July/August 2022 - 19

To address this problem, the bottom of the inner pipe was
sealed, and several 0.19 in (5 mm) diameter circular holes were
drilled into the pipe, spaced 19.6 in (500 mm) between the
circular openings. This design forced liquid nitrogen to spray
out from the inner pipe toward the outer freezing pipe,
allowing the outer one to absorb the equivalent temperature at
each depth. As a result, a uniform frozen soil was able to form.
During the freeze, liquid nitrogen was pumped into the onfloor
distribution system and freeze pipes from a liquid nitrogen
truck. Liquid nitrogen boiled as it absorbed heat from the
ground; the nitrogen gas was released to the atmosphere through a
9.8 ft (3 m) tall exhaust pipe. To use liquid nitrogen cost effectively,
the temperature of the exhausting nitrogen gas was controlled to
remain at between -76°F to -148°F (-60°C to -100°C). °F
For proper wall formation, it is critical to keep measuring
the soil temperatures using thermometric pipes to evaluate
the development of frozen soil. The temperature data reveal
the completeness and homogeneity of the freezing. In
addition, the pressure within the piping system, as well as the
internal pressure and liquid level of the liquid nitrogen on the
truck, were regularly monitored. All this data was recorded
and stored on an online platform, accessible anytime. After
five to six days, a 3.3 ft (1 m) thick frozen wall was formed.
Then we changed the freezing parameters to maintain the
freeze and carried out excavation accordingly.
Measurements from one of the thermometers in the
vertical temperature-monitoring pipes, T3, showed that soil
temperature began to drop 10 hours into the freezing process.
Except for T3-12 (G.L.-1.5 m), which had a faster cooling speed
of 14°F (-10°C) per day, the cooling speeds of T3-6 to T3-11
(G.L.-7.5 m to G.L.-2.5 m) were from between 21.2°F to 19.4°F
(-6°C to -7°C) per day. After running the freezing process for
five to six days, temperatures at all depths reached the
freezing point (32°F or 0°C), and the soil freezing progress was
now complete.
Thermal-barrier room and frozen wall
ground's initial temperature with the temperature measured
during the freezing phase, we confirmed that the ground
cooling speed was similar between the simulation and actual
conditions. Furthermore, the thermal model results indicated
an expected freeze time of approximately six days, which was
very close to the actual experimental findings.
When the excavation was finished, the 13 ft (4 m) tall frozen
wall was exposed. A thermal-barrier room was built to prevent
the frozen wall surface from melting due to heat transfer with
the environment (i.e., from air, sunshine and water). The room
also provided a safe space for verifying the shape of the ice
wall. By direct observation, the uniformity of the frozen wall
met our expectations. The in situ experiment site was then
opened for visits for six days by more than 140 members of
government agencies, industry and academia interested in the
development of LN freezing technology in Taiwan.
Conclusion
This article has briefly summarized two case studies in
Taiwan where artificial ground freezing has been applied and
discussed a recent in situ liquid nitrogen experiment. Since
1962, ground freezing has been applied to more than 600 sites
in Asia. Among these cases, the brine freezing method is used
more than the liquid nitrogen freezing method. Effective in all
soil and ground conditions, the method has been proven as a
viable, versatile construction alternative for both shield
tunnel excavation and groundwater control. CTCI has
developed several state-of-the-art techniques and practical
methods for both design and construction in ground freezing
since 2004, in addition to successfully executing the in situ
liquid nitrogen freezing experiment last year.
Measured temperature and TEMP/W analysis result
To determine the amount of time required for freezing and
simulate the development of a frozen range at the initial
design phase, we ran the finite element method thermal
model TEMP/W program. The boundary condition for a freeze
pipe was set by the properties of liquid nitrogen. The ground's
initial temperature was set at 82.4°F (28°C). By comparing the
Yung-Wen Chen is a geotechnical design engineer at CTCI Resources
Engineering. His career has involved several projects, including the excavation
design of the Taoyuan MRT Green Line GC03, seismic hazard evaluation and
analysis of an offshore windfarm. Chen was the designer and leader of the in
situ liquid nitrogen experiment.
Ting-Wei Wang is a licensed professional engineer in Taiwan who has
worked for more than 16 years for CTCI Resources Engineering. He has more
than 10 years of experience in the design and execution of ground freezing.
Wang's geotechnical consulting experience includes design of ground
improvement, deep excavation and other geotechnical projects. He currently is
a project manager of turnkey work and an internal advisor on CTCI projects.
DEEP FOUNDATIONS * JULY/AUG 2022 * 19
July/August 2022

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