Representative Projects and Development Trend of Underwater Shield Tunnels in China

XIAO Mingqing

(China Railway Siyuan Survey and Design Group Co., Ltd., Wuhan 430063, Hubei, China)

0 Introduction

The underwater tunnel is an important means to cross waterway obstacles, with main construction methods including shield method, immersed tube method, mining method and cofferdam open-cut method. Among them, the shield method, due to its advantages of safe construction, superior quality, fast construction speed, outstanding geological adaptability, favorable operating environment in caves and wide suitability and application scope, etc., has become the most widely used method[1]. In the world, about 70% of the underwater tunnels were constructed by the shield method. China encompasses numerous rivers, lakes and sea waters, but now has less and less bridge site availabilities with ever-increasing demand for cross-waterway channels. Therefore, there is a huge potential demand for underwater shield tunnels in the future.

A large number of experts and scholars have analyzed and discussed underwater shield tunnels, among which Yang[2]introduced several major underwater shield tunnels built in the world, such as the Channel Tunnel, the Tokyo Bay Highway Tunnel in Japan, the Scythe River Tunnel in Netherlands, the 4th Tube of Elbe Tunnel in Germany, the Shanghai Chongming Yangtze River Tunnel in China, and their technical characteristics. The technical development of the underwater shield tunnels was also discussed. Sun et al.[3]analyzed the reasonable cover thickness of underwater shield tunnel, the shield type selection and key parameters. In addition, Sun also summarized the key technologies of underwater shield tunnel construction, such as the stability of excavation face, the measures to prevent the eruption of slurry, prevention of segment uplifting and shield attitude control. Lu et al.[4]discussed the underwater shield tunnel construction safety and risk assessment methods. He et al.[5]summed up the current research status of the large underwater shield tunnel structures, and discussed the existing problems as well as the research trends of the related structural problems. Hong[6]analyzed the underwater shield tunneling technology and shield docking technology in hard rock. Song et al.[7]discussed the reasonable cover thickness of underwater shield tunnel. The development of design technology of large diameter shield tunnel in China was summarized by Xiao[8]. Guo[9]studied the geological exploration experience as well as the principle and method of underwater tunnel for highway. Xiao et al.[10]conducted a study of structure type of shield section for Wuhan Yangtze River Tunnel. The researches above have promoted progress and development of underwater shield tunnel in China, which mainly aim at a single project or some problems of the underwater shield tunnel. However, there is few literature on the achievements in recent years and the development trend of the underwater shield tunnel construction.

In this paper, the development and current status of China′s underwater tunnel are summarized, the technical problems and the main technical progress of the representative underwater tunnels built in the past are analyzed. Meanwhile, characteristics and difficulties of several typical underwater tunnels under construction or awaiting for construction in China are also discussed. In addition, the development trend of China′s underwater tunnel and the existing problems are proposed to further promote technological progress of China′s underwater shield tunnel.

1 Development history and status quo of China′s underwater tunnels

1.1 Development history

Originated in the 1840s, the shield method used to be mainly applied in single weak formations with minor faults. Along with the increasing demands for construction of "large diameter, high water pressure and long distance tunnels in complex geology", modern shield technology emerged at the end of 20th Century.

In China, Shanghai is the pioneer city for large-scale underwater tunnel construction, where Dapu Road Tunnel of 10.22 m outer diameter (OD) and East Yan′an Road Tunnel of 11.30 m OD were completed in 1966 and 1984, respectively, launching China′s large-scale underwater tunnels construction[11]. In the 1990s, three more large scale cross-river tunnels of 11.0 m OD were completed, including East Yan′an Road Tunnel (south line), Dalian Road Tunnel and East Fuxing Road Tunnel.

Recently, the construction of a number of underwater tunnels has advanced China′s shield method technology for underwater tunnel construction to a new level. By the end of 2016, China has built over 70 underwater tunnels of various diameters and for various purposes using the shield method, crossing Huangpu River, Yangtze River, Pearl River, Qiantang River, Xiangjiang River, Yellow River and other waters. Among the completed underwater tunnels, Shanghai Yangtze River Tunnel (8 740 m long, with two-way and six lanes) and Hangzhou Qianjiang Tunnel (4 250 m long, with two-way and six lanes) have a maximum diameter of 15.0 m. Shiziyang Tunnel of Guangzhou-Shenzhen-Hong Kong High-speed Railway (10 800 m long, with double-hole single-lane and design speed of 350 km/h) has the largest tunnel length (shield length of 9 340 m) and the highest water pressure (0.67 MPa); Yangtze River Tunnel (3 905 m long, two-way and six lanes) created the longest tunnel section with full face in sand layer, with a diameter of 14.5 m and a shield section length of 3 022 m. Wuhan Yangtze River Tunnel (3 630 m long, consisting of 2 holes of 11.0 m diameter) is the first large diameter shield tunnel crossing the Yangtze River, which was opened to traffic in December 2008. There are still many other completed representative tunnels, including Qingchun Road Qiantang River Tunnel(3 765 m in length) with 11.3 m OD in Hangzhou, which was opened to traffic in December 2010; interzone Yangtze River Tunnel (3 100 m long) of Wuhan Metro Line 2 with 6.2 m OD, which was opened to traffic in December 2011; Yangzhou Slender West Lake Tunnel (2 230 m long, with double-deck four lanes) with 14.5 m OD, which was opened to traffic in September 2014.

1.2 Representative projects and technical progresses of completed tunnels

1.2.1 Nanjing Yangtze River Tunnel[12]

Nanjing Yangtze River Tunnel is an underwater shield tunnel (Figs. 1 and 2) which has the largest diameter, the highest water pressure and the smallest thickness-span ratio among those built in high permeability and high abrasion formations in the world. The tunnel is 3 905 m long, containing 3 022 m long shield segment with two-way six-lane and a design speed of 80 km/h, was commenced in September 2005 and opened to traffic in May 2010. As the most difficult underwater tunnel in terms of construction at that time in China, the project posed a series of technical challenges such as oversized diameter (OD of 14.93 m), high water pressure (0.65 MPa), high permeability of ground (85% of the segment penetrates through silty fine sand, gravel sand and pebble stratum, with a maximum permeability coefficient of 43.2 m/d), high formation abrasion (with a maximum quartz content of 60%, a maximum pebble particle size of about 30 cm, and a maximum SPT blow counts larger than 50), long distance excavation (with continuous excavation length of 3 022 m), ultra-shallow overburden (the smallest cover is 10.49 m in river), abrupt terrain change (the secondary deep trench is next to the levee slope toe).

Fig. 1 Vertical section of Nanjing Yangtze River Tunnel

Fig. 2 Cross section of Nanjing Yangtze River Tunnel

The main technical progresses include:

(1)Established assurance technology system for structural safety for entire design, construction and operation process of super-large diameter shield tunnels, providing systematic solutions for a series of technical difficulties such as tunnel lining stability control in ground of high water pressure and permeability, durability design and preparation of segment, fire resistance performance of structure and post-disaster assessment.

(2) Developed cutter configuration technologies which adapt to high-abrasive, dense sandy cobble and gravel formations, and made innovation in cutter replacement technology and slurry filtering technology under hyperbaric condition, making breakthrough in long-distance continuous excavation of super-large diameter boring in high water pressure and high abrasive formation.

(3) Established technology system for safe excavation and stability control of excavation interface for super-large diameter slurry shield under conditions of high water pressure and permeability, shallow depth and abrupt terrain change.

1.2.2 Shiziyang Tunnel of Guangzhou-Shenzhen-Hong Kong High-speed Railway[13]

Shiziyang Tunnel is the first underwater shield tunnel for high-speed railway in the world, and also the longest underwater tunnel and the first railway underwater tunnel ever built in China (Figs. 3 and 4). This double-hole single-line tunnel has a total length of 10.8 km and an outside diameter of 10.8 m. As the critical project of Guangzhou-Shenzhen-Hong Kong High-speed Railway, it penetrates the Pearl River estuary (i.e. Shiziyang) where geological and environmental conditions are extremely complex. Its construction posed a number of technical difficulties such as high vehicle speed (design speed of 350 km/h, the highest speed for underwater tunnels in the world), long excavation distance (shield length of 9 340 m, which is the longest underwater excavation distance in China), complex stratum (large-diameter shield penetrates successively through soft formations, soil-rock composite stratum, bedrock and the fracture zone, which is the first of its kind in China), high water pressure (0.67 MPa, which is the highest water pressure during construction in domestic records), strict safety standard (high speed, high density, large traffic volume, and underwater).

Fig. 3 Sketch of Shiziyang Tunnel of Guangzhou-Shenzhen-Hong Kong High-speed Railway

Fig. 4 Cross section of Shiziyang Tunnel of Guangzhou-Shenzhen-Hong Kong High-speed Railway

The main technical achievements include:

(1) Established structural system and design method of underwater shield tunnels for high-speed railways, and made breakthroughs in structure security and smoothness control of underwater shield tunnels of high-speed railways.

(2) Developed underground shield engagement technology, and solved tunnel construction problems in deep water and wide sea area; made breakthroughs in technical challenges for large-diameter shield in long distance and continuous penetration through soft soil, sand bed, rock stratum and fracture zones.

(3) Formed the standard for clearance in 350 km/h super long double-hole tunnels and the standard for heat release rate in case of fire on high-speed trains, and constructed double-hole tunnel underwater emergency rescue station, which provided a comfortable and safe environment and reliable evacuation passages in underwater tunnels of high-speed railways.

2 Features and difficulties of representative tunnels under construction

2.1 Roadway-Railway Yangtze River Tunnel on Sanyang Road, Wuhan[14]

2.1.1 Project overview

Commenced in December 2014, it is a cross-river tunnel shared by urban roadways and subways. The road tunnel has a total length of 4.65 km (Fig.5) which contains 2.59 km of roadway-railway shield segment with 15.2 m OD. The tunnel mainly penetrates the fine silt layer, and about 1.2 km segment in the river involves cutting into bedrock(silty mudstone and weakly cemented conglomerate) with a maximum cutting depth of 9 m. The maximum water pressure is about 0.64 MPa.

From a sectional view, the shield tunnel has a three-level layout (Fig. 6); the upper level houses the road tunnel exhaust vent; the middle level is for road traffic and each tunnel contains 3 lanes; the lower level accommodates metro traffic and evacuation passages, cable corridors and metro exhaust vents.

Fig. 5 Vertical section of Roadway-Railway Yangtze River Tunnel on Sanyang Road (unit: m)

Fig. 6 Transverse cross section of Roadway-Railway Yangtze River Tunnel on Sanyang Road

2.1.2 Main features and difficulties of the project

(1) It is the first roadway-railway shield tunnel in the world.

(2) It is the largest shield tunnel under construction in terms of diameter in mainland China.

(3) It is China′s first super-large diameter shield tunnel (larger than 15 m) which penetrates soil-rock composite stratum.

(4) It is China′s first super-large diameter shield which is routed with super-small clearance for the first time, and the clearance at the north shoreside is only 5.0 m.

2.2 Shiziyang Tunnel of Foshan-Dongguan Intercity Railway[15]

2.2.1 Project overview

The tunnel has a total length of 6.15 km (Fig. 7) which includes a shield segment of 4 900 m, with a water width of about 1.8 km and a design speed of 200 km/h. The construction commenced in January 2015.

The shoreside segment covers up to 30 m thick silt and sand layers; the silt and sand layers in underwater segment is about 10-30 m in thickness, and the underlying bedrock mainly consists of argillaceous sandstone, sandstone and mudstone. The length of the tunnel penetrating the bedrock accounts for 70% of total excavation length, and the tunnel also crosses three fracture zones and two faults. The maximum water pressure is 0.78 MPa.

Fig. 7 Vertical profile of Shiziyang Tunnel of Foshan-Dongguan Intercity Railway

The tunnel has an inner diameter of 12.0 m and outer diameter of 13.1 m. It has a two-level layout (Fig. 8); the upper level accommodates traffic lanes and the lower level accommodates evacuation passages and pipeline corridors.

2.2.2 Main features and difficulties of the project

(1) It is the largest railway shield tunnel under construction in terms of diameter in China.

(2) It is one of the underwater shield tunnels that involve the highest water pressure under construction in China.

(3)It crosses a total length of 425 m of fault and fracture zone.

(4) Long distance crossing (4 900 m) is needed in the soil-rock composite formation.

2.3 Yangtze River Tunnel of Suzhou-Nantong UHV Power Transmission and Transformation Project

2.3.1 Project overview

As a critical project of the Huainan-Nanjing-Shanghai 1 000 kV AC UHV Transmission and Transformation Project, this tunnel was constructed by shield method with a shield segment of 5 468 m (Fig. 9) and maximum water pressure of 0.8 MPa. The construction commenced in August 2016.

The formations penetrated by the tunnel mainly include silt, silty clay, mixed silt, silty soil, silty fine sand, fine sand and medium coarse sand. The SPT blow counts for the fine sand and medium coarse sand exceed 50, and the sand is locally known as "steel-board sand".

Fig. 8 Cross section of Shiziyang Tunnel of Foshan-Dongguan Intercity Railway (unit: mm)

Fig. 9 Vertical section of Yangtze River Tunnel of Sutong 1 000 kV Electric Power Project

The tunnel has an outer diameter of 11.6 m and the inner diameter of 10.5 m, and has a two-level layout (Fig. 10). The upper level accommodates two circuits of GIL pipes and transportation, installation and maintenance access; and the lower level has reserved space for two circuits of 500 kV cable corridors at sides and patrol inspection access in the middle.

2.3.2 Main features and difficulties of the project

(1) It is the first large-diameter UHV power tunnel in the world.

(2) It is the first underwater power transmission tunnel in China.

(3) It is an underwater shield tunnel under construction that involves the highest water pressure in China.

(4) It is the longest tunnel that involves a dead-end heading in formations of high abrasion and large permeability in China.

3 Characteristics and difficulties of proposed representative tunnels

3.1 Pearl River Estuary Tunnel of Shenzhen-Maoming Railway

3.1.1 Project overview

Pearl River Estuary Tunnel is planned to start construction in early 2018. It is located about 5 km downstream of Humen Bridge and has design speed of 250 km/h.

The total length of the tunnel is 13.756 km (Fig. 11), including 7 225 m shield segment and 4 925 m mined segment. The elevation of the lowest rail surface is -108.8 m. The outer diameter and inner diameter of shield tunnel are 14.0 m and 12.8 m, respectively. The cross section has a two-level arrangement (Fig. 12) with the upper level as the traffic lane and the lower level as the evacuation route and pipeline gallery.

Fig. 10 Cross section of Yangtze River Tunnel of Suzhou-Nantong 1 000 kV Electric Power Project (unit: mm)

Fig. 11 Vertical section of Pearl River Estuary Tunnel of Shenzhen-Maoming Railway (unit: m)

3.1.2 Main features and difficulties of the project

(1)It is the longest underwater tunnel for railway in China. In the past, the longest underwater railway tunnel in China is the Shiziyang Tunnel of Guangzhou-Shenzhen-Hong Kong High-Speed Railway with a length of 10.8 km. The length of the Pearl River Estuary Tunnel is 13.756 km, which is 27% longer than the Shiziyang Tunnel.

(2)It is the first large diameter underwater shield tunnel crossing granite composite formation in China. In the past, large diameter underwater shield tunnels in China lacked the experiences of crossing granite complex strata. There were only a few cases of tunnel crossing granite complex strata in land, such as the deep buried tunnel (diameter of 12.8 m) for Guangzhou-Shenzhen-Hong Kong High-Speed Railway. Due to lack of ground pre-improvement and variations of strength in the granite composite formation, the large diameter shield tunneling under water faces are with greater safety risk than tunneling in land. Therefore, more specific measures should be taken in manufacturing the shield machine and controlling the tunneling parameters.

(3)It is the shield tunnel with the highest water pressure in China. Among the underwater tunnels completed in China, the highest water pressure is with the Nanjing Yangtze River Tunnel (diameter of 14.5 m, maximum water pressure of 0.75 MPa), while the maximum water pressure of this project is about 1.0 MPa. With the increase in water pressure, higher requirements are put forward for shield machine seal, segment structure self-waterproofing, joint gasket waterproofing, etc. In addition, the risk and difficulty for inspection of cutter and blade also increase.

(4) It is the first subsea tunnel constructed using both shield method and mining method in China. Previous domestic underwater tunnels generally use one single method. Based on the geological conditions and characteristics of the ground, both shield method and mining method would be used in this project, which will face difficulties in dismantling the shield machine in the mined tunnel as well as in connection and waterproofing of the two different tunnel structures.

Fig. 12 Cross section of Pearl River Estuary Tunnel of Shenzhen-Maoming Railway (unit: mm)

3.2 Shantou Bay Subsea Tunnel on Shantou-Shanwei High-speed Railway

3.2.1 Project overview

Shantou Bay Subsea Tunnel is planned to start constructim in the first half of 2018, it will cross under Shantou Bay, with design speed of 350 km/h. With total length of 9 500 m (Fig. 13), the tunnel will be constructed by both shield method and mining method. The shield segment will be 2 190 m long, crossing soft soil, granite and three faults. The mined segment will cross five faults under the sea, including the active F12 fault. The shield tunnel, with inner diameter and outer diameter of 13.3 m and 14.5 m, respectively, has two-level arrangement (Fig. 14) with the upper level for the traffic lanes and the lower level for the evacuation route and pipeline gallery.

Fig. 13 Vertical section of Shantou Bay Subsea Tunnel on Shantou-Shanwei High-speed Railway (unit: m)

3.2.2 Main features and difficulties of the project

(1) It is the first subsea double-track rail tunnel in China with design speed of 350 km/h. With the increase in the running speed, there are higher requirements for the smoothness of the rail surface, and greater difficulty in controlling the maximum settlement and differential settlement of the tunnel.

Fig. 14 Cross section of Shantou Bay Subsea Tunnel on Shantou-Shanwei High-speed Railway (unit: mm)

(2) It is the subsea rail tunnel in China with the largest inner diameter. Among the underwater railway tunnels completed in China, the largest diameter is 10.8m for the Shiziyang Tunnel of Guangzhou-Shenzhen-Hong Kong High-Speed Railway. The diameter of this tunnel is much larger than that of the Shiziyang Tunnel. With the increase in tunnel diameter, the length of the tunnel in mixed ground is increased correspondingly. Meanwhile, the difficulties of construction and the requirements for controlling tunnel deformation are further increased.

(3) It is the first subsea tunnel in China to cross an

active fault. Two aspects need to be considered at the same time when constructing a tunnel crossing active faults; i.e. to resist both ground shaking and ground offset. In addition, the waterproof performance after the ground dislocation must be considered for underwater tunnels to prevent secondary disaster of flooding.

(4) It is the first subsea rail tunnel in China located in seismic region with seismic intensity of Intensity 8. Although many tunnels have been built in earthquake regions of Intensity 8, this is the first time for buiding underwater railway tunnels. As the social impact will be huge resulting from the interruption of traffic in high-speed railway tunnels, and the time available for maintenance of high-speed railway tunnels is short (usually only about 4 hours during the night), the tunnels will need to have excellent seismic performance.

3.3 Nanjing Heyan Road Yangtze River Tunnel

3.3.1 Project overview

Located between Nanjing Yangtze River Bridge and the Second Nanjing Yangtze River Bridge, the two-way-six-lane Nanjing Heyan Road Yangtze River Tunnel is an integral part of the local expressway network. It is planned to start in early 2018.

The total length of the tunnel is 4 215 m, including approximate 2 970 m shield segment (Fig. 15). The tunnel will run across sand layers, medium weathered breccia, breccia limestone and limestone, pebbly sandstone, fracture zones in the F7 region of fault zone, and four faults. The shield tunnel, with an inner diameter of 13.3 m and outer diameter of 14.5 m, has a three-level arrangement (Fig. 16), with the upper level for ventilation, the middle level for traffic lanes, and the lower level for the evacuation route and pipeline gallery.

Fig. 15 Vertical section of Nanjing Heyan Road Yangtze River Tunnel

Fig. 16 Cross section of Nanjing Heyan Road Yangtze River Tunnel

3.3.2 Main features and difficulties of the project

(1) It is the tunnel with the largest water depth in China. For the section in south shore, the riverbed is steep and the water depth is 53 m. The tunnel has a maximum water pressure of 0.79 MPa, and runs through heterogeneous soil and rock stratum with a shallow cover consisting of soils where the depth of water is the greatest. Among underwater railway tunnels completed in China, the maximum depth of water above the riverbed is generally less than 40 m. With the increase of water depth, on the one hand, it will increase the difficulty of geological exploration. On the other hand, the requirement for controlling the accuracy of the tunneling face pressure becomes higher. Especially when the shield passes through the shallow cover soils with the maximum water depth, the risk of tunnel face instability becomes greater.

(2) The super-large diameter shield successively passes through soft ground and rock formations as well as karst caves and faults in part of the tunnel sections. As the tunnel passes through the karst cave area with a large depth (about 60 m) and with many buildings above, it is necessary to solve the problems associated with the karst cave treatment and the advanced exploration from inside the tunnel, in addition to the need for karst cave investigations from the ground surface.

4 Development trend and problems

4.1 Development trend of underwater shield tunnels in China

4.1.1 From single soft soil formation to the soil sand complex formation

Previously, the large diameter shield tunnels in China were mainly built in soft soil formation in Shanghai. Wuhan Yangtze River Tunnel, commenced in November 2004, started the history in China to build large diameter shield tunnel in soil sand complex formation. Nanjing Yangtze River Tunnel, commenced in August 2005, made the history in China to build super large diameter shield tunnel in soil sand complex formation. Shiziyang Tunnel of Guangzhou-Shenzhen-Hong Kong Express Rail Link, commenced in May 2006, started the history in China to build large diameter shield tunnel in soil rock composite formation. Nanjing Yangtze River Tunnel (Weisan Road Tunnel) and Wuhan Sanyang Road Highway-Railway Yangtze River Tunnel, commenced in May 2010 and December 2012, respectively, started the history in China to build super large diameter shield tunnels in soil rock composite formation.

4.1.2 From large diameter to super large diameter

Super large diameter shield tunnels completed include Shanghai Shangzhong Road Tunnel (14.5 m), Shanghai Yangtze River Tunnel (15.0 m), Nanjing Yangtze River Tunnel (Weiqi Road Tunnel) (14.5 m), Nanjing Yangtze River Tunnel (Weisan Road Tunnel) (14.5 m), Hangzhou Qianjiang Tunnel (15.0 m) and Yangzhou Slender West Lake Tunnel (14.5 m).

Super large diameter shield tunnels under construction include Wuhan Sanyang Road Highway-Railway Yangtze River Tunnel (15.2 m), Shanghai Hongmei Road Tunnel (15.0 m), Shanghai Riverside Expressway Yangtze River Tunnel (15.0 m), Shantou Su′ai Subsea Tunnel (14.5 m) and Jiajiang Tunnel of the Fifth Nanjing Yangtze River Bridge (15.0 m).

4.1.3 From medium water pressure to high and ultrahigh water pressure

Before the construction of Wuhan Yangtze River Tunnel, the maximum head of large diameter shield tunnels in China is no more than 45 m. However, since then, the water pressure head has increased gradually; for examples, 57 m for Wuhan Yangtze River Tunnel; 65 m for Nanjing Yangtze River Tunnel (Weiqi Road Tunnel); 67 m for Shiziyang Tunnel of Guangzhou-Shenzhen-Hong Kong Express Rail Link; 75 m for Nanjing Yangtze River Tunnel (Weisan Road Tunnel); 78 m for Shiziyang Tunnel of Foshan-Dongguan Intercity Railway; 80 m for Yangtze River Tunnel of Suzhou-Nantong UHV Transmission and Transformation Project; and 100 m for the proposed Pearl River Estuary Tunnel of Shenzhen-Maoming Railway.

4.1.4 From ordinary rock and soil to special rock and soil, and unfavorable geological conditions

The representative cases are Yangzhou Slender West Lake Tunnel which passes through expansive Q3 paleo-clay formation; Nanjing Heyan Road Yangtze River Tunnel which runs through karst formation and underwater fault; and Yangtze River Tunnel of Suzhou-Nantong UHV Transmission and Transformation Project which traverses marsh gas formation.

Tunnels running through underwater faults also include Shiziyang Tunnel of Guangzhou-Shenzhen-Hong Kong Express Rail Link and Shiziyang Tunnel of Foshan-Dongguan Intercity Railway. The proposed Shantou Bay Subsea Tunnel of Shantou-Shanwei Express Rail Link traverses an underwater active fault.

4.1.5 From seismic regions with moderate intensity to those with high intensity

Both Shantou Su′ai Subsea Tunnel under construction and the proposed Shantou Bay Subsea Tunnel of Shantou-Shanwei Express Rail Link are located in seismic regions with Intensity 8.

4.1.6 From single construction method to combination of multiple methods

The proposed Pearl River Estuary Tunnel of Shenzhen-Maoming Railway and Shantou Bay Subsea Tunnel of Shantou-Shanwei Express Rail Link will be constructed by drill-and-blast method together with the shield method.

4.2 Existing Problems

4.2.1 Geological exploration aspect

The means of exploration for underwater tunnel in China are relatively single, mainly by drilling and water surface geophysical exploration, lacking means of geophysical investigation directly from the surface of the river (sea) bed. The accuracy of geological exploration is unsatisfactory in differentially weathered karst and granite formations or tunnels with large water depth. Therefore, investigation methods shall be further improved and innovated. Meanwhile, there is no fast and advanced geological prediction technology in construction stage.

4.2.2 Design aspect

There are still many difficulties in design to be further studied and innovated. For example, (1) Regarding the reasonable embedment depth of shield tunnel in bedrock stratum, research has been done in Shiziyang Tunnel of Guangzhou-Shenzhen-Hong Kong High-speed Railway, but a practical formula for analysis has not yet been formed. (2) Regarding load calculation, the weak stratum mainly relies on the Terzaghi′s earth pressure formula, but the calculation method for soil-rock composite formation is still lacking. (3) In the structure analysis theory aspect, at present, the structure analysis is mainly aimed at the operation stage, although the effect of construction load is also considered, the different safety requirements for construction and operation state of the structure have not been adopted, which may lead to the larger reinforcement percentage for structures in the clay formation. (4) In structure for fire protection, at present, the traffic tunnels adopt different heating curves to distinguish the fire limits of the structures according to the types of vehicles (mainly RABT curves, HC curves, ISO curves, etc.) and take corresponding fire protection measures, without considering the influence of fire size, structure clearance size (the smaller the scale of the fire, the lower the temperature; the greater the structure clearance, the lower the temperature), nor considering the beneficial effects of ventilation and water spray system on reducing structure temperature, thus affecting the engineering economy and increasing the cost during the maintenance and operation period.

4.2.3 Construction and management aspect

In construction and management, construction quality problems occur from time to time, due to tight construction schedule, cost pressure, quality awareness and technology standard. For example, the oversize of segment dislocation and structural deformation, the partial cracking and so on, which will result in a serious impact on the durability of the project. In future, it is necessary to further strengthen the research on construction organization technology, refined construction technology, standardized management, etc.

4.2.4 Equipment aspect

In equipment aspect, in some cases, some shield equipment defects may lead to problems such as difficulty in driving, cracking of the cutter-head, and the excessive consumption of cutters, etc. Study shall be further strengthened in terms of provision of shield machines and analysis of geological adaptability, cutters, and cutter replacement techniques, durability technology of long-distance TBM, and multi-mode shield technology.

4.2.5 Materials aspect

In terms of waterproof gaskets for joints, in some cases, with low prices for winning bidding or some other reasons, waterproof seal gaskets were made of recycled or unqualified materials, which would reduce waterproof quality and durability. In terms of structural materials, reinforced concrete is mainly used in segments, while the fiber reinforced concrete is not commonly seen in projects. More attention shall be paid to the research and development of new concrete materials which combine structure and disaster-prevention functions, as well as new segment-joint waterproof materials.

4.2.6 Technology standards aspect

In addition toCodeforConstructionandAcceptanceofShieldTunnelingMethod(GB 50446-2017),PolymerWater-proofMaterialsPart 4: Rubber Gasket for Shield-Driven Tunnel (GB 18173.4-2010) and a few other design codes for constructions and materials, there still lacks of systematic and comprehensive design codes for shield tunneling as well as underwater shield tunnel, resulting in great safety and economy difference between different design schemes. That is to say, even for similar projects in the same area, the difference in the amount of reinforcement provided by different designers may reach about 30%.

5 Conclusions

Enormous achievements have been made for the construction of underwater shield tunnels in China, greatly promoting the development of underwater tunnel technologies in China and in the world. For some time in the future, underwater tunnels in China will be developed at high speeds; however, increasingly complicated construction conditions and growing technological challenges need constant improvements and innovations in specifications, standards, designs, constructions, equipment, materials and management, laying foundation for building more high-quality projects.

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