Combined freeze-sealing and New Tubular Roof construction methods for seaside urban tunnel in soft ground

Combined freeze-sealing and New Tubular Roof construction methods for seaside urban tunnel in soft ground

Tunnelling and Underground Space Technology 58 (2016) 1–10 Contents lists available at ScienceDirect Tunnelling and Underground Space Technology jou...

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Tunnelling and Underground Space Technology 58 (2016) 1–10

Contents lists available at ScienceDirect

Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust

Combined freeze-sealing and New Tubular Roof construction methods for seaside urban tunnel in soft ground Yongshui Kang a, Quansheng Liu b,⇑, Yong Cheng c, Xiaoyan Liu b a

Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, State Key Laboratory of Geomechanics and Geotechnical Engineering, Wuhan, Hubei 430071, China Key Laboratory of Rock Mechanics in Hydraulic Structural Engineering of Ministry of Education, School of Civil Engineering and Architecture, Wuhan University, Wuhan, Hubei 430072, China c China Communications Construction Company Second Highway Consultant Co., Ltd., Wuhan, Hubei 430056, China b

a r t i c l e

i n f o

Article history: Received 6 July 2015 Received in revised form 20 March 2016 Accepted 11 April 2016

Keywords: Seaside urban tunnel Freeze-sealing New Tubular Roof (NTR) method Soft soil

a b s t r a c t Construction of seaside urban tunnels is significantly difficult due to the densely adjacent buildings and underground facilities in conjunction with unfavourable geological conditions. For this, this paper investigates the construction methods used for Gongbei Tunnel, which is a typical seaside tunnel connecting Hong Kong-Zhuhai-Macau Bridge in China. The construction methods and stability of the undercutting section will be focused. In the tunnel section of interest, dozens of thick steel tubes are jacked into the soft soil, and freeze-sealing method is applied to form a thick water-proofing wall. In this study, the combined freezing and New Tubular Roof (NTR) method are simulated by thermo-mechanical coupling analysis. The temperature field obtained in the freezing process indicates that the thickness of frozen wall grows approx. 2.0 m after 50 days of freezing. Besides, the stability of the surrounding ground and supporting structures in the bench-cutting stage are also investigated. Then the thawing process is simulated and associated suggestions for post-grouting to prevent excessive thawing settlement are proposed. The numerical results show that the tunnel is stable and the influence of tunnel excavation on adjacent buildings is within the permissible range. It also shows that the designed construction methods can be used to adequately meet safety and stability standards when adopting the proposed construction and supporting system. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction With the rapid economic development over the past decades, China is now observed with large cities developing quickly. Some cities are becoming increasingly crowded due to rapid increase of urban population. In order to fully utilize the space in urban areas and relieve the pressure due to heavy traffic and living spaces, construction of underground space has developed quickly in recent decades (Li et al., 2014; Yang et al., 2014). However, increasing urban underground structures, such as urban tunnels and subways in China, have led to some serious geotechnical problems, for example the stability of underground tunnels with large crosssections. Similarly, some specially developed excavation and support methods should be proposed to meet the requirements on stability of adjacent buildings or underground facilities. Shallow excavation could inevitably induce ground movements and surface settlements in urban areas (Jongpradist et al., 2013; ⇑ Corresponding author. E-mail address: [email protected] (Q. Liu). http://dx.doi.org/10.1016/j.tust.2016.04.001 0886-7798/Ó 2016 Elsevier Ltd. All rights reserved.

Raphael et al., 2015). This might cause excessive deformation and damage to nearby existing structures and facilities, such as underground pipelines and buildings (Fang et al., 2011; Hou et al., 2015). The interaction between soil, tunnels, and underground pipelines is very complex (O’Rourke and Trautmann, 1982; Sirivachiraporn and Phienwej, 2012), especially in the seaside areas where ground water table is higher than that of in-land locations. Attewell et al. (1986) showed that the bending moment due to soil deformation is the main factor result in the damage of pipelines. For serviceability purposes, deformation of surrounding soils of the tunnels should be adequately controlled. Moreover, noise and vibration problems associated with these underground tunnels also need to be solved. Therefore, there is an urgent need to develop effective and safe construction methods for urban underground structures. Artificial freezing method proposed by Poetsch in the 1880s is often used in tunnel excavation, mining and underground space excavation in soft ground and rocks. In 1906, it was used in a subway tunnel crossing beneath a river in France (Wu, 1988). From the 1950s, this method has been widely used and improved in many countries, including Switzerland, USA, Australia, and China, etc.,

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(Rojo et al., 1991; Chen et al., 2000). In the 1960s, engineers in Japan began to improve freezing techniques on railway tunnels and other underground structures in water-rich soft soils (Andersland and Ladanyi, 2004; Gianpiero et al., 2015). New Tubular Roof (NTR) method (also called tubular-roof preconstruction method) is a good choice for construction of urban tunnels and long-span underground structures, where the deformation of surrounding environments should be strictly controlled. In this method, large-caliber steel tubes are jacked into target position to shield inner excavation space. The NTR method is an improved version of the pipe roof method (InnJoon et al., 2005). It is becoming increasingly popular for the construction of subways and tunnels around the world. In the 1970s, Antwerp Central Station in Belgium utilized tubular jacking method (Hemerijckx, 1983; Hisatake and Ohno, 2008). Since then the NTR method has been commonly used and improved in many countries. In the 1970s, tubular roof method was used in Kawase–Inae in Japan. In the 1990s, Italy further developed this method and constructed 29 m wide Venzia Station tunnel (Lunardi, 1991). In 1994, the method was used in the USA. In the late 1990s, the NTR method was introduced to Korea and was greatly improved in subsequent years (Kim et al., 2002). In the 2000s, the NTR method was used in construction of subway stations in Beijing and Shanghai in China. Several improved construction methods, based on the NTR method, such as endless self–advancing (ESA) method and roof–box jacking method, have been proposed in Japan in the last decade. It is a challenging issue to study the stability of the surrounding ground when using the NTR method for the construction of large cross-sectional seaside tunnels, such as the Gongbei tunnel in Hong Kong-Zhuhai-Macau Bridge. The geological condition of seaside urban tunnel is quite complex, and the soils are soft and rich in water. However, few engineering experiences or studies are reported on the application of combined freeze-sealing and NTR method in such a long-span seaside tunnel. In order to prevent serious groundwater seepage and improve the strength of surrounding soil, freeze-sealing method is applied in combination with NTR method in this tunnel. Several freezing pipelines are installed inside the jacking tubes and a frozen wall around the tubes can be formed during freezing process to prevent water seepage. The stability of tunnel should be evaluated prior to excavation. Firstly, the evolution of temperature field for determination of the thickness of frozen wall is investigated. Then freezing construction scheme and growing thickness of frozen wall are determined for the optimum freezing time. Next, freeze-thawing action under multi-field coupling process is investigated. The water-rich soil would expand upon freezing and shrink after thawing, which would impose additional deformation and pressure on the tunnel surrounding soil and structures. Finally the stress state of the supporting structures and ground movement under the proposed excavation and support methods is calculated. During the numerical analysis, a three-dimensional (3D) model of the tunnel using combined freeze-sealing and NTR construction methods for seaside urban tunnel in soft ground is proposed in this research.

2. Project overview Gongbei Tunnel, located in China’s Zhuhai district adjacent to Macau, is the key part of the Hong Kong-Zhuhai-Macau Bridge, which connects three important cities, as shown in Fig. 1. This tunnel starts from the Gongbei Bay area and is connected to the seacrossing bridge, with a total length of 2741 m. The left line (14.25 m wide and 5.1 m high) starts from chainage No. ZK1+150 to No. ZK3+891; while the right line starts from chainage No. YK1+515 to No. YK3+890. The NTR method is used in the middle part (255 m long) of the tunnel. Two work shafts are set in the

two ends of the buried section. The open-cut method is applied for the rest sections of the tunnel. Diaphragm wall and steel or reinforced concrete supports are the major support methods for the open-cut sections. The self-stability of the soil layers is poor and the geological condition is rather complicate due to the soft ground near seaside. In addition, various buildings and underground facilities of different types, such as electric pipelines, water supply pipelines, and telecommunication facilities, are quite densely distributed near this tunnel regime, which attribute to the difficulties of construction. In this condition, the tunnel safety should be ensured and the ground deformations are within acceptable limits during tunnel construction. The soil at the site is mainly land filled and constitutes layers of medium sand, silty clay, gravelly sand, and muddy clay among others. The water content of the soil is generally high and several soil layers are rich in humus. The sludge layer is basically of liquid-plastic state with high compressibility, sensitive thixotropy and high porosity. As a result, the soil strength is significantly low. The chemical composition of the groundwater is quite similar to that of the seawater because it mainly constitutes chlorides of calcium and magnesium (Cl1ACa2+Mg2+). The permeability of sands such as medium sand and gravelly sand is very high. While the sludge, clays, and residual soils have relatively low permeability. The maximum salinity contents of sulfate, magnesium salt and Cl1 are 595, 91, and 690 mg/kg, respectively. Comprehensively, the ground water can be evaluated as a corrosive medium for the concrete structures. The main mechanical parameters of the soils are listed in Table 1. 3. Construction of the tunnel section using the NTR method Gongbei tunnel is constructed by various methods at different sections, including open-cut method and undercutting method (combined freeze-sealing and the NTR method). Structural styles are also diverse as shown in Fig. 2. This paper mainly focuses on the undercutting section using combined freeze-sealing and the NTR method. 3.1. Design of major supporting structure The total length of the section using the NTR method is 255 m. A couple of work shafts are set at the ends of this section. The left and right lines of the tunnel intersect and overlap in this section. Thirty-six large-caliber steel tubes are jacked into the soils along the tunnel contour as shown in Fig. 3. Cross-sectional area of the tunnel section is 330 m2. The height and width are 20.8 m and 18.7 m, respectively. Combined freezesealing and the New Tubular Roof (NTR) method is used. The major supporting system of the tunnel is shown in Fig. 4. There are 36 steel tubes (1.62 m in diameter) jacked into the soil as advancing tube roofing. Freezing pipelines are installed in the tubes (see Fig. 5), and a frozen wall around the tubes will be formed to prevent the water seepage in freezing process. C25 shotcrete with double mesh reinforcement and steel arch made of 22b joist steel are used to form the primary supporting structure. The secondary lining is made of light steel frame (0.30 m thick) with cast-in-place C35 concrete. The third lining is made of corrosionresistant reinforced concrete lining structure and the thickness of the third lining in invert arch, side wall, and floor is 0.6 m, 0.7 m and 0.5–1.5 m, respectively. 3.2. Design of the freeze sealing construction The main purpose of freezing construction is to form a water proofing wall around the profile of the tunnel. The arrangement

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(a) distribution and surrounding constructions

(b) sketch of geological profile and the tunnel (the horizontal scale is 1/10 of the vertical scale) Fig. 1. Location of Gongbei tunnel.

Table 1 The serial number and main mechanical parameters of the soil layers. Soil layers

Gravity (kN/m3)

Natural porosity ratio

Elasticity modulus (MPa)

Poisson’s ratio

Cohesive (kPa)

Frictional angle (°)

Artificial earth fill c-1 silt c-2 silty clay c-3 silver  Gravelly sand d-2 silty clay d-3 muddy-silty clay e-1 silty clay  Silt e-2 silty-fine sand e-3 silty clay  clay f-2 silty sand g-1 sandy clay g-1 gravel clay h-1 completely decomposed granite h-2 intensely decomposed granite

17.7 15.2 19.5 19.5 19.0 17.7 19.3 19.4 19.3 19.6 17.7 19.6 17.8 20

0.40 2.24 0.73 0.53 0.71 1.24 0.81 0.66 0.83 0.48 1.06 0.48 0.99 1.06

20 1.5 20 25 28 4 30 35 28 50 45 60 300 2000

0.3 0.4 0.28 0.25 0.32 0.35 0.3 0.25 0.3 0.25 0.25 0.25 0.25 0.3

12 10 16 0 16 10 18 0 15 0 20 0.5 50 200

15 4 10 24 14 5 15 33 8 34 20 10 30 40

of freezing pipes is shown in Fig. 5. The main freezing pipes are laid in the steel jacking tubes filled by concrete, and the limiting pipes are longitudinally divided into three sections to control different freezing regions. Secondary freezing pipes are welded in the remaining jacking tubes. The freezing process is mainly controlled by the main freezing pipes. The limiting pipes are used for adjusting temperature field to prevent serious frost floor heaves. They are timely activated according to monitored ground temperature and displacement. The temperature of the salt water in the freezing pipes is set to 25  30 °C in active freezing stage and 22  25 °C in the secondary freezing stage. According to the bench-cut design, the cross

section is vertically divided into five freezing regions. The soil is completely frozen in the freezing stage while thaws gradually in the thawing stage. Massive volume deformation of frozen wall is prevented by grouting in the thawing stage. The main construction sequence is as follow: (1) Jacking the steel tubes into target ground from work shaft; (2) Conducting freezing process by main freezing pipes for several days (active freezing stage); (3) Adjusting temperature of the freezing ground by limiting pipes and secondary freezing pipes (Secondary freezing stage); Serious frost heave would be prevented in this step. (4) Reinforcing soil inside the tunnel to prevent collapse; (5) Bench cutting (shown in Fig. 4(b)) and installing primary support,

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(a) parallel section

(b) open overlapped section

(c) closed overlapped section

(f) parallel section

(e) transitional overlapped section

(d) NTR section

Fig. 2. Typical cross section style of the tunnel.

(a) sketch of the structure

(b) jacking tubes in the work shaft

(c) jacking machine

Fig. 3. Pictures of supporting structure in the NTR section.

(a) detailed supporting structure

(b) sketch of primary support and bench cut method Fig. 4. Main supporting methods for the tunnel.

waterproof materials, and secondary lining; (6) Constructing the third lining, and removing temporary joist steel supports; and (7) Stop freezing and grouting to prevent thawing settlement of the ground. 4. Numerical simulations of combined freezing and NTR construction There are many buildings and underground pipelines near Gongbei tunnel. High local temperature and abundant rainfall make the construction process very difficult and complicated. Since the site is located near seaside, soil is rich in ground water

and the sedimentary condition of the surrounding soil is very complex leading to poor self-stability to the surrounding ground. Numerical simulation of seaside tunnels constructed using combined freezing and the NTR method has not gained much attention of researchers in the past. Therefore, we developed a threedimensional numerical model for the Gongbei tunnel and carried out coupled thermo-mechanical analyses. The simulation mainly focuses on the change of temperature field, stress state of supporting structure, and stability of the surrounding ground under the proposed supporting system. The model is built and calculated by FLAC3D which is a three-dimensional explicit finite-difference program for engineering mechanics computation.

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According to Eq. (1), the frozen ratio is an exponential function associated with time under heat insulation condition. Temperature boundary condition in freezing construction is usually fixed by freezing pipes. Generally speaking, the soil can be considered to achieve heat balance state when frozen for two hours at certain temperature. Thus, according to Eq. (1), the frozen ratio which determines the frozen state of the soil can be considered as varying linearly with the freezing temperature. Therefore, the main mechanical properties can be simplified correlated linearly with the freezing temperature in effective freezing stage. This conclusion has been verified by the results of laboratory tests. The parameter function curve is simplified as shown in Fig. 6. In order to identify the changes in mechanical properties due to freezing, a series of laboratory tests were conducted before designing the freezing construction. According to the thermal and mechanical tests on the soil samples collected from drilling holes, the following conclusions are drawn:

(a) structure sketch

(b) picture of freezing pipe Fig. 5. Arrangement of freezing pipes.

4.1. Modification of mechanical properties of freezing soils The main purpose of the freezing construction is to form water proofing wall outside the excavation space. Freezing would induce significant changes in mechanical properties of soil and it should be appropriately taken into account in the numerical model. For instance, the strength of frozen soil will increase because the ice can combine the soil grains. The phase transition in soil is a complex process which involves coupling of thermal-hydraulicmechanic fields. Moreover, additional deformation would be induced as volume expansion of up to 9% could occur when water freezes. Frozen ratio that represents the percentage of water phase changed into ice is a key variable that controls the freeze/thaw effect, and it is determined by several factors such as temperature and frozen time. Both heat conduction and latent heat transformation are time-related processes. The frozen ratio of continuous porous medium can be expressed by an exponential function as follows (Kang et al., 2013):

ui ðtÞ ¼

  k t ð1  nÞC s qs  wi ðT 0  TÞ 1  e ð1nÞCs qs Lnqw Sr

ð1Þ

(1) The specific heat of soil is between 1.34 and 1.56 J g1 K1. The thermal conductivity of soil at normal temperature is varied between 1.066 and 1.537 kCal/mh°C, while this value increases to 1.51 and 2.06 kCal/mh°C at the freezing temperature. The freezing point of soil is between 0.4 °C and 1.8 °C. (2) Measured freezing pressure during the tests is between 0.73 and 0.93 MPa, while the volume expansion ratio was between 1.08% and 3.41%, meaning that the frozen expansibility of the soil is quite weak. (3) Uniaxial compressive strength of soil is linearly correlated with the freezing temperature. The strength of freezing soil increases from 0.093 MPa to 0.282 MPa when temperature decreases by 1 °C. Modulus of elasticity of frozen soil can increase from 2.504 to 9.318 MPa when temperature decreases by 1 °C. As freezing temperature decreases, compression strength of soil increases gradually while Poisson’s ratio reduces. The Mohr–Coulomb circles from triaxial tests on some frozen soil at 5 °C to 15 °C are shown in Fig. 7, which indicate that the shear strength of frozen soil increases when freezing temperature decrease. Therefore, the soil in the frozen wall can be strengthened by intensified freezing. 4.2. Modeling and analysis method A three-dimensional model of the tunnel and the surrounding soil is developed by FLAC3D as shown in Fig. 8. The size of the model is 120  80  50 m. In order to investigate the typical section of the tunnel, curvature of the tunnel is neglected. All of the soil layers are considered as equivalent continuous isotropic

where, ui(t) is the frozen ratio of water-ice system, ui ¼ mi =ðmi þ mw Þ, 0 6 ui 6 1; mi and mw denote the mass of ice and unfrozen water respectively; L is the latent heat coefficient of water-ice system (J/kg); km is the heat conductivity of the soil grain [kJ/(m s K)]; T is temperature; T0 is the freezing point of the water; Ts0 is the initial temperature of soil grain before freezing; Cs is the specific heat of soil grain [kJ/(kg K)]; qs and qw respectively denote the density of soil grain and water (kg/m3); n and Sr are the porosity and saturation degree, respectively; The initial value of frozen ratio is assumed to be zero, ui(t = 0) = 0. Specially, when T 0  T s0 > DT se and t P t e , ui = 1, where, DT se is defined as critical temperature change; te is a critical time variable.

t e ¼ B ln

A A1



ð1  nÞC s qs ð1  nÞC s qs ;A ¼ ðT 0  T s Þ kwi Lnqw Sr

5

ð2Þ Fig. 6. Simplified parameters function.

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(a) filling

(b) medium sand

(c) silty clay

(e) gravelly sand

(d) muddy-silty clay

(f) gravelly clay

Fig. 7. Mohr-Coulomb circles form tests on frozen soil at 5 °C to 15 °C.

(a) the model (120×80×50m) and soil layers Fig. 8. Calculating model for the tunnel.

(b) elements of the supporting structure

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(a) primary support

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(b) excavation steps

Fig. 9. The elements of primary support and bench cut.

(a) 10 days

(c) 30 days

(b) 20 days

(d) 50 days

Fig. 10. Distribution of temperature field during freezing process (Unit: °C).

Table 2 The mechanical parameters of major supporting structure. Supporting structure

Gravity (kN/m3) Elasticity modulus Poisson’s (GPa) ratio

Jacking tube (steel) 78.5 Lining (reinforced concrete) 25.0 Primary support (steel) 78.5

210 30 210

0.22 0.20 0.22

mediums and are assumed to follow the Mohr–coulomb law. The top boundary of the model is free while bottom boundary is fixed in both vertical and horizontal direction. Besides, the vertical faces

of the model are fixed in horizontal direction. The jacking steel tubes are modeled as elastic elements. Mechanical and thermal properties of soils are assigned according to the laboratory tests (Table 1). The properties of major supports are shown in Table 2. Half of the tubes are filled by plain concrete before freezing and excavation. The temperature of the freezing tubes is fixed to 30 °C for 50 days in active freezing stage. A coupled thermomechanical analysis is conducted. A frozen wall forms and grows thicker gradually as the freezing process continues. Mechanical properties of the frozen zones are modified every 1000 steps. The model is firstly self-balanced and then the displacements in all directions are initialized to zero before freezing and excavation.

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(a) surrounding ground (cut plane x=0)

(b) jacking tubes (odd number filled with concrete)

Fig. 11. The calculated vertical displacements.

(a) minimum principal stress

(b) maximum principal stress

Fig. 12. Distribution of principal stress of the third lining (negative stress compression; Unit: Pa).

Fig. 13. Horizontal displacements data collected from vertical cut planes near the edge of piles of adjacent buildings (final temperature 15 °C).

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(a) thawing for 20 days

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(b) thawing for 30 days

Fig. 14. Distribution of temperature filed during thawing process.

Then the steel tubes are jacked into target zone, and the odd numbered tubes are filled by plain concrete. Afterwards, the soil in the tunnel was excavated stepwise and primary supports are installed as shown in Fig. 9. Finally, the secondary and third linings are built as shown in Fig. 8(b).

the soil is generally weak, the thawing deformation cannot be neglected. Stepwise grouting is required to prevent ground settlement while thawing. Besides, Fig. 14 shows that the outer fringe of the frozen wall thaws earlier than the inner part. Therefore, it is better to gradually grout from outside to inside.

4.3. Calculating results

4.4. Safety assessment of adjacent buildings

4.3.1. Temperature field in active freezing stage Designed thickness of the frozen walls before excavation is 1.8–2.0 m. The temperature field when freezing continued for 10–50 days are shown in Fig. 10. The results indicate that the thickness of the frozen wall grows gradually in the active freezing stage. The thickness of the outer frozen wall (where the temperature value is lower than 2 °C) is approx. 2.0 m after 50 days freezing, which can well meet the design requirements.

According to the Standard for appraiser of reliability of civil buildings of China, the maximum allowable roof displacement in an adjacent building due to excavation is 26 mm. According to the Code for design of building foundation of China, the maximum allowable inclining ratio of the buildings in adjacent to a tunnel construction site is 4‰. The adjacent building is about 12 m high, and the maximum ground settlement is 8–10 mm. Therefore, the computed inclining ratio due to the differential settlement of the building is lower than 0.8‰, meaning the additional displacement caused by the tunnel excavation is within the permissible range.

4.3.2. Stability in bench cut stage In order to investigate the stability of the surrounding ground and the supporting structures, displacements of the model during the bench cut process are monitored as shown in Fig. 11. The soil in the inner part of the tunnel is reinforced because it’s pregrouted to prevent collapse. Fig. 11 shows that the displacement of the surrounding ground is controlled at low level. Nonetheless, the displacement of the middle layer in the side wall exceeds 1.5 cm due to the low strength and compression modulus of the muddy-silty clay (d-3), shown as the blue colored region in Fig. 11(a). The displacements of the jacking tubes are very low (in the order of 101 mm), as shown in Fig. 11(b). 4.3.3. Stability after building the third lining The distribution of principal stresses of the third lining is shown in Fig. 12. Obvious stress concentration occurs in the corners and spandrel of the lining. Inner surface of the floor and lower part of the side wall are in tension, while the top area stays in compression. We can conclude that the supporting structure of the tunnel mainly bears the horizontal pressure from two sides of the surrounding ground. Horizontal displacement data collected from the cut planes near the edges of piles of adjacent buildings are plotted in Fig. 13. This figure confirms that the horizontal displacement mainly occurs in the depth of 5–28 m, and the maximum value of the horizontal displacement is about 5 mm. 4.3.4. Thawing stage and proposal for post grouting In order study the variation of temperature field after ceasing freezing process, the temperature of linings and tubes are set to 15 °C for thawing simulations. The temperature fields after thawing for 20 and 30 days are shown in Fig. 14. Ice crystal would shrink when phase changed to liquid water (temperature rises to 2 °C). Although laboratory tests showed that the frozen expansibility of

5. Conclusions Seaside urban tunnels with large cross sections are extremely hard to construct, because many factors should be taken into account during construction, such as soft soil, rich ground water, adjacent buildings, and underground utilities. The New Tubular Roof (NTR) method is a good choice for the construction of urban tunnels when the surrounding environment is restricted and is not suitable for open-cut method. Freeze-sealing is also necessary for seaside tunnels with large cross sections, although the combined construction methods might be more complicated and expensive. There are several problems which should be addressed based on geological survey before construction. The change of temperature field is a key factor to predict the thickness of a frozen wall. The stability of the supporting structures during bench cut process should also be taken into account to evaluate the safety condition of the tunnel. In addition, serious frozen heaves and thawing ground settlements should be prevented in the construction. In this paper, we have analyzed some of the critical problems of the combined construction methods and have provided recommendations for future improvements. The findings of this study are expected to have a significant impact on the urban tunnel design and construction industry. Acknowledgements This research is financially supported by the National Natural Science Foundation of China (nos. 41130742 and 41302237), and China National Key Basic Research Development Plan (973 Plan, no. 2014CB046904).

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