Ground freezing

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The salvage pit of the collapsed historical archive of the city of Cologne is frozen with liquid nitrogen. In the background on the left is the tank for storing liquid nitrogen; The nitrogen used is released back into the atmosphere through the pipes on the right.

The soil freezing or ground freezing is a construction method in civil engineering , in which the floor is solidified by freezing artificial soil water, and made water-impermeable. The resulting frost body gives the excavation a certain degree of stability and protects it from the ingress of water until these functions can be taken over by the structure itself.

General

To freeze the ground, freezing pipes are inserted into the ground. A coolant flows through the freezing pipes , which extracts the heat from the surrounding soil. This creates cylindrical frost bodies around the freezing pipes, which combine with the freezing bodies of the neighboring freezing pipes to form frozen cubatures . Precise freezing drill holes and powerful freezing units are required. The prerequisite for using this method is a sufficiently high water content in the soil and no excessive groundwater velocities (up to around 4 meters per day for brine, up to around 11 meters per day for nitrogen).

The frozen soil has a compressive strength between 0.6 and 0.8 MN / m 2 in cohesive soils (see subsoil ), and between 1.2 and 1.5 MN / m 2 in non-cohesive soils , with the strength decreasing Temperatures of the frozen body increases and can also have far greater strengths. In the case of icing, an average temperature of the frozen body of −10 ° C to −20 ° C is usually assumed. During the freezing process, unwanted uplifts can occur, and when the frozen body thaws, it can settle .

Two different icing methods can be used:

  • Ground freezing with brine : brine freezing
  • Bottom freezing with liquid nitrogen: nitrogen freezing

Sometimes both types of process are combined with one another: First, the frozen body is built up in a relatively short time by means of nitrogen freezing, and then the (cheaper) brine freezing takes over the maintenance of the freezing body.

An example of extensive ground freezing was the construction of the subway under the Vienna Danube Canal using the New Austrian Tunneling Principle . The ground freezing had the following objectives: Sealing of the excavated cross-section against the groundwater, the Danube Canal and against unknown pathways of the water, caused e.g. B. by sand lenses, old outcrops that have not been damned or waterways along the wooden posts of the lock island, the base of the Kaiserbad lock and the left pre-quay. Construction of a temporary auxiliary vault in the longitudinal and transverse direction in order to be able to safely create the excavated cross-section.

Brine as a coolant

The cold is generated in a closed cycle process. Three cycles can be distinguished:

Refrigerant circuit

The coolant is a brine , mostly aqueous salt solutions such as calcium chloride , sodium chloride or magnesium chloride with a temperature of around −50 ° C are used. The coolant circulates in the freezing pipes in the ground and thereby extracts the heat from the soil.

Refrigerant circulation

In order to keep the coolant at constantly low temperatures, gases such as carbon dioxide , whose sublimation point is −78.5 ° C, and ammonia with a boiling point of −33.4 ° C are used as refrigerants . In the so-called evaporator, the liquid refrigerant comes into contact with the freezing pipes emerging from the floor, in which the heated refrigerant flows. The refrigerant evaporates and the necessary energy is extracted from the refrigerant, which is then cooled down to the required temperature and can be used again. The now gaseous refrigerant is compressed in the compressor, but still remains gaseous and therefore continues to heat up. In the condenser , enough heat is extracted from the refrigerant until it becomes liquid again. The refrigerant circuit closes when the evaporator re-enters.

Cooling water circuit

With a recooling system, this circuit ensures that the refrigerant in the condenser is cooled down to the desired temperature and liquefied.

Since this is a closed system and therefore neither coolants nor refrigerants are used, it is particularly suitable for larger and longer-lasting construction projects. The long lead time (up to six weeks and longer) that is required until the frozen body has reached the required size and the constant supply of energy must be taken into account. When the frozen body builds up, energy is constantly required to maintain the refrigerant cycle. When the freezing process is finished, the maintenance of the frozen body works through intermittent energy dissipation. The temperature of the freezer body is continuously measured via temperature sensors in the floor in addition to the freezing pipes and heat is only extracted automatically when required.

Liquid nitrogen as a coolant

Nitrogen used to freeze the salvage pit of the collapsed historical archive of the city of Cologne is released back into the atmosphere.

In cohesive soils that are at risk of frost, slow freezing processes pose the risk of ice lens formation (see ice lens ) and the associated undesirable uplift of the soil. The use of liquid nitrogen is recommended for such soils , as the freezing process takes place here much faster and at much lower temperatures.

In this open process, liquid nitrogen at a temperature of −196 ° C is fed into freezing lances. The nitrogen evaporates in the freezing lances and removes its heat from the surrounding soil. The low temperature leads to a large temperature gradient and freezes the soil water very quickly. The nitrogen cannot be used again and escapes into the atmosphere.

In a rare variant, the liquid nitrogen can also be brought into direct contact with the surrounding soil by means of perforated lances.

Since the nitrogen is a consumable item, it is delivered in highly insulated tank trucks as required and temporarily stored in also highly insulated double-walled tanks. For economic reasons, the method is particularly suitable for short-term and rapid icing (up to a maintenance time of about 3 months). The lead times to achieve the desired frost body circumference are considerably shorter than with brine freezing (duration: about one week), and the method can be used both in frost-sensitive soils and at higher groundwater speeds (up to about 11 meters per day).

Possible applications

The implementation of a ground freezing offers itself among others in the following cases:

  • when building tunnels under water,
  • when building (short) tunnels, cross cuts (connection between two tunnel tubes) or emergency exits in water-bearing loose rock,
  • when pressing through railway underpasses,
  • for sealing slotted or bored pile walls ,
  • as sealing of joints between segment and diaphragm wall,
  • for deepening floor slabs under the groundwater (elevator),
  • for taking undisturbed soil samples.

Artificial icing is also used in mining, when sinking shafts. One then speaks of the freezer shaft method .

literature

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