Freezing Method of Ground Improvement

Basic Function:

• Ground freezing is a process by which the in-situ pore water is converted into ice. Like the cement in concrete, the ice bonds the soil particles together, imparting strength and impermeability to the frozen soil mass.
• Ground freezing is based on the withdrawal of heat from the soil. Continuous energy is usually required to establish and maintain a frozen soil body.
• For the build-up of a frozen soil body either a row or multiple rows of vertical, horizontal or inclined freeze pipes have to be drilled into place. An open-ended inner pipe, sometimes referred to as the down-pipe, is inserted into the center of the closed-end freeze pipe.
• The down pipe is used for the supply of the freeze pipe with a cooling medium, usually brine or liquid nitrogen. In case brine is used, it is also supplied by the inner pipe.

Applications of Ground Freezing:

• Temporary underpinning of adjacent structure and support during permanent underpinning
• Ground support and seepage control during tunnelling
• Large open excavation, Retaining structures for open excavation
• Tunnelling through a full face of granular soil
• Sinking and lining of deep mineshafts up to depth of more than 600 m
• Cross-passages between shafts and tunnel tubes or between tunnel tubes, respectively
• Temporary soil improvement under foundations
• Temporary sealing of leakages
• Temporary water cut-off for connections at the interface between existing and new underground structures
• Strengthening or stabilizing the ground
• Controlling the seepage
• Increase shear strength of soil (20 times of unfrozen soil)
• To collect soil  core sample.

General Description:

A series of freeze pipes are typically drilled and installed around the perimeter of a proposed excavation, usually a shaft or tunnel.  There are two methods of freezing as shown in the figures.  Figure 1 referred to as circulating coolant (brine solution) where a refrigerated fluid is circulated through a closed system.  A mobile freeze plant is typically used to chill the fluid usually less than minus 30oC which is then circulated through the series of pipes and returns to the plant.  Heat is extracted from the ground and the frozen wall forms. The freezing time depends on geological and hydrogeological conditions usually takes six to eight weeks.

Another method as shown in Figure 2, direct expansion, uses a cryogenic fluid such as liquid nitrogen.  The fluid flows through the pipes, with sufficient latent heat, changes state and the gas is vented to the atmosphere. Liquid nitrogen has a temperature of -196oC resulting in the formation of the frozen structure usually in ten to 14 days but the freezing time depends on geological and hydrogeological conditions

With either method, once the frozen earth structure is formed excavation can begin.  It is necessary to maintain the freezing during the excavation until the final structure is completed and the excavation backfilled.

A complex instrumentation system is required to monitor and record ground temperatures, groundwater levels, coolant temperature and flow, as well as freeze plant performance.  If the ground freezing project is in an urban area near structures and utilities heave and deformation measurements are necessary.

Geologic Applicability:

• It is applicable to a wide range of soils, but it takes considerable time to establish a substantial ice wall and the freeze must be maintained by continued refrigeration as long as required.
• Ground freezing may be used in any soil or rock formation regardless of structure, grain size or permeability.
• Ground freezing however is limited by groundwater velocity. Lateral groundwater flow greater than 1m per day introduces heat into the system and can retard, or even prevent the formation of a frozen earth structure.