Two Kinds of Ground Freezing Techniques for Tunnel and Open Excavations

Ground freezing is a geotechnical method that solidifies in situ pore water into ice in order to fuse subsurface soil and rock particles together. Peripheral freezing is used to create a stable and impermeable wall a little distance around large open excavations and horizontal tunnels to cut off groundwater, prevent landslides, and reduce the amount of frozen ground to be removed.

Two techniques are used in order to freeze the ground: brine freezing and liquid nitrogen freezing. Depending on the type and length of the excavation project, geotechnical engineers may use any of these methods to temporarily solidify the ground surrounding the worksite.

Brine freezing is applied for large, long-term projects. The technique uses heat extraction to reduce ground temperature. Calcium chloride brine is chilled between 5ºF and -13ºF before it is pumped into a drop tube within a freeze pipe installed in the ground. The coolant then absorbs ground heat as it flows upward through the pipe. The brine is then taken to a refrigeration plant where it is cooled before it is recirculated.

The freezing process takes six to eight weeks for smaller diameter excavation sites, and 10 to 12 weeks for larger ones. As soon as ground temperature reaches approximately 32ºF (0ºC), the temperature is slightly reduced to maintain the freeze. Liquid nitrogen freezing, on the other hand, is used for short-term projects and emergency situations where there is loose soil or disturbed ground conditions. Unlike brine, liquid nitrogen is not recycled; as the coolant is pumped into the drop tube, it immediately boils at -320ºF, and exhaust gas escapes into the atmosphere through vents. Because of its extremely low temperature, liquid nitrogen accelerates the freezing process, creating a very strong frozen wall in just a few days.

Reliable geotechnical contractors can provide ground freezing services for open and tunnel excavations.