Artificial ground freezing is a geotechnical technique used in constructing shafts, mines, and tunnels to provide temporary earth support and groundwater control when other conventional methods, such as dewatering, shoring, grouting, or soil mixing, are not feasible.
Common uses
Overview
Ground freezing also provides regional groundwater barriers around mining operations for gold and other minerals, oil sands, or oil shales. It is often called ground freezing, soil freezing, or freeze wall. Ground freezing involves drilling, installing a series of relatively closely spaced pipes, and circulating a coolant through these pipes. The refrigerated coolant extracts heat from the ground, converting the soil pore water to ice, resulting in an extremely strong, impermeable material. It is the most positive ground improvement method used in the underground construction and mining industries.
The freezing process is an entirely closed system. No chemicals are injected into the ground. Coolants can be environmentally friendly glycols, calcium chloride brine, or liquid nitrogen. These coolants are chilled with different electrically powered refrigeration (freeze) plants. In the case of liquid nitrogen, the liquid is delivered to the project site in tankers and vented to the atmosphere immediately after circulating through the pipes.
Once the frozen earth structure is formed, excavation or mining may commence. The freezing is maintained until the project is completed. After completion, the frozen soil thaws, and the groundwater regime returns to its original state.
Why choose Keller for ground freezing?
Keller is a global leader in ground freezing solutions, offering unmatched expertise and innovation in geotechnical engineering. Our team has successfully implemented ground freezing across various projects from urban infrastructure to energy and transportation sectors.
We tailor each ground freezing system to site-specific conditions, ensuring optimal performance, safety, and minimal environmental impact. Whether dealing with soft soils, high groundwater pressure, or sensitive adjacent structures, Keller’s ground freezing solutions deliver reliable results.
Applications of ground freezing
Deep shafts are the most common application of ground freezing. The freeze pipes are drilled and installed around the perimeter of the proposed shaft to the required depth, typically through an impermeable rock or soil stratum to ensure base stability. Liquid refrigeration, typically calcium chloride brine, is circulated through these close pipes, extracting heat from the ground and forming the frozen earth structure.
Once the structure is formed, excavation of the unfrozen material in the shaft center can commence. There are two approaches to excavating and lining the shafts. On shafts extending to 100m or less, the excavation proceeds to the shaft invert while insulating the surface of the frozen ground with polyurethane foam. Typically, the insulation is applied in 3m intervals.
On deeper shafts, those extending to 600m or more, the fifinal lining is installed as the excavation progresses. A specially designed Galloway facilitates excavation at one level while completing the lining on a higher level. The excavation and lining process continues to the shaft invert. In both approaches, the ground freezing system remains operational until the concrete for the final lining has cured and reached design strength.
Ground freezing is used extensively in the tunneling industry. Tunnel applications use several different approaches. The most common involves drilling the freeze pipes around the tunnel perimeter horizontally, similar to the frozen shaft approach. This horizontal configuration is used to tunnel beneath roads or railways or to construct safety cross passages between two existing tunnels when tunnel lengths are 40m or less.
If the horizontal requirements exceed 40m, a vertical approach is used to freeze the entire alignment solid and mine through a frozen soil mass. Mining then proceeds through the frozen mass using Sequential Excavation.
Methods. Using this approach, the length of the tunnel that can be frozen is unlimited. As with the shafts, once a lining is installed that can withstand both earth and hydrostatic loads, the freezing system can be turned off. The termination of the freezing can occur in sequence with the mining operations.
Ground freezing is also used with Tunnel Boring Machines (TBMs). The ground in front of or around the TBM can be frozen in advance to create a pre-planned safe haven for tunneling interventions or used in emergencies for TBM repair.
Ground freezing has been proposed for regional groundwater barriers up to 10km long to isolate groundwater from mining operations instead of large-scale dewatering operations with environmental consequences or requiring complex and expensive treatment operations.