Method and apparatus for artificial ground freezing
Joseph A. Sopko and George Briley
Each ground freezing project requires an evaluation to determine the appropriate spacing between the freeze pipes. Increasing the spacing between pipes results in a longer time required for the ground to be frozen to form the barrier. Three to six weeks of freeze time is typical for the freeze zone to be completed with the necessary permeability. The time can be reduced by either using a colder circulating fluid or by reducing the pipe spacing. However, if the pipe spacing is reduced, more drilling is required. Because drilling is the single most costly aspect of a ground freezing project, it is highly undesirable to space the pipes close together. Conversely, the overall cost can be reduced significantly by increasing the pipe spacing to decrease the drilling requirements. With increased distance between pipes, the only way for an effective frozen barrier to be formed in a reasonable time period is to decrease the temperature of the coolant that is circulated through the freeze pipes.
On some projects, coolant temperatures must be approximately -52°C (-62°F) or less to allow a pipe spacing that is consistent with a reasonably low drilling cost. However, conventional circulating fluids such as calcium chloride brine or ethylene glycol cannot attain such a low temperature.
This invention encompasses a method and apparatus for ground freezing that makes use of cooling techniques resulting in circulating fluid temperature of -50°C (-58°F) or less. This has the great advantage of allowing the freeze pipes to be spaced relatively far apart while still creating an impermeable frozen earth barrier in a reasonable period of time. The savings in drilling cost that results from the need for fewer freeze pipes creates a major economic benefit making ground freezing practical for very large projects.
The heat transfer circulating fluid is preferably aqua ammonia (ammonium hydroxide) with 27-30% ammonia, which has the advantage of being readily available at a low cost and the ability to serve as an efficient heat transfer fluid. Equally important, aqua ammonia (ammonium hydroxide) has a very low viscosity (actually less than water at -52°C) so that it can be easily pumped through the freeze pipes to minimize pumping costs and difficulties.
The refrigeration plant used to cool the circulating fluid may advantageously employ low and high stage vapor compression refrigeration systems arranged in a cascade relationship with one another. The low stage system may use carbon dioxide as its refrigerant with its condenser arranged to discharge its heat to the evaporator of the high stage system. Ammonia is preferably the refrigerant in the high temperature system. However, R-22 or other refrigerant may be employed. In this way, the low temperature system can cool the circulated fluid to the requisite temperature -52°C (-62°F) or less and thus allow the freeze pipes to be spaced relatively far apart.