July/August 2022 - 17

other works including excavation, structure construction and
flood control, thus requiring a careful consideration of how to
approach the ground freezing works. Due to cost
considerations, a conventional closed-system brine freezing
technique was selected. The freeze was performed for up to
650 days and involved two vertical freezing works for the
launching shaft and four horizontal freezing works for the
reception shaft.
In this first case study, CTCI applied lithification
technology and refrigeration during facilities fabrication, and
provided supervisory control of the brine freezing equipment
and pipe design. Despite the high complexity and difficulties
in boring and piping in a harsh environment, the ground
freeze successfully cut off water ingress.
The tunnel shaft was 417 ft (127 m) long, retained by a 6.5 ft
(2 m) thick diaphragm wall and an internal bracing system.
Six shield tunnels were connected to the structure, with the
southwest face built for launching the shield tunnel machine,
while the northwest and east faces served as breakthrough
points. Groundwater posed a threat whenever a diaphragm
was broken to enlarge the tunnel eye for flexible joint
installations. The project adopted multiple protection
mechanisms to complete the six tunnel connections, with
ground freezing technology serving as a key measure to
mitigate risk associated with the deep excavation and high
hydraulic pressure. Vertical freezing was recommended for
the launching side, while horizontal freezing was used for the
reception sides.
Protection mechanisms for shield tunnel arrival side
Horizontal freeze pipes were drilled and installed to form
frozen cylinders for excavation support and groundwater
control during cross passage construction. The refrigeration
system for circulation of the cooling medium was located
above ground, with the coolant supplied to the cross passage
via manifolds, using a specially designed pumping system.
An extensive instrumentation system provided monitoring
of ground temperatures, soil and water pressures, coolant
flow rates and pressures, as well as refrigeration plant
processing information.
The challenges of this project concerned drilling horiz2
ontal
boreholes under more than 10,443 lbf/ft (500 kN/m )
water pressures, as well as the logistical requirements inside
the shield tunnel. Ice walls were fully formed within 50 days
from the beginning of the freezing process in October 2016.
Once the ground was frozen, cross passage could be safely
constructed. Throughout the construction period, temperatures
were constantly tracked until the cross passage was
subsequently constructed with concrete. The freezing system
was turned off in January 2017.
2
Turnout structure of Taipei Metro Xinzhuang Line
Power Company Project - The second case study
involves applying artificial ground freezing for the
construction of a shaft and tunnel cross passage on a power
utility project beneath downtown Taipei in 2016. The cross
passage was approximately 164 ft (50 m) below groundwater
level.
Liquid Nitrogen Experiment
The liquid nitrogen (LN) freezing method differs from the brine
method in several ways, including using much lower
temperatures. The technique uses -320.8°F (-196°C) coolant
that circulates in an open system (also known as a direct expansion
system). In addition, the approach provides a faster
freezing speed, with fewer equipment requirements. This method
is suitable for small-scale exercises or rescue engineering.
To develop the LN freezing technology for use in Taiwan,
CTCI conducted a two-week in situ experiment in Yilan County
(from August to September 2021). The purpose of the
experiment was to carry out a 13 ft (4 m) deep excavation with
sheet piles and a 3.3 ft (1 m) thick frozen wall. Also, we wanted
DEEP FOUNDATIONS * JULY/AUG 2022 * 17
July/August 2022

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