Horizontal freezing study for cross passage of river-crossing tunnel

Ming Zhang , LingMin Wang , BaoWen Wang , HongBao Lin

1. John Finlay Engineering & Technology Group of Companies, 28F, CITIC Building, No.19 Jiangguomenwai Street, Beijing 100004, China

2. China Coal Special Drilling Engineering (Group) Corporation Limited, No.131, Dongshan Road, Huaibei, Anhui 235044, China

Horizontal freezing study for cross passage of river-crossing tunnel

Ming Zhang1*, LingMin Wang2, BaoWen Wang1, HongBao Lin1

1.John Finlay Engineering & Technology Group of Companies, 28F, CITIC Building, No.19 Jiangguomenwai Street, Beijing 100004, China

2.China Coal Special Drilling Engineering (Group) Corporation Limited, No.131, Dongshan Road, Huaibei, Anhui 235044, China

To determine the appropriate soft foundation treatment for a river-crossing tunnel, freezing reinforcement design and technology were introduced based on the channel tunnel design and construction practice. Through finite element analysis and engineering practices, two rows of horizontal perforated freezing pipes were designed and installed on both sides of a passage for tunnel reinforcement, which produced the thickness and strength of frozen crust that satisfied the design requirements. These information are valuable for guiding the design and construction of river-crossing tunnels in coastal areas.Keywords: river-crossing tunnel; cross passage; horizontal freezing; soft soil foundation

1. Foreword

In recent years, as underground engineering construction has rapidly developed in many cities, the horizontal freezing reinforcement technique (Yangetal., 2006) has been widely adopted for soft soil foundation reinforcement. This technology was applied in many river-crossing tunnels, including Chongming Island, Dalian Road, Xiangyin Road, the South Road in Tibet, Renmin Road, Xinjiang Road, and the Yangtze River in Nanjing and Wuhan. This paper presents research on the difficulties and treatment strategies of freezing design technology for the Renmin Road river-crossing tunnel, provides valuable information for the design and construction of future river-crossing tunnels and ensures the safety of side passage and peripheral environments.

2. Engineering geology conditions

The Renmin Road river-crossing tunnel in Shanghai is one of the important channels for both sides of the Huangpu River transportation junction. The second side passage is located at NK+809.242 to the north tunnel and SK+800.000 to the south tunnel; it is about 28.4892 m to the center of the shield-driven tunnel; the side passage is -32.520 m to the north of the designed elevation center of the pavement arch,and -32.693 m to the south. The tunnel runs below the Huangpu River, its bottom elevation is -9.46 m; and the average high tide level of where the line crosses is 3.12 m.The side passage consists of a bell mouth and a horizontal channel, both of which are of a circular-shaped arch structure.

The soils inside of the second side passage (-29.310 to-33.883 m) were Layer 1, a dark-green-yellow color clay layer; Layer 2a, a caesious-olive-drab color sand-silty layer;and Layer 2b (the lower layer of freezing soil), an olive-drab-grey color powder sand. The physical parameters of the soils are shown in Table 1.

3. Horizontal freezing design

3.1. Freezing pipe arrangement

The diameter of the side passage common channel was 1.3 m, the diameter of the bell mouth was 1.6 m, and the thickness of the support was 0.3 m. For the large diameter of the channel, radiating of concrete and steeling pipe was much easier than doing that on soil. There was serious influencing-freezing velocity nearby tunnel duct piece, so resistance to overturning and leak-freeness of freezing soil wall may be influenced. After design research, two horizontal rows of the perforated freezing pipes were installed in both sides of the side passage; the angle of the perforations was parallel with the center attachment of the two tunnels to ensure the thickness of the freezing soil wall was correct and the duct pieces were well glue-jointed. The freezing pipe arrangement is shown in Figure 1.

Table 1 Physical-mechanical parameters of the second side passage soils

Figure 1 Arrangement of the freezing holes in soils

3.2. Freezing parameters

The main freezing parameters and data of soils are as follows: the active freezing period was 50 d, the single hole flow was acquired no less than 5 m3/h; the saline temperature for the active-freezing 7 d should be under -20 °C; the saline temperature for excavation should be under -28 °C,and the temperature differences between the go and back circuit were no more than 2 °C. The Φ108×820 mm mild-steel seamless pipe was adopted as freezing pipe.

3.3. Freezing wall calculation

The freezing wall was designed as a -13 °C isothermal body; the freezing wall thickness of the common tunnel was 2.2 m; and the freezing wall thickness of the bell mouth was 2.5 m. The Young’s modulus was 150 MPa and Poisson’s ratio was 0.25 for -13 °C soils (Table 2). The compression strength was 4.0 MPa, the flexural strength was 2.3 MPa and the shear strength was 1.8 MPa.

Three-dimensional finite element analysis (Wangetal.,1996; Lietal., 2007a, b, c, d; Li and Wang, 2008a, b) was adopted for stress analysis and deformation calculation of the freezing wall. A one-quarter side passage model was created by symmetry, 60 m along the side passage axial direction, 60 m along the tunnel axial direction, and(top-down) 50 m deep along the strata. The water weight,which was up to the side passage, was considered as a fixed load. The finite element model created is shown in Figure 2 and the numerical simulation results are shown in Figure 3 and Table 3.

According to the side passage freezing technical regula-tions of the Shanghai Engineering Construction Code, the compression strength safety factor should be no less than 2.0,the flexural strength safety factor should be no less than 3.0,and the shear strength safety factor should be no less than 2.0. The results of our three-dimensional finite element calculation agreed with these requirements.

Table 2 Parameters of the materials

Table 3 The calculated stress and displacement of the freezing wall

Figure 2 The calculation model

4. Horizontal freezing construction

The construction of the horizontal freezing hole (Fanget al., 2009) is the key element of artificial soil freezing, so the deviation of the freezing borehole must be controlled and the watertightness of the freezing device must be ensured during the construction period.

(1) Deviation of the freezing borehole is mainly caused by drill rocker stress conditions, adjustments, and errors of the open pore. The following technical measures could be adopted to ensure drilling precision (Yueetal., 2006):

The Φ108×8 mm freezing pipe was used as the drill rocker. In this case, the freezing pipe was cut into small pieces of steel bar,i.e., 1 m, 1.5 m, 2 m, and 2.5 m lengths.Appropriate bars were chosen during the progress of drilling.Threading was used to connect the freezing pipes. In order to ensure the concentricity and strength of the welds, the joints were manually arc-welded after the threads were fastened.

Figure 3 Numerical results

(2) The distance between the boring machine and the opening was increased. Guide device and guide pipe in the hole-opening height were erected to control the direction of drilling. In order to reduce the influence of the vibration from the boring machine, any error of the hole opening was reduced and the boring machine was fixed and leveled.

(3) The hole was drilled with a diamond core drill. The freezing pipe was installed during the drilling progress. In order to assess the stability of the strata, the small bores within the bore were opened before the freezing holes were opened. Either serious leakage or arenaceous conditions were found, concrete-sodium silicate was injected into the hole to improve the soil stability around the hole firstly; then,the freezing bore was drilled (each drill bore had an orifice tube) and the drilling sealing device was installed. The compressive strength of the sealing device was 0.6 MPa. (Slurry drilling without slurry could have been done using a powerful horizontal drilling rig; if the drilling slurry was washed out, the slurry would be made up in time.) When the freezing was done, the pipe mouth was welded shut with a steel plate to prevent water leakage after the project completed.

(4) As recommended by Lietal. (2004), the freezing process was carefully monitored. To accomplish this,thermionic holes, pressure-release holes, and monitoring holes were arranged in the freezing wall. The freezing condition of the soils near the duct piece was the key safety element of the freezing wall, considering of this, thermion holes were arranged along with the duct piece to monitor the freezing progress.

(5) A pre-tested supporter was installed in both sides of the joint channel to prevent deformation or destruction of the tunnel when the steel duct piece of the join channel was opened. Opening the performed steel duct piece of the right side after the structure level of join channel was constructed.The injected pipes were embedded in the duct piece of join channel to compensate for soil thaw. The injection should be coordinated to melt process of freezing wall. The clay cement paste could be drilled firstly.

(6) The freezing wall would have an obviously creep process before being damaged because of the intensive creep properties of the freezing soil. The security of the freezing wall could be judged by freezing wall deformation during the excavation progress. So the deformation of the freezing wall and the corresponding temperature must be observed promptly during the excavation periods. If obvious deformation of freezing wall occurred, the freezing wall should be supported with steel bearer and plank lagging immediately.The excavation constructing processes should be adjusted and the freezing should be strengthened at the same time.

(7) The construction time of excavation and supporting join channel was very short, it was much shorter than melting time of the freezing wall. According to mine shaft freezing engineering practice, the accidental freezing stop would not produce greater impact on the excavation safety. But in order to improve further construction safety, the following measures should be taken: the reliable freezing construction machinery should be chosen; enough spare equipment should be installed; monitoring of the freezing wall should be strengthened when freezing was stopped; also the lining construction should be made as soon as possible.

The testing and controlling methods on construction quality are shown in Table 4.

Table 4 Construction quality inspection and control methods of the freezing hole

5. Conclusion

The side passage of the Renmin Road river-crossing tunnel was frozen in April, 2009, and the excavation acceptance was completed 6 months later (Zhangetal., 2010).The freezing effect on the soil behind the freezing wall was good after excavation, and the freezing wall did not collapse.The high strength of the freezing wall provided sufficient construction conditions for excavation and support of the side passage. Through summarizing the monitoring data and the construction experience, the design scheme of the freezing method was continuously optimized and the freezing method was made more safely and economically.

Fang JH, Zhang ZH, Zhang JY, 2009. Application of artificial freezing to recovering a collapsed tunnel in Shanghai metro No.4 line. China Civil Engineering Journal, 42(8): 124-128.

Li DW, Wang RH, 2008a. Frozen soil creep model based on statistical damage theory. Chinese Journal of Applied Mechanics, 25(1): 133-136.

Li DW, Wang RH, 2008b. Nonlinear rheological constitutive model for frozen sand under complex stress paths and its engineering application. Chinese Journal of Geotechnical Engineering, 30(10):1496-1501.

Li DW, Wang RH, Hu P, 2007a. Study of frozen soil creep damage-coupling constitutive function. Journal of Glaciology and Geocryology, 29(3):446-449.

Li DW, Wang RH, Hu P, Cui H, 2007b. Study on two yield-surface rheological model of frozen soil by unloading state. Rock and Soil Mechanics, 28(11): 2337-2342.

Li DW, Wang RH, Lin B, 2007c. Viscoelastic plastic model of frozen soil and numerical calculation soft matrix applied. Journal of Glaciology and Geocryology, 29(2): 321-326.

Li DW, Wang RH, Zhao YH, Hu P, 2007d. Research on parabolic yield-surface creep constitutive model of artificial frozen soil. Rock and Soil Mechanics, 28(9): 1943-1948.

Li DY, Lu AZ, Zhang QH, Chen YK, 2004. Analysis of freezing method for construction of connected aisle in Nanjing metro tunnels. Chinese Journal of Rock Mechanics and Engineering, 23(2): 334-338.

Wang RH, Yu CH, Xiong SY, Wang GC, 1996. Simulating experiment study on stability of frozen soil retaining wall. Chinese Journal of Geotechnical Engineering, 18(5): 52-57.

Yang P, Ke JM, Wang JG, Chow YK, Zhu FB, 2006. Numerical simulation of frost heave with coupled water freezing, temperature and stress fields in tunnel excavation. Computers and Geotechnics, 33:330-340.

Yue FT, Qiu PY, Yang GX, Shi RJ, 2006. Design and practice of freezing method applied to connected aisle in tunnel under complex conditions.Chinese Journal of Geotechnical Engineering, 28(5): 660-663.

Zhang M, Lin HB, Zhou XZ, Xiao YS, 2010. Design and calculation of the frozen wall of a river-crossing tunnel by means of horizontal freezing.Journal of Glaciology and Geocryology, 32(4): 773-777.

10.3724/SP.J.1226.2011.00314

*Correspondence to: Dr. Ming Zhang, Engineer of John Finlay Engineering & Technology Group of Companies, 28F, CITIC Building, No.19, Jiangguomenwai Street, Beijing 100004, China. Tel: +86-10-65254865; Email: 0556zm@163.com

14 January 2011 Accepted: 4 March 2011