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GROUNDFREEZING TECHNICAL PAPERS
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 Back to Technical Papers
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Artifical Ground Freezing – For Environmental Remediation
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Printable Version 
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Artificial ground freezing, a technique which has been used extensively for groundwater control and
excavation support in the underground construction industry for over one hundred years, has recently been
used for environmental applications. The versatility of this process and the advent of more powerful and
energy-efficient refrigeration equipment provides a promising technology which has now been demonstrated
and made readily available. While the primary concept of converting soil pore water to ice is relatively
simple, its applications to remediation projects requires the complex integration of the thermal, structural
and hydraulic properties of the soil as well as the construction know-how and specialized equipment.
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Artificial ground freezing, as it is used today, was originally developed by F.H. Poetsch in 1883. Poetsch's
process involves the circulation of a refrigerated coolant through a series of subsurface pipes to extract
heat, thus converting the soil water to ice, creating a strong, watertight material. The material is so
strong in fact, that it is routinely used as the only method of groundwater control and soil support for the
construction of shafts, hundreds of feet into water-bearing soils.
Most ground freezing systems are quite similar in principle, with subtle differences in the engineering
aspects of the individual sites. It is these differences which have been addressed and reported in the
literature, and learned (in some cases the hard way)
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Figure 1. Typical scenario of freeze pipe spacing and indication which can be connected to a portable
refrigeration plant, or liquid nitrogen tanker.
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through extensive field experience. The single most
important component of a ground freezing system is the subsurface refrigeration system, consisting of a
series of refrigeration pipes, installed with various drilling techniques. The quantity, spacing, depth
and size of the refrigeration pipes are unique to each site, and determined on the basis of the thermal
and hydraulic properties of soils, construction schedules and cost effectiveness.
Within each of the freeze pipes, a smaller diameter feed pipe is installed permitting the downward circulation of
the cooling medium which then flows to the surface through the annulus of the larger pipe. The cooling medium varies
depending on the required application. Where very rapid freezing is required for applications such as containment
after a spill, liquid nitrogen is used with temperatures well below -I50o C. For most applications however, a
secondary coolant such as calcium chloride (brine) or ethylene glycol is used. This secondary coolant is chilled
using large portable refrigeration plants which employ ammonia as a primary refrigerant. These refrigeration plants
are typically mounted on conventional over-the-road trailers and are electrically powered using commercially
available electricity or by diesel generators.
Once the system has been drilled and installed, it operates continuously as a closed system requiring constant
monitoring with occasional plant adjustment and coolant flow modifications. After the initial freezing has been
completed and the frozen barrier in place, the required refrigeration capacity is significantly reduced to maintain
the frozen barrier.
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Barrier Walls
Frozen earth barriers are designed and built to prevent the migration of contaminated groundwater. The frozen
earth barrier can be used to contain the contamination during remediation activities on a temporary basis or can
be installed for long-term use. For the purpose of this discussion, it is assumed that no excavation will be
required, eliminating the need for structural considerations. The actual geometry of the barrier wall is extremely
flexible, permitting locations near existing structures, utilities and right-of-ways. Successful construction
applications using a frozen barrier to provide watertight excavations when subjected to over 20,000 pounds per
square foot of hydrostatic pressure proves in no uncertain terms that the frozen earth is impermeable.
The preliminary design of barrier walls must consider the subsurface soil properties, time requirements, and cost
factors related to both items. Each item is discussed in greater detail to identify how it relates to the design
of the ground freezing system.
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Subsuface Soil Properties
Most environmental projects in the remediation phase have considerably more subsurface investigation and
characterization than conventional construction freezing projects which sometimes have nothing more than one sampled
boring. As a minimum, the following information should be provided in the geologic report:
1. Identification of all soil strata;
2. Identification of contaminants and respective concentrations;
3. Index properties of soils including unfrozen water contents and grain size distribution;
4. Permeabilities of subsurface strata from field tests; and
5. Seasonal ground water levels.
The surface location and installation of the freeze pipes is based predominately on the geometry of the zone requiring
isolation. The spacing between two adjacent refrigeration pipes and the depth of each pipe however, is dependent on
the subsurface soil properties and groundwater conditions.
Soil strata identification is critical in determining the required depth of the frozen earth wall. In about half of
the applications reviewed thus far, the barrier wall can be extended into a low permeability soil or rock, preventing
vertical migration of the contaminant. In the remaining applications, angled, horizontal or directional drilling
techniques are required to install the refrigeration pipes in a manner to isolate the bottom of the containment area.
High permeability strata must also be identified as they can have a negative impact on the freezing process if not
identified in the conceptual design stages.
Groundwater flow through permeable strata can retard and in some cases prevent the formation of the frozen barrier.
In most applications however, it is possible to overcome the adverse effects of moving groundwater by either lowering
the temperature of the refrigeration medium or reducing the spacing between the adjacent refrigeration pipes. Other
subsurface properties, particularly the seasonal groundwater levels and permeability of soils must be considered in
the evaluation of moving groundwater and are discussed later.
The type and concentrations of the contaminants are usually identified in the early phases of the investigation and
are key factors in the design, long-term performance and construction of the frozen earth barrier. During the design
phase, it is imperative that laboratory tests be conducted to determine the freeze point temperature. Undisturbed,
contaminated samples should be retrieved from areas as close to the actual location of the proposed frozen earth wall
as feasibly possible. This freeze point is used in the design phase to determine the required coolant temperature.
There are basically two situations that are frequently observed in designing ground freezing systems in contaminated
soil. One situation is that in which the freeze pipes are installed in the contaminated soil and results of the
freeze point laboratory tests are used to determine freeze temperatures. In this example there is a high level of
confidence that once frozen, the wall will remain frozen and impermeable since it takes considerably more energy to
form the wall initially than it does to maintain the wall in the frozen state
However, another situation which requires consideration is that in which the frozen earth wall is installed and
formed in clean, uncontaminated soil, but at a later date will come in contact with the contaminated groundwater.
In order to ensure that this contact will not erode or melt the frozen wall, rendering it unsuitable, laboratory
investigations are required. The most accurate method in evaluating this situation is through a testing program
which includes frozen soil permeability tests.
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Figure 2. Formation of frozen earth barrier develops at different rates depending on the thermal and hydraulic
properties of each stratum. Typically, rock and coarsegrained soils freeze faster than clays and silts.
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In a frozen soil permeability test, an undisturbed sample of each soil stratum is frozen in a triaxial cell
in which the cell fluid is ethylene glycol refrigerated to a temperature consistent with the outer fringe
of the frozen earth wall. The permeate in this test should be the strongest concentration of contaminated
groundwater that is expected in the field.
The back pressure on the sample as well as the head pressure on the permeate should be as close to field
conditions as possible. This test should be conducted in a long-term mode, with careful measurements being
recorded frequently. If it appears that the frozen material has failed, then it will be necessary to start
with
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a fresh sample and conduct the test at a colder temperature. Subsequent tests should be conducted at
increasingly colder temperatures until a satisfactory test is completed.
The index properties of the soils, particularly the water content and grain-size analyses are needed to
conduct thermal analysis. This analysis is used to determine the required time to freeze and also to
assist in determining the spacing between refrigeration pipes. Typically the larger grained cohesionless
sands and gravels freeze much quicker than the fine-grained clays and silts. For each soil type, thermal
properties vary depending on the water content. Energy is required for the phase change from water to ice,
therefore, the higher water content, the more energy or longer duration required to freeze. The majority
of ground freezing projects conducted since the early 1900's have been in the construction industry and
require refrigeration pipe spacing at an approximately 3.5-foot spacing, with an average coolant
temperature of – 25oC.
As previously discussed, the potential for moving water must be carefully evaluated prior to the initiation
of freezing procedures. Evaluation of the permeabilities should be completed using in-situ slug or pumping
tests. Following this evaluation, piezometric levels should be verified across the entire area to be
contained. Any observed gradients should be correlated with permeability values in order to evaluate
groundwater velocities. These velocities are then used to determine the refrigeration pipe spacing and
required coolant temperatures. Care should be taken when evaluating the piezometric levels that seasonal
and / or tidal fluctuations are accounted for, as they can induce groundwater movement which can be
detrimental to the freezing process.
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Thermal Analysis
The thermal analysis is the most important component of designing the ground freezing system, during the construction
of frozen earth barrier walls. Four independent parameters comprise the factors in completing the thermal analysis,
assuming hydrostatic conditions are relatively static. Moving groundwater conditions are discussed in a subsequent
section.
The four parameters are:
- Thermal properties of the soil;
- Required time to freeze;
- Coolant temperatures; and
- Freeze pipe spacing
The thermal properties of the soil are the single constant parameter of the analysis. Analysis may begin once this
determination has been completed. The required parameters and typical values for both a sand and clay are presented
in Table 1.
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TABLE 1
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Parameter
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Water
Content (%)
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Unit
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Clay Value
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Sand Value
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Unfrozen Conductivity
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25
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BTU/ft. hr. oF
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0.92
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1.2
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Unfrozen Conductivity
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25
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BTU/ft. hr. oF
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0.92
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1.2
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Frozen Conductivity
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25
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BTU/ft. hr. oF
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1.1
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1.0
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Unfrozen Specific Heat
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25
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BTU/ft3. hr. oF
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41.5
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41.5
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Frozen Specific Heat
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25
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BTU / ft3. hr. oF
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29.3
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29.3
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Latent Heat
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25
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BTU / ft3.
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3600
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3600
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Engineering Design
The thermal properties comprise the most constant parameter in this phase of the analysis, while the required
time to freeze is the most variable and dependent on the other three. With the soil parameters remaining
constant, it can be generally assumed that the warmer the coolant and the greater the spacing between freeze
pipes, the longer it will take to freeze.
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The coolant temperatures are a function of the refrigeration system capacity. There are two basic types of
refrigeration plants used for frozen barrier construction. The single phase screw compressor capable of
generating coolant temperatures to approximately -35oC and the two-phase machines capable of generating
temperatures to -70oC. The electric energy required to produce these temperatures increases with the colder
temperatures.
Each individual refrigeration plant is limited to the total linear footage of freeze pipes it can
efficiently cool. For the single phase screw compressors, experience has shown that they can freeze
between 7,000 to 10,500 feet of freeze pipe. As the footage increases, the maximum
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Figure 3. Finite element model grid of a section of shaft prior to freezing.
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coolant temperatures are higher during the initial freeze. As previously noted, the warmer temperatures will
require a longer time to freeze.
The spacing between adjacent freeze pipes also effects the required time to freeze, but constant
refrigeration capacity must be optimized. Greater spacing, results in less pipes, which results in cooler
brine requirements. Tighter spacing requires more pipes, but the coolant will be warmer. Since installation
costs are significant when adding pipes,generally the designer will use the greatest spacing possible to form
the freeze with the required time frame.
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Figure 3. Finite element model
grid of a section of shaft prior
to freezing.
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The method of thermal analyses routinely used by the author involves a time-dependent heat transfer finite
element system. The particular program used has water-to-ice phase change capabilities and has been used
in over 30 construction applications. Three or four different models are used by the designer to estimate
the temperature regime throughout the wall at any given time. For each model, the required time to freeze
is computed by assuming constant coolant temperatures. The results are used to evaluate the time required
freeze duration for each spacing.
After determining the spacing and corresponding number of pipes, the refrigeration capacity is computed.
This process is based on a system of several trials, which are then evaluated in a cost comparison analysis.
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Hydraulic Analysis
The thermal analysis was based on the assumption that the groundwater was essentially static. Groundwater
velocities of less than 5 meters/day generally do not have a significant impact on the freezing process. When
velocities are greater than 5 meters/day, design adjustments must be made, either by lowering the coolant
temperature or decreasing the spacing between two adjacent freeze pipes. Increasing the required time to freeze
will not compensate for moving groundwater.
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Installation Procedures
The most significant aspect of the construction of a frozen earth barrier wall is the drilling and placement of
each individual freeze pipe. The pipes must not only be installed quickly, but within close tolerance to the
vertical. Pipes installed with 3.5-foot spacings on center could have considerably greater spacings at depth
if there is no alignment control during drilling.
During the drilling, it is necessary to keep the borehole open throughout the entire depth until the freeze
pipe is lowered. Several methods of drilling have been used including hollow stem augers, mud rotary and
variations on the casing advancer system. A newer method which was recently used on a deep shaft construction
project in New York City is the reverse rotary dual tube method. This system is capable of advancing the drill
bit at rates of up to 100 feet per hour in glacially deposited subsoils. After the initial borehole is drilled,
a slightly larger diameter borehole casing is then drilled permitting the retraction of the bit while keeping the
hole open. The freeze pipe is then lowered into the cased hole by welding threaded 20 to 40 foot sections.
After the pipe is lowered, the casing is removed and each individual freeze pipe is pressure tested for leaks
and surveyed for verticality using an orientable inclinometer. Should any two pipes have large deviations
exceeding allowable tolerances, an additional pipe is installed.
After completion of the drilling and pipe installation, the system is connected to the refrigeration plant via a
supply and return manifold consisting of 6-inch diameter pipes located around the perimeter of the barrier wall.
After the pipes have been connected, the entire system is charged with the circulating coolant, either calcium
chloride or environmentally safe ethylene glycol. This coolant is chilled via the refrigerant plant and circulated
through the manifold system and corresponding freeze pipes. The circulation is maintained by a series of
centrifugal pumps located at the refrigeration plant and within the manifold system, if required.
A key point often overlooked in the operation of ground freezing systems, is that the coolant serves as a heat
transfer medium, extracting the heat from the ground as it circulates through the pipes. The volume and rate of
coolant flow through the entire system, including the refrigeration plant, is equally as critical to the freezing
process as the temperature. A computer simulation of the coolant hydraulics, much like that used for network pipe
analysis is often used to check coolant velocities on those projects that have unusually long supply and return
manifolds.
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Instrumentation
Once the freezing process has begun, careful monitoring is required to ensure formation of the barrier wall and
also to verify when freezing is complete. The most direct method of evaluating the frozen barrier formation is
ground temperature measurement. During the drilling process, several temperature monitoring pipes are installed.
These pipes are identical to the freeze pipes, except they are not connected to the coolant circulation system.
Instead, they are filled with calcium chloride to prevent freezing while a series of thermocouple sensors are
lowered into the pipes. The thermocouples are placed at specific depths within each pipe to correspond to the
various subsurface strata.
The coolant flow, temperature and pressure are monitored at various locations throughout the manifold to verify
the results of the hydraulic network analysis. Quite often it is necessary to make adjustments to various flow
valves to balance hydraulics thus ensuring the uniform formation of the barrier wall. In some cases where there
are measurable flow deficiencies, it is necessary to install additional pumps within the system.
During the freezing, continuous monitoring of the refrigeration plant(s) is also required. The more modern
refrigeration plants used for ground freezing are electrically driven trailer-mounted mobile units. Ammonia is
used as the primary refrigerant which chills the calcium chloride or ethylene glycol in a large heat exchanger.
Most plants require 480 volts, three phase electrical power which is available using commercial electricity or
diesel driven generators. Monitoring of plant amperage, refrigerant pressures and process temperatures is
required several times each day. The data are evaluated and compared to the ground temperature response to
verify that the system is operating in accordance with the design.
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Completion
Complete formation of the frozen earth wall is usually indicated by the ground temperature data, and confirmed
by piezometric levels. In construction applications, a pump test is usually conducted within the contained
one. External piezometers are measured and their failure to reflect the pumping operations confirms the
integrity of the barrier. Once this formation is confirmed, the plant operation and corresponding electric power
is significantly reduced. For frozen walls without a side exposed to the air as in a construction excavation, the
maintenance may only require the plant to run two or three days every three weeks. In Milwaukee, Wisconsin,
there have been several instances where frozen ground for deep shaft construction has remained frozen well over
one year after the freezing has been terminated.
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Application II Solidification
In certain remediation activities, the removal of contaminated soils can be more easily accomplished when solidified.
Solidification can not only facilitate the removal process, but it can greatly reduce the volatilization of components
and eliminate hazardous vapors. The basic design and installation procedures are similar to those used on the barrier
walls with the exception of the thermal analysis.
When using freezing to solidify a contaminated area, the thermal finite element method model is a critical tool to
determine the required placement and spacing of the freeze pipes. The solidification process has been used recently
in conjunction with a deep shaft excavation. Ground freezing was selected as the excavation support system for
constructing several shafts on a tunnel construction project. Prior to the shaft excavation, a sludge like material
was encountered in the shaft vicinity at a depth of approximately IS feet. Four sampled borings were conducted to
retrieve samples for analytical testing. Results of the laboratory tests are presented in Table A-1 attached in the
Appendix.
The consistency of the sludge material and the foul odors prompted concern from the general contractor. The high
levels of lead and reactive sulfides made disposal of the excavated material much more difficult than conventional
excavation. Solidification using artificial freezing was selected as the method to expedite the sludge removal and
to reduce the potential of volatilization during excavation of the shaft. Initial plans called for complete freezing
during the late fall with removal during the winter, thus taking advantage of the colder temperatures to keep the
waste solid after excavation. Once excavated, the soil would be stored on site while analytical testing would be
conducted.
Design modifications to the original freezing system were required to ensure that solidification could be completed
within the contractor's required time frame. Finite element thermal analyses were completed and indicated that in
order to minimize the need for addition freeze pipes, coolant temperatures of at least – 31oC would
be required. Modifications were made to the refrigeration plant which resulted in the production of the much colder
calcium chloride.
The freezing period lasted approximately 8 weeks, with ground temperature measurements taken daily. This particular
project had considerably less total freeze pipe footage than the maximum capacity afforded by the refrigeration plant.
The cooler weather increased the efficiency of the refrigeration system, producing brine coolant temperatures of
approximately – 32oC.
Excavation was initiated using pneumatically powered spades which fractured the frozen waste into small pieces ranging
from 4 to 12 inches in average diameter. After spading, the fractured waste was removed from the excavation using a
crane-mounted excavation bucket.
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The frozen ground concurrently served as a groundwater barrier and excavation support system. Excavation proceeded on
a 24 hour basis and lasted approximately 21/2 weeks. During this time, constant air quality monitoring was conducted.
Monitoring included organic vapors, methane, oxygen content and explosive potential. During the early phases of the
excavation, the organic vapor analyzer indicated extremely high levels of organic vapor. These levels were the maximum
detectable by the OVA, thus requiring the suspension of removal activities pending laboratory analysis.
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Figure 5. Schematic of the ground freezing
system used for remediation of contaminated soils.
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The excavation level at this point was approximately five to eight feet above the zone that boring information had
indicated the presence of contaminated soils. The laboratory analysis of these soils found no detectable levels of
any contaminant. Further investigation revealed that gas used to prevent the air
hammers from freezing was causing the OVA to show the abnormally high levels of organic gases.
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After this discovery, OVA readings were taken periodically, after the air equipment was briefly removed from the
excavation. This periodic monitoring permitted the anti-freeze gas to evacuate and virtually eliminated any indications
of the presence of organic vapors. The high levels of reactive sulfides in the boring samples caused concern related to
the presence of hydrogen sulphide gas within the excavation. At no time during the excavation of the frozen material
were any concentrations of this gas detected.
As the excavation progressed, samples were collected with a frequency of approximately one sample for each foot of
excavated soil. An interesting perspective related to the nature of soils occurred during the sampling process. While
the majority of the material collected and placed in the sample jars appeared like a clayey soil material upon removal
from the excavation, it rapidly melted to a sludge like material when the sample jars were transferred into the project
engineers trailer prior to shipment to the laboratory. This confirmed the engineers assumption that it would be
necessary to keep the material frozen until it could be transferred to a special waste landfall.
Excavation through the frozen sludge material progressed rapidly as crews worked continuous 24-hour shifts. Visual
observation of the excavation clearly indicated when the extent of the frozen sludge had been reached. The excavated
material was temporarily stored on site, then transferred to a special waste landfall. Excavation proceeded
continuously into the clean soils without any significant changes in procedure except for the elimination of the health
and safety program and continuous sampling. Excavated soils from the remainder of the shaft were used as fill for
various earthwork projects.
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Conclusion
The frozen ground concurrently served as a groundwater barrier and excavation support system. Excavation proceeded
on a 24 hour basis and lasted approximately 21/2 weeks. During this time, constant air quality monitoring was conducted.
Monitoring included organic vapors, methane, oxygen content and explosive potential. During the early phases of the
excavation, the organic vapor analyzer indicated extremely high levels of organic vapor. These levels were the maximum
detectable by the OVA, thus requiring the suspension of removal activities pending laboratory analysis.
The excavation level at this point was approximately five to eight feet above the zone that boring information had
indicated the presence of contaminated soils. The laboratory analysis of these soils found no detectable levels of any
contaminant. Further investigation revealed that gas used to prevent the air hammers from freezing was causing the OVA
to show the abnormally high levels of organic gases.
After this discovery, OVA readings were taken periodically, after the air equipment was briefly removed from the
excavation. This periodic monitoring permitted the anti-freeze gas to evacuate and virtually eliminated any indications
of the presence of organic vapors. The high levels of reactive sulfides in the boring samples caused concern related to
the presence of hydrogen sulphide gas within the excavation. At no time during the excavation of the frozen material
were any concentrations of this gas detected.
As the excavation progressed, samples were collected with a frequency of approximately one sample for each foot of
excavated soil. An interesting perspective related to the nature of soils occurred during the sampling process. While
the majority of the material collected and placed in the sample jars appeared like a clayey soil material upon removal
from the excavation, it rapidly melted to a sludge like material when the sample jars were transferred into the project
engineers trailer prior to shipment to the laboratory. This confirmed the engineers assumption that it would be necessary
to keep the material frozen until it could be transferred to a special waste landfall.
Excavation through the frozen sludge material progressed rapidly as crews worked continuous 24-hour shifts. Visual
observation of the excavation clearly indicated when the extent of the frozen sludge had been reached. The excavated
material was temporarily stored on site, then transferred to a special waste landfall. Excavation proceeded continuously
into the clean soils without any significant changes in procedure except for the elimination of the health and safety
program and continuous sampling. Excavated soils from the remainder of the shaft were used as fill for various earthwork
projects.
The case study briefly described in this paper demonstrates how the innovative use of this readily available technology
has been used successfully for environmental remediation. Additional applications on other projects throughout the
country are currently being considered, and offer excellent opportunities for the growth of this technology.
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Typical Freeze Application – Shaft Construction
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1. Assembling freeze pipes.
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2. Installation of freeze pipes.
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3. Application of freeze with our electronically controlled refrigeration plant.
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4. Frost development on freeze pipe headers.
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5. Excavation following completion of freeze wall.
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6. Construction of concrete liner. Once completed, refrigeration can be shut down.
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7. Opening the portal revealing freeze pipes to be cut and capped prior to tunnel construction.
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Appendix A
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SUMMARY OF METALS - TOXICITY CHARACTERISTIC LEACHING PROCEDURE
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Parameters
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Units
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Boring 1
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Boring 1
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Boring 2
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Boring 2
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Boring 3
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Boring 3
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Boring 4
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Boring 5
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Boring 5
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S-1
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S-2
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S-1
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S-2
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S-1
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S-2
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S-1
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S-1
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S-2
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Depth
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Ft
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9.5
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19.0
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20.0
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40.0
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20.0
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30.0
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20.0
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16.0
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26.0
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Material
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--
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Sludge
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Silty Clay
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Sludge
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Silty Clay
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Sludge
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Silty Clay
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Sludge
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Silty Clay
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Sludge
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Arsenic
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ag/kg
(ppm)
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< 0.1
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< 0.1
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< 0.1
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< 0.1
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< 0.1
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< 0.1
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< 0.1
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0.14
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0.09
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Barium
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0.55
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0.14
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0.13
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0.57
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0.75
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1.08
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0.52
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0.47
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1.30
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Cadmium
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< 0.01
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< 0.01
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0.02
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< 0.01
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< 0.01
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< 0.01
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< 0.01
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< 0.01
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< 0\.01
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Chromium
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< 0.02
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< 0.02
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< 0.02
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< 0.02
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< 0.02
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< 0.02
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< 0.02
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< .1
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< .1
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Copper
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10.4
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< 0.02
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< 0.02
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< 0.02
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< 0.02
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< 0.02
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< 0.02
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< 0.02
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< .31
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Lead
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0.56
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0.93
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0.80
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< 0.15
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9.21
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< 0.15
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< 0.15
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< 0.1
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< .1
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Mercury
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< 0.0002
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< 0.0002
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< 0.0002
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< 0.0002
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< 0.0002
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< 0.0002
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< 0.0002
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< 0.0002
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< .0002
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Nickel
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< 0.5
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< 0.5
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< 0.5
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< 0.5
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< 0.5
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< 0.5
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< 0.5
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< 0.1
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< 0.5
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Selenium
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< 0.1
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< 0.1
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< 0.1
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< 0.1
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< 0.1
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< 0.1
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< 0.1
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< 0.02
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< 0.1
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Silver
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< 0.02
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< 0.02
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< 0.02
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< 0.02
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< 0.02
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< 0.02
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< 0.02
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< 0.02
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< 0.02
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Zinc
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3.65
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3.28
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3.29
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0.39
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15.2
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0.32
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0.11
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0.10
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4.30
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 TOP
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