrastructure