TM 5-815-5/AFM 88-5, Chap 6/NAVFAC P-418
orable site for artificial freezing is where the water ta-
ble is high, the soil is, e.g., a running sand, and the wa-
ter 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
15- to 20-foot centers have been used to dewater cais-
best way to control water in some excavations, e.g.,
sons and mine shafts 75 to 250 feet deep. An example
deep shafts.
of the design of a deep-well system supplemented with
(2) Frozen soil not only is an effective water bar-
vacuum in the well filter and screen to dewater a
rier but also can serve as an excellent cofferdam. An
stratified excavation for a shaft is shown in figure
example is the frozen cofferdam for an open excava-
D-8, and an example to dewater a tunnel is shown in
tion 220 feet in diameter and 100 feet deep in rub
figure D-9.
bishy fill, sands, silts, and decomposed rock. A frozen
d. In designing a well system to dewater a tunnel or
curtain wall 4000 feet long and 65 feet deep has been
shaft, it should be assumed that any one well or pump-
successfully made but only after some difficult prob-
ing unit may go out of operation. Thus, any combina-
lems had been solved. Mine shafts 18 feet in diameter
tion of the other wells and pumping units must have
and 2000 feet deep have been excavated in artificially
sufficient capacity to provide the required water table
frozen soils and rocks where no other method could be
lowering or pressure relief. Where electrical power is
used. Any soil or fractured rock can be frozen below
used to power the pumps being used to dewater a shaft
the water table to form a watertight curtain provided
or tunnel, a standby generator should be connected to
the freeze-pipes can be installed, but accurate site data
the system with automatic starting and transfer equip-
are essential for satisfactory design and operation.
ment or switches.
b. Design. As with the design of any system for sub
4-11. Permanent pressure relief sys-
surface water control, a thorough site study must first
tems. Permanent drainage or pressure relief systems
be made. Moving water is the factor most likely to
can be designed using equations and considerations
cause failure; a simple sounding-well or piezometer
previously described for various groundwater and flow
layout (or other means) must be used to check this. If
conditions. The well screen, collector pipes, and filters
the water moves across the excavation at more than
should be designed for long service and with access
about 4 feet per day, the designer must include extra
provided for inspection and reconditioning during the
provisions to reduce the velocity, or a curtain wall may
life of the project. Design of permanent relief or drain-
never close. If windows show up in the frozen curtain
age systems should also take into consideration poten-
wall, flooding the excavation and refreezing with add-
tial encrustation and screen loss. The system should
ed freeze-pipes are nearly always necessary. A knowl-
preferably be designed to function as a gravity system
edge of the creep properties of the frozen soils may be
without mechanical or electrical pumping and control
needed; if the frozen soil is used as a cofferdam or
equipment. Any mechanical equipment for the system
earth retaining structure, such can be determined
should be selected for its simplicity and dependability
from laboratory tests. Thermal properties of the soils
of operation. If pumping equipment and controls are
can usually be reliably estimated from published data,
required, auxiliary pump and power units should be
using dry unit weight and water content.
provided. Piezometers and flow measuring devices
should be included in the design to provide a means for
c. Operation. The ground is frozen by closed-end,
controlling the operation and evaluating its efficiency.
steel freeze-pipes (usually vertical, but they can be
driven, placed, or jacked at any angle) from 4 to 6
4-12. Freezing.
inches in diameter, spaced from 3 to 5 feet in one or
a. General.
more rows to an impervious stratum. If there is no im-
(1) The construction of a temporary waterstop by
pervious stratum within reach, the soil may be com-
artificially freezing the soil surrounding an excavation
pletely frozen as a block in which the excavation is
site is a process that has been used for over a century,
made, or an impervious stratum may be made artifi-
not always with success and usually as a last resort
cially. In one project, a horizontal disk about 200 feet
when more conventional methods had failed. The
across and 24 feet thick was frozen at a minimum
method may be costly and is time-consuming. Until re-
depth of 150 feet. Then, a cylindrical cofferdam 140
cent years far too little engineering design has been
feet in diameter was frozen down to the disk, and the
used, but nowadays a specialist in frozen-soil engineer-
enclosed soil was excavated without any water prob-
ing, given the site information he needs, can design a
lem.
freezing system with confidence. However, every job
(1) Coaxial with each freeze-pipe is a 1 to 2-inch
needs care in installation and operation and cannot be
steel, or plastic, supply pipe delivering a chilled liquid
left to a general contractor without expert help. A fav-
(coolant) to the bottom of the closed freeze pipe. The
4-48
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