Soil freezing

Soil freezing or soil solidification is a temporary reinforcement of water-containing soil by means of ice walls. The concept is to convert pore water into frozen water (ice). The frozen pore water acts as a bonding agent, fusing together particles of soil thus increasing significantly the compressive strength of the frozen soil and make it impervious to water seepage. Frozen ground may be used to provide stabilization, resistance and a watertight barrier during deep excavations, cross passages between tunnels or excavations in urban areas.

Construction sequence

1. In the soil zone to be frozen, borehole drilling is carried out across the full height of the aquifers and spaced according to a defined grid pattern consistent with the shape of the area to be treated.

2. Installation of a series of steel freeze pipes in the bores. These small diameter, closed-end freezing tubes are equipped with so-called drop tubes or smaller, inner supply tubes, open-ended in their lower portion.

3. Initial freezing by the continuous circulation of a refrigerated fluid. The coolant is pumped down the drop tube to the bottom of the freeze pipe and flows up the annulus (space between the two pipes), extracting heat from the surrounding soil.

4. Progressive and radial formation of a frozen zone of soil in the shape of a cylinder around each pipe. The zones enlarge with time and finally merge to form a continuous, stable and impermeable frozen earth barrier. As the freezing process continues and additional freezing occurs, the design thickness is reached.

5. The condition is maintained during the construction phase.

Fields of application

• Presence of sufficient groundwater essential, but no major groundwater flow

• Complex excavations in urban environment, difficult to access and in tight working conditions (limited surface)

• Protection for gallery passage, circular and deep excavations, cross passages between tunnels.

Environmental impact

• Space required for the refrigeration equipment

• Heave or settlement from ground freezing operations may occur (control of pipes or existing structures necessary)

• Proper know-how of the freezing method: strict control of the soil freezing operation, avoid stopping and turning on of the refrigeration unit.

Technical specifications

• Freezing of the groundwater increases the mechanical characteristics of the frozen soil massive allowing to resist the forces generated during the excavation work.

• A cooling medium is circulated through the pipes placed in the boreholes, thus freezing the pore water.

• High cost (borehole drilling and maintaining the freeze wall during the construction works)

• Information on and determination of the thermal conductivity of the soil.