Principle of ground freezing

    Ground freezing technology makes use of the properties of artificially frozen ground. To achieve this, freeze pipes are bored or driven into the ground in specified intervals. A refrigerant is passed through the freeze pipes absorbing the heat of the surrounding ground as it circulates. This leads to the formation of cylindrical frost bodies surrounding the freeze pipes which gradually coalesce to form frozen walls or slabs. As a rule, frost bodies can be produced in any shape by placing the freeze pipes in the appropriate positions. The frost bodies are monitored by temperature sensors placed within the frozen body or in the ground adjacent to the frozen area.

    Freezing with brine

    For artificial ground freezing with brine, a saline solution, usually calcium chloride is used as the refrigerant. The brine is cooled to temperatures of -30 to -40°C by freezing units and then pumped through an insulated circuit to the freeze pipes and back. The cold brine flows through the freeze pipes withdrawing heat from the soil around the pipe. With progress of time, cylindrical frost columns grow around the freeze pipes. These columns connect and form frozen walls or plates. The layout of the freeze pipes defines the geome-try of the frozen area.

    Depending on various factors affecting the freezing phase, this process can take up to 20 to 40 days to complete. Expansion as well as the temperatures within the frozen area are measured using temperature measuring pipes which are fitted with sensors. The collected data is recorded and stored on an online platform and can be accessed at all times. Max Bögl owns all the necessary equipment for brine freezing, including cooling towers, storage tanks for brine, pumps and insulated pipes of various dimensions.

    Compared to freezing with liquid nitrogen, brine freezing is more cost-effective for frozen areas with large volumes and/or a long maintenance phase. An economical alternative is to use liquid nitrogen during the freezing phase and to switch to brine freezing for the maintenance phase.

    Freezing with liquid nitrogen

    Artificial ground freezing with liquid nitrogen uses cryogenic liquefied nitrogen with a temperature of -196°C as a refrigerant. The liquid nitrogen is delivered by specialized trucks to the construction site where it is temporarily stored in vacuum-insulated tanks. The liquid nitrogen flows through insulated pipes to the freeze pipes and via a down-pipe to the deepest points of the pipes. As the nitrogen exits the downpipe it evaporates and flows back through the freezing pipe. The escaping gas is then collected in exhaust pipes and discharged into the atmosphere through an exhaust tower.

    Heat is extracted from the soil and frozen areas are formed during evaporation. Temperaturecontrolled magnetic valves discharge the liquid nitrogen into each freeze pipe.
    The freezing process, the frozen area and frost expansion is monitored with measuring pipes equipped with sensors. The collected data is recorded and stored on an online platform and can be accessed at all times.

    The advantages of ground freezing with liquid nitrogen over brine are the short mobilisation time for the equipment and a substantially shorter freezing time (usually 3
    to 7 days). For frozen areas with a small volume and short maintenance phase, it can be an economical alternative even though nitrogen is a consumable. A combination of both methods is also an option.

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