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Artificial Ground Freezing for Rehabilitation of Tunneling Shield in Subsea Environment

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Abstract

Artificial ground freezing method was employed in the rehabilitation project of a subsea tunnel. To ensure safety of the subsea rehabilitation work, special design and research were conducted considering the unfavorable influence of the salt in seawater had on freezing effect, such as thickness thinning and strength loss of the frozen wall. A shell-shaped frozen soil wall was designed to cut off the leakage channel into the shield. Double rows of vertical freezing pipes with limited-depth freezing were settled in front of the cutter head, and auxiliary freezing pipes were settled at the sides of the shield to achieve the design goal. Results of analyzing monitoring data on frozen soil temperature showed that the design was reasonable for shield rehabilitation in subsea stratum.
Artificial Ground Freezing for Rehabilitation of Tunneling Shield in
Subsea Environment
Hu Xiangdong
1, a
, Zhang Luoyu
2,b
1
Room 723, Geotechnical building, Tongji University, Yangpu district, Shanghai, China
2
Room 715, Geotechnical building, Tongji University, Yangpu district, Shanghai, China
a
anton.geotech@tongji.edu.cn,
b
zly110dark@126.com
Keywords: artificial ground freezing; shield rehabilitation; subsea environment; seawater freezing;
temperature monitoring.
Abstract. Artificial ground freezing method was employed in the rehabilitation project of a subsea
tunnel. To ensure safety of the subsea rehabilitation work, special design and research were conducted
considering the unfavorable influence of the salt in seawater had on freezing effect, such as thickness
thinning and strength loss of the frozen wall. A shell-shaped frozen soil wall was designed to cut off
the leakage channel into the shield. Double rows of vertical freezing pipes with limited-depth freezing
were settled in front of the cutter head, and auxiliary freezing pipes were settled at the sides of the
shield to achieve the design goal. Results of analyzing monitoring data on frozen soil temperature
showed that the design was reasonable for shield rehabilitation in subsea stratum.
Introduction
When an incident occurs during the construction of a subsea tunnel, rehabilitation work would
be more complex and difficult than that of ordinary underground tunnel due to the unfavorable
influence of subsea environment on tunnel construction. This paper briefly introduces a rehabilitation
project of a subsea shield in China adopting artificial ground freezing method (AGF).
AGF is a method which freezes the water in the ground by artificial refrigeration method to
create a high-strength and watertight frozen soil wall. It is now widely used in tunnel construction and
rehabilitation projects[1]. Ground freezing in seawater differs from ordinary AGF due to the
influences of seawater on freezing effect. With the increase of salinity, the freezing temperature and
thermal conductivity of soil become lower, resulting in thinner frozen soil wall and longer necessary
freezing time. What’s more, the higher the salinity, the lower the strength of the frozen soil. So it is of
great importance to research whether the undersea ground freezing can meet the design and
construction requirements for rehabilitation of the undersea shield tunnel.
Project outline
The cross-sea section of the high-voltage line of Huaneng Shantou Power Plant Second Phase
was laid through an undersea tunnel. This cross-sea tunnel was located at the west of the Shantou Bay
Bridge The southern receiving shaft of the subsea tunnel was located at the inner side of the ash
embankment of Huaneng Shantou Power Plant, and the northern working shaft was in Zhuchi Port
area at the bank of the pier of Shantou Port Group Third Company. When the shield advanced to the
118th ring, mud spurting occurred in the tunnel. A steel bulkhead was installed immediately at the
location of the +3th ring to protect the tunnel and shafts from damage. Mud and water flooded the
tunnel and submerged the shield. Parameters of this project are: cut mileage of the shield:
K0+128.466; elevation of tunnel center: -26.8m; ground elevation: +2.8m; the highest sea tide
elevation: +0.0m; elevation of the beach: -1.0m; diameter of the shield: 3.55m; outer diameter of
tunnel segments: 3.4m; inner diameter of tunnel segments: 2.9m (shown in Fig. 1).
Advanced Materials Research Vols. 734-737 (2013) pp 517-521
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Considering all aspects of the condition, AGF method was adopted to reinforce the soil in front
of the shield cut. When the frozen soil wall met the design requirements, construction team could
open the steel bulkhead, clean the tunnel and recover the shield.
117
Elevation of the shield
head: -26.8m
Elevation of the ground: +2.8m
Frozen
region
Sea level
116115 114 1 13 112 111 110 109
A
B C D
Seabed
collapse crater
Offshore platform
Working platform
Freezing pipes
Fig. 1 The shield and the freezing plan
Freezing plan design
This was the first domestic project in which AGF was employed in subsea ground (seawater
stratum). To ensure the cutting grid of the shield can be repaired successfully, a cylindrical frozen soil
shell was designed covering the shield head. This frozen soil shell was designed to withstand the
water and soil pressure in front of the shield after the tunnel was cleaned as well as to resist water
during rehabilitation of the shield. So the key factors of this freezing design were: the bearing capacity
of the frozen soil wall in front of the shield, which was decided by the average temperature and the
thickness of the frozen wall, and the sealing length of seepage path.
According to the structural and hydrogeological material, the stratum was completely
seawater-saturated, strength of frozen soil there was significantly lower than that of the none-saline
frozen soil due to the high salinity of pore water[2]. Therefore, the design average temperature for
frozen soil wall in this project was designed to be -15°C, much lower than usual. And the thickness of
the frozen soil wall was decided to be 1.5m based on related researchs [3][5].
Water sealing capacity of the frozen soil shell was determined by the longitudinal length of
frozen soil covering the shield head, which was also the sealing length of seepage path along shield
skin into shield head. Theoretically, the sealing length does not need to be very long on the premise of
ensuring mechanical strength. Estimating based on experience, the sealing length should be greater
than 20cm, better if greater than 50cm. Thus, the total thickness of the frozen soil wall should be
0.2~0.5 m more on the basis of 1.5m, that is, 1.7~2.0 m
As is shown in Fig. 1 and Fig. 2, to achieve the design goals, double rows of freezing pipes were
installed in staggered arrangement. The two rows of freezing pipes were 0.8m apart and pipes in each
row were installed at 0.7m spacing, and the space between shield cut and the row close to it (Row A)
was 0.3m. And on each side of the shield near the cut face, an additional freezing pipe was settled (C1
and C2) to create a frozen column connecting the frozen soil wall. Besides, an auxiliary freezing pipe
was settled on each side of the shield rear (D1 and D2) to form two frozen columns that could restrain
the shield to limit the displacement of shield during the rehabilitation process. The layout of freezing
pipes was shown in Fig. 2. Counting from elevation of the platform, the depth of freezing holes was
34.71 m; regarding the position of shield as the center, the vertical freezing height was calculated to
be 8.05 m, located at the lower part of the freezing pipes; the upper 26.66m of the pipes did not need
to freeze.
518 Resources and Sustainable Development
Shield head
B1
B2
B3
B4
B5
B6
A1
A2
A3
A4
A5
A6
A7 C2
T2
T3
T4
T5
T1 D1
D2
C1
N
Fig. 2 Layout of freezing pipes and temperature measuring holes
Monitoring plan of freezing
Real-time automatic monitoring system based on the “1-wire bus” technique for AGF was
employed in this project[6]. As is shown in Fig. 2, five measuring holes were settled to monitor the
temperature field of the frozen soil wall. The 3 measuring holes settled at the inner side of the shield
cut were T1, T2, T3, among them, T2 was at the center of the shield, at the depth of 28.1m, and the
distance from the upper shell of the shield to T2 was 0.4m. The 2 measuring holes settled beside the
shield cut were T4, T5, among them, T5 was at the center of the tunnel, whose depth was the same as
T2. The depth of T1, T3 and T4 was 34.71m, extending to the bottom of the freezing depth. About 10
measuring points were contained in each measuring hole. The spaces between the measuring points
were reasonably arranged according to the importance of the measured parts.
Analysis of Monitoring Data
Began in September 20, 2008 as active freezing, and stopped in January 9, 2009, the freezing
process took 110 days, during which refrigeration system functioned normally. The brine temperature
of the supply main was maintained at about -30 °C. The average difference between delivery and
return temperature of the brine of each group was about 0.8 °C.
5.1 Development of temperature of the frozen soil wall Measuring points in holes T1, T3 and T4
covered the entire freezing depth of 8.05m, among them, T1 and T3 were at the inner side of the
frozen wall, while T4 at the outer side. The temperature changes in T1 and T3 were similar because
the layouts of measuring points in these two holes were same. Monitoring data of T1 and T4 is
analyzed below. The curves depicting temperature change of measuring points over freezing time in
T1 and T4 are shown in Fig. 3 and Fig. 4 respectively.
-20
-15
-10
-5
0
5
0 20 40 60 80 100 120
Temperature (°C)
Freezing time (d)
T1-01
T1-02
T1-03
T1-04
T1-05
T1-06
T1-07
T1-08
T1-09
T1-10
Fig. 3 Temperature curves of the temperature
hole T1
-20
-15
-10
-5
0
5
0 20 40 60 80 100 120
Temperature (°C)
Freezing time (d)
T4-01
T4-02
T4-03
T4-04
T4-05
T4-06
T4-07
T4-08
T4-09
T4-10
Fig. 4 Temperature curves of the temperature
hole T4
Advanced Materials Research Vols. 734-737 519
It can be concluded from Fig. 3 and Fig. 4 that the temperature changing trend of measuring
points in T1 and T4 were similar, specifically, temperature in the 1.5m range of the top and the bottom
part of the frozen wall dropped comparatively slowly, while the temperature of the 6m range in
middle part of the frozen wall dropped rapidly. In the period of active freezing, temperature of the
frozen soil wall dropped approximately linearly, the average dropping rate was about 0.4 °C /d. After
50 days of freezing, the dropping rate decreased, and the temperature in the middle part of the frozen
wall (at the position of shield head) maintained at about -15 °C. The temperature of measuring points
T1-02 and T4-02 dropped fastest. And in the 6m range of the middle part of the frozen wall,
temperature of the lower part dropped faster than the upper part.
5.2 Check of the thickness of the frozen soil wall
The development of the thickness of the frozen
soil wall could be analyzed based on the monitoring data of the profile of T2-T5. According to the
monitoring data on November 1, 2008 (after 40 days of freezing), the spatial distributions of
temperature of holes T2 and T5 are shown in Fig. 5 and Fig. 6 respectively. It could be seen in the two
figures that of the design thickness of the frozen soil wall above the shield, the top part had
comparatively high temperature (higher than -11 °C), while the part close to the shield had lower
temperature (around -17 °C). Calculating results of the frozen soil wall are shown in Table. 1.
22
24
26
28
30
32
34
36
-20 -17 -14 -11 -8 -5 -2 1 4
Depth (m
)
Temperature (°C)
The spatial distribution of temperature of T2
The shield
Frozen soil
Fig. 5 Temperature distribution of T2 on Nov. 11
22
24
26
28
30
32
34
36
-20 -17 -14 -11 -8 -5 -2 1 4
Depth (m)
Temperature (°C)
The spatial distribution of temperature of T5
Frozen soil
The shield
Fig. 6 Temperature distribution of T5 on Nov. 11
Table. 1 Calculation for the frozen wall on profile of T2-T5 after 40 days of active freezing
Depth
(m)
Thickness
at T5 (m)
Thickness at
T2 (m)
Total
thickness (m)
Effective
thickness
(m)
Covering
length (m)
Average
temperature
(
°
C
)
26.0 0.555 0.727 2.082 1.655 0.427 -14.9
26.5 0.590 0.866 2.256 1.690 0.566 -15.2
27.0 0.597 1.155 2.552 1.697 0.855 -15.4
27.5 0.637 1.195 2.632 1.737 0.895 -15.6
28.1 0.660 1.168 2.628 1.760 0.868 -15.5
Average 0.608 1.022 2.430 1.708 0.722 -15.32
It can be concluded from Table. 1 that after 40 days of active freezing, the average thickness of
the frozen soil wall was 2.43m, with the effective thickness of 1.73m, 0.21m more than the design
length of 1.5m; the average temperature of the frozen wall was -15.32 °C, lower than the design
temperature of -15 °C. The covering length of frozen shell over the shield head was 0.72m, which
reached the design requirement of 0.2~0.5 m. Thus, it could be concluded that after 40 days of active
freezing, all parameters of the frozen soil wall had reached the design requirements, cleaning of the
tunnel could be carried out.
520 Resources and Sustainable Development
Summary
Based on the foregoing analysis, it could be concluded that the freezing plan of creating a “frozen
shell” was very effective:
(1) The shell-shaped frozen soil wall bore earth pressure, and covered the shield cut with an
impervious barrier, providing protection for the rehabilitation of the tunnel.
(2) The freezing was very effective. Temperature of the frozen soil wall changed faster in the
upper part than the lower part, and the auxiliary freezing pipes on both sides of the shield head
contributed significantly to the forming and maintaining of the frozen shell. So the freezing plan was
reasonable, and the parameters were accurate.
(3) The design conception, design parameters and theoretical analysis method specifically for
the freezing of saline ground under seawater was effective, ensuring the success of the first domestic
AGF project in undersea stratum.
Acknowledgements
The authors would like to thank the National Natural Science Foundation of China funded
projects (50578120, 51178336) and the Science and Technology project of the Zhejiang Department
of Transportation (2010H02) for providing support to our research.
Literature References
[1] Chen Tao, Hu Xiang-dong, Application of Vertically Freezing Reinforcement in Recovery
Works of Shield Tunnel, Journal of Hefei University of Technology. 32(2009) 1542-1546.
(in Chinese)
[2] Hu Xiangdong, Experimental Research on Properties of Artificially Frozen Soil in the Port
Area for Project of the Subsea Cable Tunnel in Shantou. Shanghai, Dept. of Geotechnical
Engineering, Tongji University.(2008) (in Chinese)
[3] Hu Xiangdong, Huang Feng, Bai Nan, Models of Artificial Frozen Temperature Field
Considering Soil Freezing Point, Journal of China University of Mining and Technology.
37(2008) 550-555. (in Chinese)
[4] Hu Xiangdong, Zhao Junjie, Research on Precision of Bakholdin model for Temperature
Field of Artificial Ground Freezing, Chinese Journal of Underground Space and Engineering.
6(2010) 96-101. (in Chinese)
[5] Zhang Jun, Li Muhan, Hu Xiangdong, Safety State Assessment Method of Artificially Frozen
Soil Curtain in Saline Strata and Its Application, Chinese Journal of Underground Space and
Engineering, 6(2010) 189-192. (in Chinese)
[6] Hu Xiang-dong; Liu Rui-feng, Temperature Monitoring System for Freezing Method based
on “1-Wire Bus”, Chinese Journal of Underground Space and Engineering, 3(2007) 937-940.
(in Chinese)
Advanced Materials Research Vols. 734-737 521
... The Tokyo Bay Tunnel in Japan and the Storebaelt Tunnel achieved shield by through ground freezing [136,137]. The early construction of the Shanghai Metro Line 2 [138] bypass and pumping station in China used horizontal freeze lining technology to ensure safe passage ( Figure 22). ...
... The recently constructed Qiongzhou subsea Tunnel, Xiamen Metro Line 2 and Line 3 projects all used the freezing method to achieve safe construction of undersea tunnels with the aid of open chamber solutions. The Tokyo Bay Tunnel in Japan and the Storebaelt Tunnel achieved shield by through ground freezing [136,137]. The early construction of the Shanghai Metro Line 2 [138] bypass and pumping station in China used horizontal freeze lining technology to ensure safe passage ( Figure 22). ...
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