The driving of metro tunnels at helsinki with the aid of ground freezing

Twin 6.5 m diameter tunnels were driven through saturated soil at a depth of 25 m under a street intersection with the aid of horizontal freezing. The frozen soil section was about 30 m long. Because of groundwater pressure the horizontal freeze holes were drilled with sealed casings. A new drill bit was introduced to form a tight plug at the end of the casing. The soil around the perimeter of the tunnel bores was frozen by vaporizing freon in the freeze tubes. A 2.5 m thick ice wall in sand and moraine was developed during the freezing period of 55 days. The tunnel bores were excavated under protection of the frozen arch by cautious drilling and blasting. The advance was 1.2–1.8 m per round. Permanent tunnel linings were constructed by bolting together cast iron segments.