Freezing method for stabilising soils
Freezing can be used as a method of stabilising water-saturated soils and preventing collapse next to excavations. By freezing the soil until it becomes impervious, it enables workers and plant to operate safely inside the ‘ice wall’ that is formed. In this way, deep earthworks can take place.
In order to produce the low temperatures, steel freeze pipes are installed at approximately 1 m centres around the site that is to be excavated. The pipes are comprised of two tubes. The outer tube is 100-150 mm in diameter, is sealed at the bottom and connected at the top to a return pipe. The inner tube is 38-75 mm in diameter, is open at the bottom and connected to the flow pipe at the top.
These pipes carry chilled brine which is pumped down the inner tube. A refrigeration plant is used to cool the liquid, constantly re-circulating it through the pipes. The brine temperature ranges from -15 to -25-degrees, although the freezing medium must have a freezing point that is well below this temperature range, meaning that a solution of calcium chloride or magnesium chloride is often used.
For the method to be employed there must be moisture content of 8% of the voids. Using a brine medium, the time to obtain a wall of ice depends on the spacing of the pipes, the refrigeration quantity and the type of soil. For example, with pipes spaced at 1 m centres, a frozen wall measuring 1 m thick in sand and gravel takes 10-12 days, and 15-17 days in clay. An observation borehole in the centre of the treated area is sunk to determine whether the frozen wall is completely continuous and excavation work can begin.
Where rapid freezing is required, liquid nitrogen is used as the freezing medium, and this can reduce the freezing time to a few days.
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