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Solidifying and sealing is daily business in the construction industry. Under certain conditions, ground freezing with liquid nitrogen is the best solution compared to conventional soil treatment methods.
During the last two decades soil freezing with liquid nitrogen (LIN) has developed from an exotic gas application with lots of uncertainties into a standard procedure for treating unstable soil and leakages.
Using this process together with our experts offers numerous advantages:
The set-up of a LIN-freezing plant can be done quickly as large quantities of specialist hardware is available in stock
The investments for a LIN freezing plant is only a fraction of the amount to install a brine freezing unit
The temperature of the frozen soil is substantially lower compared to the use of a brine freezing plant, which increases stability
The low temperature of LIN (-196°C or -320.80oF) allows freezing in about 4 to 7 days which is substantial faster than the brine freezing process which easily can take a month
Environmental friendly process, no hazardous substances, no vibrations and no ground water pollution or extraction
Automatically working process
Flexible in design of frozen soil
Combination of sealing and static support
High tolerance to soil humidity (5 – 100%)
Frozen soil is 100% watertight, there is no leakage
The hardness of frozen soil is comparable to concrete
The solidification of the soil is only temporary. After shutting down the LIN-supply the frozen soil will melt in a few weeks.
The liquefied nitrogen (LIN) is produced in air separation plants. LIN has a temperature of -196°C (-320.80oF) and is stored in vacuum insulated vessels with some 10.000 litres volume.
Process Description Installation
The copper freeze pipes with a standard diameter of 54mm are installed with an average distance of 0.5 to 0.8 metres. On the inside, downpipes (diameter 10 to 12mm) are installed.
The LIN is fed into the pipes through insulated supply lines. Hereby the LIN vaporises, whereas 1kg of LIN is extracting an energy of abut 200kJ out of the surrounding soil which cools down and freezes.
The vaporised cold GAN (gaseous nitrogen), also called exhaust gas, additionally extracts about 100kJ out of the ground. The temperature of the exhaust gas is used for controlling a solenoid valve. This way there is guaranteed to be a steady flow and the most efficient use of LIN.
After a while the growing frozen areas around the freeze pipes touch, merge and finally grow further as a closed and watertight wall. In about one week this process forms a frozen wall with an diameter of about 1 metre.
This so called establishing phase normally lasts four to seven days. The total LIN consumption in this time is about 1500 to 2500 litre LIN for 1m³ of frozen soil. Geological influences (thermal sources, water flow etc.) can influence this value.
Maintaining the Freeze
In the subsequent maintenance-phase the LIN-supply is reduced, and the frozen soil stops growing and keeps its volume. To maintain 1 m³ of frozen soil it takes about 90 litre LIN/day.
Stopping the Freeze
When the LIN-supply is stopped, the frost body starts melting and will be gone in a few weeks.
Designing a Freeze Site
When making a freeze design for static support and/or sealing purpose a lot of parameters have to be taken into consideration.
The two main goals we try to achieve are:
- Maximum safety
- Minimised LIN consumption.
With experience from more than 100 freeze project and many basic research studies in the last three decades we are able to offer the best solution.
We do not only deliver liquid nitrogen. According to customer needs we can offer:
- Static calculations
Together with our specialised industrial service group we can guarantee quality globally. Our recent projects in Singapore, already five, are running according to schedule despite of very difficult conditions (45m below surface, soil temperature 26°C 978.80oF, blasting in the tunnel).
A Case Study
In Bielefeld, Germany, a tunnel had to pass through several different geological formations.
In one part the roof of the tunnel was partially driven through quaternary layers, where an alluvial groove consisting of saturated silty fine sand appeared. This faulty area with a length of about 50m was driven under a shelter of nitrogen freezing.
Points that had been taken into consideration were the sub-crossing of a park with old oak trees. Also the small overlay of only 7m and the keeping of the draining function of the soil were important facts for the decision to use nitrogen freezing.
As freeze design a roof-like form was chosen. The inclination was 45°. The freeze pipes stretched into the watertight boulder clay. To prevent the roots of the oak trees from frost damages the upper parts of the freeze pipes were insulated. This also decreased the volume of the frozen soil and reduced the amount of liquid nitrogen.
For an increase in safety five bulkheads were designed. The resulting five sections with a length of 10m each, decreased the number of control circuits. One section had about twenty four freeze pipes for the roof and approximately eight freeze pipes for the bulkhead.
These thirty two control circuits were handled and documented by means of our special designed containerised process control system.
Parallel to the progress in excavation the freeze installation was moved from one section to the next to build a shelter for the tunnel works. After having passed the alluvial groove the freezing equipment was removed. Now the park with the old trees shows no signs of this severe construction work.