Only projects that could bear the cost of major refrigeration plant construction and expensive drilling costs were able to benefit from this technology.

As freezing and drilling technology advanced, more applications in civil construction emerged. In the 1950s, still expensive and not very transportable, freezing became known as the “problem solver” for the most difficult construction projects. Construction issues with extremely difficult soil conditions could be remedied with great success, and many were.

Shafts from several feet to over 2,000 feet deep have been constructed using freezing as the only method of shoring and water cut-off.

Foundation shoring, cofferdams, underpinning, and temporary access roads using soil freezing have become more and more common.

History

It was developed in the 19th century by the German engineerFriedrich Poetsch. His patent for ground freezing was granted in 1883. The method was developed for shaft sinking to get through water bearing soilsdown to the hard rock and coal seams. It was the only safe method to construct shafts with depths of more than 50 m in water saturated soil. The deepest freezing shaft in Germany was completed in Rheinberg with a depth of more than 600 m.

If not for artificial soil freezing, major deposits of coal, ash, salt, nickel, lead, and gold would still be inaccessible.

The first soil freezing techniques, while effective, were very costly and time consuming.

History

Principle of Ground Freezing:

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 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.

The inner pipe is connected to the supply line and the outer pipe to the return line (when brine is used) or the exhaust line (when liquid nitrogen is used).

The coolant flows through the inner pipe to its deepest point. On its way back through the annulus between inner pipe and freeze pipe, the coolant picks up heat and is warmed up.

Due to the flow of the coolant the frost penetrates the soil and a ring of frozen soil occurs around the freeze pipes.

Depending on the arrangement of thefreeze pipes location and directions one can achieve all shapes of frozen soil walls (bodies) as required for the individual task.

Applications of ground freezing

sinking and lining of deep mineshafts up to depth of more than 600 mdeep excavations (shafts)tunneling using the sequential excavation method SEM under the protection of a structural and watertight frozen soil bodycross-passages between shafts and tunnel tubes or between tunnel tubes, respectively large open excavations, retaining wallstemporary soil improvement under foundationstemporary sealing of leakages temporary water cut-off for connections at the interface between existing and new underground structures

Brine freezing:requires a closed circulation system and the use of refrigeration plants The brine (usually

calcium chloride CaCl2), which is warmed upduring circulation, flows back through the insulatedsurface manifold system before returning tothe freeze plant station for recooling.The brine supply temperature T generally rangesfrom T = -20 °C to -37 °C.

The entire freezing plant consists of:

1.The required number of freeze units,

2.Several additional components like low voltage switch- gears,

3.Tank for the brine backflow4.The recooling machine.

Several freeze units can be combined in a more powerful freeze plant. To minimize fresh water consumption special recooling systems should be connected for heat exchange with the air. Currently, it is state of the art to use ammonia as cooling agent within the freeze unit (not as coolant in the freeze pipe system).Ammonia is much more environmentally friendly than hydrocarbon fluoride.

Liquid nitrogen (LN2) freezing

Liquid nitrogen freezing is a process by which heat is extracted from the soil through direct vaporization of a cryogenic fluid (LN2) in the freeze pipes.

From an on-site storage tank or directly from atank truck, the LN2 is fed through an insulated surfacemanifold system, usually consisting of copperpipes and quick-connect cryogenic hoses, into theinner pipes

Ground freezing is a technique that has been used extensively for:1.Groundwater control 2.Excavation support in the underground construction The process involves the circulation of a refrigerated coolant through a series of subsurface pipes to convert soil water to ice, creating a strong watertight material.

The material is so strong, in fact, that it is routinely used as the primary method of groundwater control and soil support for the construction of shafts hundreds of feet into water-bearing soils.

Most ground freezing systems are quite similar in principal.

The most important component of a ground freezing system is the subsurface refrigeration system, consisting of a series of refrigeration pipes installed with various drilling techniques.

Depending upon the application, the coolant can be brought to temperatures well below -150 degrees celcius. Ground freezing can be achieved by using either a large portable refrigeration plant or liquid nitrogen. After the initial freezing has been completed and the frozen barrier is in place, the required refrigeration capacity is significantly reduced to maintain the frozen barrier.

Peripheral Freezing:

The general intent of peripheral freezing is:to minimize the amount of frozen ground to be excavated.

The frozen wall, of appropriate thickness and strength, is constructed for the most part outside of the excavation but extends some distance inside the future excavation surface so that the freshly exposed face is stable. Peripheral freezes must be formed with sufficiently watertight bottoms to ensure that excessive groundwater leakage will not develop as an upward flow into the unfrozen ground inside the frozen barrier.

Typical candidates for peripheral freezes are shafts, large circular, open excavations, horizontal tunnels and small connections between structures.

ShaftsVertical shafts are the most common application for peripheral freezes. For deep mines, no better method of sinking production shafts through deep, water-bearing ground has yet been established. In mine shafts on the order of 10 to 20 feet in diameter, excavations have been carried out to depths of over 2,700 feet within the protection of un-braced frozen walls.

There are several advantages of ground freezing unique to the construction of shafts:The freeze can be implemented through the soil/rock interface, which is often the most difficult geology in which to create a groundwater cut-off by other methods. A frozen wall, by design, is continuous into the underlying cut-off and resists the loads imposed by full groundwater and soil pressures. Proper instrumentation can provide assurance of the integrity of the freeze to full depth prior to excavation.    

Large Circular Open ExcavationsConceptually, this application is very similar to shaft freezing, but shallower and wider. Pump stations and other structures up to 200 feet in diameter have been constructed within frozen walls of this type. For larger excavations, multiple rows of freeze pipes may be necessary to develop the required wall thickness. For rectangular structures, an elliptical shape is employed to mobilize the compressive strength of frozen soil.

Tunnel ExcavationsSince frozen ground can be created with any freeze pipe orientation, ground freezing is a very effective stabilization tool for tunneling operations. Vertical freeze pipes can be installed to create a frozen arch through which tunneling can proceed. The sides of the arch extend to an underlying cut-off below the tunnel invert, while short pipes are used to freeze above the tunnel crown. Vertical pipe installation can also be used to create a full face tunnel freeze.Horizontal pipes installed from jacking pits can be used to create a frozen cylinder that is parallel to the axis of the tunnel. In this instance, a mass freeze approach may be preferable to a peripheral freeze, except for relatively large Diameter tunnels, to avoid the mix of frozen and unfrozen ground encountered during excavation.

ConnectionsGround freezing can be utilized to facilitate connections between non-interlocking or disjointed structures such as a cross passage between two tunnels or a mined connection between deep structures. The frozen ground will conform to adjoining subsurface installations or obstructions, if necessary, to provide a composite cut-off structure. Freezing works well in these situations because small, irregularly-shaped, hand-mined excavations can be performed under the cover of the frozen ground without internal lining or support and without the need to handle seepage water from within the restricted or confined excavation.

Case Study : Subway Section 3.4H, Düsseldorf, Germany

Background: As part of the Düsseldorf mass transit subway system expansion, four 40 m long tunnels were excavated directly below buildings and a major roadway. All four tunnels were advanced using the Sequential Excavation Method (SEM) formerly called New Austrian Tunneling Method (NATM).

Peculiarity: 1.There was very little space between the roof of the tunnel and the bottom of overlying building foundations. As a result, any ground loss or other causes of settlement due to tunneling,would have lead to direct and adverse movement to the existing building foundations.

2.The individual tunnels are located in non-cohesive soils. The general soil profile consists of intermittent changing quaternary sand and gravellayers. Underlying this stratum is very dense tertiary fine sand.

The Application:For the driving of three of the tunnels, the gravel and sands were stabilized and the groundwater was controlled by ground freezing using a brine coolant

The data of the 3 frozen soil bodies are listed following:Top of frozen soil D = 0.6 / 1.8 / 6.7 m below foundationExcavation zone A = 46 / 42 / 75 m²Frozen soil length L = 48 / 40 / 40 mFrozen soil volume V = 1,600/2,600/2,900m³Frozen soil thickness d = 1.5 / 1.5 / 2.2 m

The data of the 3 frozen soil bodies are listed following:Top of frozen soil D = 0.6 / 1.8 / 6.7 m below foundationExcavation zone A = 46 / 42 / 75 m²Frozen soil length L = 48 / 40 / 40 mFrozen soil volume V = 1,600/2,600/2,900m³Frozen soil thickness d = 1.5 / 1.5 / 2.2 m

To ensure that the soil mass to be frozen had an adequate bearing capacity, water was injected into the soil.To do this, vertical cut-off walls were grouted along the sides of the tunnel to reduce the run-off of water injectedinto the soil.

Water injection and freezing-up were conducted in four phases

All of the tunnels were driven without incident and with only negligible subsidence to the buildings and the main road directly above the tunnels.

very close distance from the buildings to the tunnel of track 1 and the highlydemanding urban conditions.

Access shaft of the tunnel of track 1