(1) The construction of a temporary waterstop by artificially freezing the soil surrounding an excavation site is a process that has been used for over a century, not always with success and usually as a last resort when more conventional methods had failed. The method may be costly and is time-consuming. Until recent years far too little engineering design has been used, but nowadays a specialist in frozen-soil engineering, given the site information he needs, can design a freezing system with confidence. However, every job needs care in installation and operation and cannot be left to a general contractor without expert help. A fav
orable site for artificial freezing is where the water table is high, the soil is, e. g., a running sand, and the water table cannot be drawn down because of possible damage to existing structures of water (in a coarser granular material). The freezing technique may be the best way to control water in some excavations, e. g., deep shafts.
(2) Frozen soil not only is an effective water barrier but also can serve as an excellent cofferdam. An example is the frozen cofferdam for an open excavation 220 feet in diameter and 100 feet deep in rubbishy fill, sands, silts, and decomposed rock. A frozen curtain wall 4000 feet long and 65 feet deep has been successfully made but only after some difficult problems had been solved. Mine shafts 18 feet in diameter and 2000 feet deep have been excavated in artificially frozen soils and rocks where no other method could be used. Any soil or fractured rock can be frozen below the water table to form a watertight curtain provided the freeze-pipes can be installed, but accurate site data are essential for satisfactory design and operation.
b. Design. As with the design of any system for subsurface water control, a thorough site study must first be made. Moving water is the factor most likely to cause failure; a simple sounding-well or piezometer layout (or other means) must be used to check this. If the water moves across the excavation at more than about 4 feet per day, the designer must include extra provisions to reduce the velocity, or a curtain wall may never close. If windows show up in the frozen curtain wall, flooding the excavation and refreezing with added freeze-pipes are nearly always necessary. A knowledge of the creep properties of the frozen soils may be needed; if the frozen soil is used as a cofferdam or earth retaining structure, such can be determined from laboratory tests. Thermal properties of the soils can usually be reliably estimated from published data, using dry unit weight and water content.
c. Operation. The ground is frozen by closed-end, steel freeze-pipes (usually vertical, but they can be driven, placed, or jacked at any angle) from 4 to 6 inches in diameter, spaced from 3 to 5 feet in one or more rows to an impervious stratum. If there is no impervious stratum within reach, the soil may be completely frozen as a block in which the excavation is made, or an impervious stratum may be made artificially. In one project, a horizontal disk about 200 feet across and 24 feet thick was frozen at a minimum depth of 150 feet. Then, a cylindrical cofferdam 140 feet in diameter was frozen down to the disk, and the enclosed soil was excavated without any water problem.
(1) Coaxial with each freeze-pipe is a 1% to 2-inch steel, or plastic, supply pipe delivering a chilled liquid (coolant) to the bottom of the closed freeze pipe. The
coolant flows slowly up the annulus between the pipes, pulls heat from the ground, and progressively freezes the soil, (A typical freeze-pipe is shown in fig. 4-37.) After a week or two, the separate cylinders of frozen soil join to form the barrier, which gradually thickens to the designed amount, generally at least 4 feet (walls of 24-foot thickness with two rows of freeze-pipes have been frozen in large and deep excavations in soft organic silts), The total freeze-time varies from 3 to 4 weeks to 6 months or more but is predictable with high accuracy, and by instrumentation and observation the engineer has good control. Sands of low water content freeze fastest; fine-grained soils of high water content take more time and total energy, although the refrigeration horsepower required may be greater than for sands.
(2) The coolant is commonly a chloride brine at zero to -20 degrees Fahrenheit, but lower temperatures are preferable for saving time, reducing the amount of heat to be extracted, and minimizing frost – heave effects (which must be studied beforehand). In recent years, liquid propane at -45°F has been used in large projects, and for small volumes of soil, liquid nitrogen that was allowed to waste has been used. (These cryogenic liquids demand special care-they are dangerous.) Coolant circulation is by headers, commonly 8-inch pipes, connected to a heat-exchanger at the refrigeration plant using freon (in a modern plant) as the refrigerant. The refrigeration equipment is usually rented for the job. A typical plant requires from 50 horsepower and up; 1000 horsepower or more has sometimes been used, Headers should be insulated and are recoverable. Freeze-pipes may be withdrawn but
are often wasted in construction; they are sometimes used for thawing the soil back to normal, in which case they could be pulled afterward.
d. Important considerations. The following items must be considered when the freezing technique is to be used:
(1) Water movement in soil.
(2) Location of freeze-pipes. (The spacing of freeze-pipes should not exceed the designed amount by more than 1 foot anywhere along the freeze wall.)
(3) Wall closure. (Freeze-pipes must be accurately located, and the temperature of the soil to be frozen carefully monitored with thermocouples to ensure 100 percent closure of the wall. Relief wells located at the center of a shaft may also be used to check the progress of freezing. By periodically pumping these wells, the effectiveness of the ice wall in sealing off seepage flow can be determined.)
(4) Frost-heave effects-deformations and pressures. (Relief wells may be used to relieve pressures caused by expansion of frozen soil.)
(5) Temperature effects on buried utilities.
(6) Insulation of aboveground piping.
(7) Control of surface water to prevent flow to the freezing region.
(8) Coolant and ground temperatures. (By monitoring coolant and soil temperatures, the efficiency of the freezing process can be improved.)
(9) Scheduling of operations to minimize lost time when freezing has been completed.
(10) Standby plant. (Interruption of coolant circulation may be serious. A standby plant with its own prime movers is desirable so as to prevent any thaw. A continuous advance of the freezing front is not necessary so that standby plant capacity is much less than that normally used.)