Groundwater has traditionally been an essential source of fresh water throughout the world.
It is clean, simple to pump and accessible to anybody.
However, when it comes to construction activities, groundwater is usually an unwelcome condition.
As a result, it is critical to remove groundwater from construction sites.
Groundwater dewatering and ground freezing are two strategies utilized to attain this goal.
This article will discuss ground-freezing technologies.
What is ground freezing?
Artificial ground freezing is a construction technique used in the building of shafts, mines and tunnels to provide temporary earth support and groundwater management when other traditional methods such as dewatering, shoring, grouting, or soil mixing are impractical.
Ground freezing is a technique that uses property changes caused by lowering the temperature below the freezing point.
The ground becomes waterproof and substantially stronger.
The approach is typically employed as temporary support and waterproofing, or simply as a watertight barrier against groundwater flow.
The underground building and mining sectors benefit greatly from this form of ground enhancement.
when it can be used?
Deep hole excavation
Ground freezing is most commonly used in deep holes.
The freeze pipes are bored and placed around the intended shaft’s perimeter to the desired depth.
The coolant is circulated until a frozen zone extending from one to ten meters is produced.
The shaft is subsequently dug and lined and the freezing system is switched off.
In tunneling industry
The tunneling industry makes considerable use of ground freezing.
Several techniques are used in tunnel applications.
The most frequent method includes horizontally drilling the freeze pipes around the tunnel circumference, much like the frozen shaft approach.
This horizontal layout is used to create safe cross-passageways between two existing tunnels or to tunnel beneath highways or trains.
Another way of tunnel construction is to solidify the entire alignment and mine through a frozen mass of dirt.
Excavation beneath the groundwater table
Ground freezing should be considered for waterproofing or stabilization if water is leaking into the excavation or if water entry would render the ground unstable and dangerous.
Similarly, if the surroundings are sensitive to settlements and cannot support groundwater withdrawal.
Ground freezing creates a safe and stable building pit or soil tunnel while also controlling groundwater.
The construction area’s groundwater effect area is limited.
Exclude groundwater from mining activities
Ground freezing has been proposed for regional groundwater barriers up to 10km long to separate groundwater from mining activities rather than large-scale dewatering operations that have environmental implications or necessitate sophisticated and costly treatment procedures.
Ground freezing methods
Ground freezing techniques are classified into two forms based on the freezing agent: liquid nitrogen and brine.
Nitrogen-type ground freezing
The liquid nitrogen-type ground freezing device freezes the target ground by continuous heat exchange between the ground and the liquid nitrogen-filled double tube.
The liquid nitrogen-type ground freezing device has advantages in that it takes up little space, is simple to use and freezes the ground quickly.
Furthermore, when compared to the brine-type freezing system, it can be used with an appropriate modification where the groundwater flows at a relatively high speed.
Brine-type ground freezing
The brine-type ground freezing system, on the other hand, uses a solution of calcium chloride or magnesium chloride called “brine” for the freezing ground.
A refrigerating machine regulates the temperature of the brine at 20-40°C and the temperature-controlled brine is circulated through the freezing pipe.
The brine type system, as opposed to the previously described liquid nitrogen type, is ideal for a large-scale building site with a long construction duration due to its substantial facilities, which include a refrigerating machine and brine storage.
It has disadvantages such as taking longer to freeze a certain area of ground than liquid nitrogen and not being suited for high-velocity groundwater movement.
Additional ground-freezing technologies utilized for temporary tunnel support such as:
Indirect method
This method makes use of two coolants.
Ammonia is the first coolant, while calcium chloride is the second.
In the freezing unit, ammonia is compressed and chilled.
The condenser changes ammonia from a gas to a liquid.
The water is cooled in the cooling unit.
This liquid ammonia is passed through and cools the secondary coolant.
The cooled brine solution is channeled through parallel-driven tubes parallel to the ground.
The tubes freeze and the surrounding ground also freezes.
This method is repeated until the required ground area freezes.
Direct method
This approach is further classified into two kinds:
By circulation of the primary refrigerant fluid through the ground tubes
In this type of direct ground freezing strategy, only ammonia is used in the freezing process.
The process is identical to that of the indirect approach, except that just one coolant is used.
The compressed ammonia is injected into the subterranean tubes.
This ammonia causes the tubes to freeze, forcing the surrounding ground to freeze as well.
By injection of a coolant into the ground, as liquid nitrogen
This approach does not necessitate the use of a refrigeration facility.
Ammonia is transported to the site under moderate pressure and stored in insulated tanks on-site.
Tubes are pushed into the earth, with return pipes to exhaust into the atmosphere.
There is a benefit for emergency usage, namely fast freezing without the need for expensive fixed plant and equipment. This may be doubly useful in locations far from power sources.
Ground freezing advantages
1-Can be carried out at a large depth.
2-Can be done in tough, disturbed, or sensitive terrain.
3-Temporary underpinning and support of nearby structure while permanent underpinning is completed.
4-Shaft sinking through the water-filled ground.
5-The shaft was built entirely beneath non-cohesive moist earth.
6-Tunneling through a granular soil face.
7-Tunneling across mixed terrain.
8-Stabilization of the soil.
9-It has an environmental benefit because it reduces CO2 emissions and energy usage.
Ground freezing disadvantages
1-Extremely costly.
2-Continuous monitoring is required.
3-Water expands in volume during freezing, causing soil heave and thaw settlement.
References
1- Ground Freezing [online] Available at: https://www.geoengineer.org/education/web-class-projects/cee-542-soil-site-improve-winter-2014/assignments/ground-freezing
2- Ground Freezing Technique for Soil Stabilization -Methods, Advantages, Applications [online] Available at: https://theconstructor.org/geotechnical/ground-freezing-technique/16944/
3- Ground freezing [online] Available at: https://www.keller-na.com/expertise/techniques/ground-freezing
4-Ground freezing in construction [online] Available at: Ground freezing in construction – groundfreezing.com
5-Development of Ground Freezing System for Undisturbed Sampling of Granular Soils [online] Available at: https://www.hindawi.com/journals/ace/2018/1541747/
6-GROUND FREEZING [online] Available at: https://www.geofrost.no/en/about-ground-freezing/
