TAN Zhongsheng， HE Weiguo， WANG Mengshu

(1. Research Centre of Tunneling and Underground Works, Beijing Jiaotong University, Beijing 100044, China; 2. China Railway Tunnel Survey & Design Institute Co., Ltd., Tianjin 300133, China)

0 Introduction

Hainan Island is located in the southernmost end of China, separated by the Qiongzhou Strait from Guangdong Province in the north, sitting opposite to Vietnam along Beibu Gulf in the west and facing Taiwan Island and next to the Philippines, Brunei and Malaysia across South China Sea. As an important marine traffic hub under the "the Belt and Road" national development strategy (see Fig. 1), transportation between Hainan Island and the inland can only be made by ferry due to the existence of the Qiongzhou Strait. Transportation and communication of goods and personnel are greatly constrained. In order to solve the transportation dilemma and promote the economic development across the strait, in-depth studies have been carried out on the route, form (bridge or tunnel) and feasibility based on meticulous work over 20 years in the early stage, since Guangdong Provincial Communications Department conducted researches on the cross-sea channel for the Qiongzhou Strait in 1994, followed by the Ministry of Railways and the Ministry of Transportation and China Railway.

1 Necessity of construction of a cross-sea channel for the Qiongzhou Strait

The cross-sea channel for the Qiongzhou Strait is not only a solution for transportation, but also a major project that involves multi-fields such as energy, national defense, science and technology, opening up and reform, comprehensive utilization, politics, economy and so on. It is a significant project to enhance China′s comprehensive national strength, defend the country′s territorial integrity and promote regional economic development.

1.1 The Qiongzhou Strait tunnel is a strategic passageway for safeguarding the sovereignty of the territory, airspace and waters of the islands in South China Sea

As the second largest island of China, Hainan Island is a strategically important area connecting the inland with South China Sea and a crucial military base of China. The construction of the Qiongzhou Strait cross-sea channel can strengthen the bond between the mainland and Hainan Island. It also holds strategic significance for the successful solution of the South China Sea issues and ensuring the territorial safety as well as exploitation of oil and gas resources in South China Sea. At present, due to poor transportation between Hainan Island and the inland, it is difficult for Hainan Island to shoulder the important task as the rear base for the solution of South China Sea issues and exploitation of oil and gas resources in the area. Therefore, it is in urgent need to open up a convenient and efficient external transportation channel.

1.2 The Qiongzhou Strait tunnel is crucial for securing the regional economic development and national strategy

Three economic zones, including the Yangtze River Delta, Bohai Rim and Southeast Coast, have been formed in the coastal development zone following the reform and opening-up policy in China. Currently, the sluggish development of transportation infrastructures is one of the key factors restricting further development of the coastal area. The construction of a land passageway that connects the northern, central and southern economic circles brings about many benefits: exchange and complementation of north-south economy, improvement of bottleneck constraining economic development and ensuring in-depth development of the "the Belt and Road" national strategy. The Tongsan Line (from Tongjiang of Heilongjiang Province to Sanya of Hainan Province), one of the "five longitudinal and seven transverse" main highways in plan, along with the coastal railway and one of the "eight longitudinal and eight transverse" main railways in plan, is the artery connecting the three coastal economic circles. However, this "artery" is disconnected by Bohai Strait, Yangtze River Estuary, Hangzhou Bay, Pearl River Estuary and Qiongzhou Strait, which leads to increase in transportation energy consumption, delay in transportation time, and increase in transportation cost. Consequently, the economic and social development is restrained. Therefore, it is imperative to construct a railway tunnel across the Qiongzhou Strait. As an offshore island, the prosperity and development of Hainan Province depends in a large degree on the surrounding areas. An efficient passageway connecting Hainan Island to the inland is required so as to break the existing regional restrictions and promote the exchange of goods, energy and human resources. The existing "administrative regional economy" is expected to be transformed into an "economic regional economy" to promote the economic integration and speed up the economic development of the island.

1.3 The Qiongzhou Strait tunnel is an important component of the coastal railway

The operation of the Hainan west rapid railway loop at the end of year 2015 marked the formation of a closed loop of rapid railways in Hainan Island, which can promote the construction of railway network in Hainan Island. However, due to the existence of the Qiongzhou Strait, connection with the inland railway network still depends on ferries, which greatly limits the speediness of coastal railway. In order to complete the coastal passageway as one of the "eight longitudinal and eight transverse" main railways, railway construction in Zhanjiang and Haikou has to speed up. The Qiongzhou Strait railway tunnel, as an important component of the coastal railway, plays a crucial role in the whole process and its construction will determine the formation and speediness of the railway network.

2 Engineering geological conditions of the Qiongzhou Strait

The engineering geological condition of Qiongzhou Strait is very complex. Many related issues have to be considered carefully for construction of such large-scale subsea tunnel, especially engineering dynamic geological problems, such as the stability of rock and soil, the active fault, earthquake and volcano, etc. These problems will be briefly discussed in this paper[1-2].

2.1 Seabed morphology and sediments

The Qiongzhou Strait is about 80 km long in the east-west direction and 30 km wide in the south-north direction, which makes it the smallest one among the three major straits in China. The narrowest part of the strait is located in the middle and is about 18.6 km wide. The strait is relatively shallow in the west and deep in the east. The middle part of the strait is a more than 50 m deep, 10 km wide and 70 km long water basin. The middle axis of the basin is a roughly 80-114 m deep trough. There is an uneroded hill (or isolated hill) in the middle of the strait, which is less than 50 m deep and about 10 km long in the east-west direction, as shown in Fig. 2.

Fig. 2 Seabed topography of Qiongzhou Strait

A number of steep ridges exist in the southern and northern sides of the strait, with the largest elevation up to 70 m and the largest slope angle up to 22°-24°. A series of shoals and scour troughs lie alternately in the eastern narrow of the strait. The water depth is only 20-30 m at some locations. The west narrow of the strait is a huge undersea delta, with water depth of 40-50 m. There are many ridges at the bottom of the gorges between headlands and many deep troughs between gulfs. The parallel scour troughs and sand ridges approximately in the east-west direction are well formed due to strong bottom currents.

The composition of the strait seabed is quite different from the east to the west. The shoal in the east consists of gravels and coarse sand, while the undersea delta in the west consists of muddy sand. The scour troughs in the middle are composed of Tertiary calcareous cemented sandstone without being covered by Quaternary sediments.

2.2 Strata lithology

As shown in Fig. 3, the seabed strata of the Qiongzhou Strait are mainly the Quaternary and Tertiary marine sediments. The upper part consists of Quaternary silt, sand clay, clay or interbedded soil. The lower part consists of thick layered Tertiary clay, silty sand or interbedded soil with a total thickness up to a few hundred meters.

Fig. 3 Geological section of Qiongzhou Strait

The near-field engineering geology can be divided into the following zones:

(1) The terrestrial basalt zone: It is distributed on both sides of the strait. The terrain is gentle with high rock and soil strength. The coastal areas are affected by high-velocity tidal flows and storm surges. Some areas are severely eroded and gravitational sliding occurs intensively.

(2) The sandbar terrace zone: It is generally distributed in the front edge of the basalt zone. The main landforms are sandbar and level-Ⅰ marine deposition terrace. As a result of frequent and strong typhoons, movement of sand dunes often causes damage or burial of engineering facilities. In addition, the loose coastal area is intensively eroded by storm surges.

(3) The coastal shoal zone: It is located in the transi-tional zone of land and sea, which is partially above the sea level at low tide, and is underwater at high tide. Hydrodynamic factors in this area, such as coastal currents, tides, waves and bottom currents, are particularly strong, which often result in scouring, erosion of submarine soils and engineering facilities as well as rapid accumulation of marine sediments, reduced stability of the seabed and the coast. If encountered with a once-in-a-hundred-year storm surge, the seabed and coastal rock and soil may be instable.

(4) The loose beach zone: This zone is located in the shoal area with water depth less than 50 m. The terrain is relatively flat with little change in slope gradient. The upper strata are mainly loose sand or muddy soil, and the lower strata are hard clay and interbedded silty sand.

(5) The deep seabed basin zone: This area is located in waters with water depth more than 50 m. It is 70 km long and about 10 km wide. The terrain fluctuates greatly with large-scale uplifts and collapses and the marine erosion is intensive. The soil is mainly hard Tertiary gray clay. The unfavorable factors such as seabed landslides, faults, shallow gas, surging currents and seawater corrosion are present in this area.

2.3 Geological structures

The Qiongzhou Strait is the boundary between the South China plate and the South China Sea plate, usually called Leiqiong Rift Valley. The northern boundary is the Jiepao-Huangpu Fault and the southern boundary is Wangwu-Wenjiao Fault. There are mainly three groups of active faults in the Leiqiong area: the approximately EW trending faults (five main faults), the NE trending faults (seven main faults) and the NW trending faults (six main faults). These three groups of faults form a reticular tectonic framework, as shown in Fig. 4. All the faults are normal faults. The approximately EW trending faults are the main controlling structures. The NE trending faults are of the largest scale, but non-active lately because of their early formation. The youngest NW trending faults are the most active structures and also the main seismogenic structures.

Fig. 4 Main active faults in Leiqiong area

Among these faults, the following ones have greater impacts on determination of the tunnel route:

(1) The Qiongzhou Strait Fault (F3): It is distributed in the middle of the Leiqiong fault zone trending in the EW direction, dipping toward the south in a stepped manner. It exhibits the normal fault activity with the northern plate rising and the southern plate descending. The fault may form at the end of the Cretaceous period: the rift valley was in strong expansion in the Neogene period, leading to partial submergence of the Leiqiong fault basin and separating the Hainan Island and Leizhou Peninsula. The activity gradually became weaker in the early Pleistocene period. The fault cuts through the entire crust and reaches the Moho surface. According to the seismic sounding data by the South China Sea Western Oil Company, the western section of the fault at the middle of the strait was formed during the early late-Tertiary period, while aM5.5 earthquake occurred on June 26, 1871 in the eastern section, indicating that the fault was active and the activity in the eastern section is stronger than the western section.

(2) The Maniao-Puqian Fault (F4): It is distributed approximately in the EW direction. The known on-land part is 100 km long, dipping NNW with a dipping angle of about 80°. The fault cuts deep through the crust. In the early time, it was a reverse fault with the northern plate rising and the southern plate descending. After the Cretaceous period, it became a normal fault with the northern plate descending and the southern plate rising. This fault was the main controlling structure of the Qiongshan earthquake in 1605. Large-area collapse occurred at the north side due to the earthquake. In the eastern section, there are submerged ancient villages. It can be seen that the fault is still active.

(3) The Puqian-Qinglan Fault (F21): It is distributed in NW335°-345° direction and dips toward SW with a steep dipping angle. It is a normal fault with the eastern plate rising and the western plate descending. The fault is deep and the northern part may extend to Leinan and Xuwen. Seismic sounding confirmed that this fault also appeared in the Upper Tertiary and Quaternary strata. The distribution of the isoseismal curves for the Qiongshan earthquake in 1605 shows that the Puqian-Qinglan Fault was one of the seismogenic structures, and the epicenter was located at the intersection of the Puqian-Qinglan Fault and the Maniao-Puqian Fault. The fault was formed in the Mesozoic period, with the eastern part rising and the western part descending for a long time. The normal fault activity has become more obvious since the Pleistocene period.

(4) The Haikou-Yunlong Fault (F20): It is distributed in the NW direction. The on-land part is 25 km long. The fault is still active according to the ruptures developed in the early and middle Pleistocene strata near the Haikou Hostel and the occurrence of several moderate earthquakes in Haikou.

(5) The Changliu-Xiangou Fault (F19): It is parallel to the Puqian-Qinglan Fault and extends along the NW direction to the western part of Leizhou Peninsula. It strikes in NW330° and dips SW with a dipping angle of 60°-80°. This fault is shallow and has undergone normal fault movement with the eastern part rising and the western part descending since its formation in the Mesozoic period. According to the record since 1356, earthquakes have ever occurred along the fault, mainly with magnitudes of 3-3.75, indicating that the fault is still active.

2.4 Earthquakes and volcanoes

Since 1400, the south-central coastal seismic belt has experienced two active periods. The first period was 1400-1710, which lasted for 310 years; the second period started from 1711. It has been 306 years since then, and is currently at the late stage of the second period. Each activity period can be clearly divided into four phases: the quiet phase, the accelerated release phase, the large release phase and the residual release phase. From 1400 to 1995, a total of 31M≥4.75 earthquakes were recorded in this area (including the Qiongshan earthquake aftershock in 1605). Among the 31 earthquakes, 9 had a magnitude greater than 6.0, and the most intensive earthquake was theM7.5 Qiongshan earthquake in 1605. TheM6.1 earthquake in December 1994 and theM6.2 earthquake in January 1995 in the Beibu Gulf showed that the southeastern coastal areas entered a period with relatively high seismic activity. The seismic belt in this area is in the residual release phase of the second active period and will enter the quiet period of the next active period in about 10 years. In the next 100 years, the seismic activity will be in the residual release period and the quiet period of the next cycle, which means that the seismic activity is in the adjustment and energy accumulation period. Therefore, it is estimated that the strongest earthquake in the Leiqiong area in the next 100 years will be a moderate earthquake with magnitude about 6.0.

The volcanic activities in this area lasted for very long time, and can be divided into four active periods from the Neogene period to the Holocene period. The volcanic activities in the early period were not prevalent; the middle period was the Cenozoic period with most active volcanic activities; the activities in the late period became weaker; recent activities are more limited. No volcanic activity has been recorded in written history.

3 The Qiongzhou Strait cross-sea channel

(1)The Qiongzhou Strait is characterized by typhoons, large span, great water depth, poor strata condition and so on. In addition, there are 100 000-ton class or above vessels sailing in this region, which is another unfavorable factor for construction of cross-sea bridges and transportation. The cross-sea bridge scheme is restrained by many factors and technically difficult. Furthermore, the cross-sea bridge is more vulnerable to wars (especially modern warfare) and other natural disasters, if compared to tunnels. Considering the possibility of a territorial war in future because of territorial claims over South China Sea from many countries around, the cross-sea channel for the Qiongzhou Strait should be more discreet and shall not be easily destroyed so as to function as the traffic artery in a war. Therefore, a tunnel scheme would be an inevitable choice and has an irreplaceable significance. Therefore, a tunnel scheme is recommended as the Qiongzhou Strait cross-sea channel.

(2) The construction of the Qiongzhou Strait tunnel will face many technical difficulties including high water pressure, high seismic intensity, high corrosivity, long-distance construction and long-distance ventilation and evacuation. After investigation and in-depth study, it is found that other than the long-distance ventilation and evacuation problems of highway tunnels, which are yet to be solved at present (multiple ventilation shafts are required in the middle section), the remaining technical difficulties, however, would not limit the tunnel construction. In this regard, a railway tunnel scheme is recommended for the cross-sea tunnel and cars can be shuttled through the tunnel. According to the status quo and planning of the railway network on both banks, a two-lane level-Ⅰ electrified railway is recommended. The design speed is 200 km per hour and the railway line is shared by passenger and freight trains.

4 Alignment of the Qiongzhou Strait railway tunnel

Based on the natural and geological conditions of the Qiongzhou Strait and the layout of Yuehai railway network, the tunnel alignment schemes were designed and compared. The seabed landform of the Qiongzhou Strait changes dramatically, and the water depth varies from 10 m to 120 m. The seabed geology is complex. Faults and disastrous geological conditions are distributed extensively and are difficult for survey. This brings about great difficulty for reasonable selection of tunnel alignment. The tunnel alignment has a direct impact on the technical and economic feasibility. The preliminary design of four possible tunnel routes and their connection with the existing railways are shown in Fig. 5. From the east narrow to the west narrow are Line Ⅰ, Line Ⅱ, Line Ⅲ and Line Ⅳ, respectively.

Fig. 5 Alignment plan of Qiongzhou Strait railway tunnel

4.1 Line Ⅰ

The southern end of the undersea section of the tunnel line is in Haikou, the northern end in the Paiwei Cape of Xuwen. The plan is a straight line. The sea surface is 19.2 km wide. The maximum depth of the Strait is 82 m in the north and 90 m in the south, and the water depth within 5 km from the south bank of the Strait is less than 30 m.

The main seabed strata are the Quaternary (Q) and the Neogene (N2) strata. The main lithology of the Quaternary stratum is silt, clay, silty sand and weathered basalt, with a thickness of up to 50 m and less than 10 m at the location with the greatest water depth. The Neogene stratum is mainly thick-layered clay and clay sand or silty sand, with the maximum thickness up to 200 m.

The main faults near the tunnel line include the NE trending Longtang-Qianshan Fault in the northern end, the approximately EW trending Qiongzhou Strait Fault in the middle, the EW trending Changliu-Haikou Fault and the NW trending Haikou-Yunlong Fault in the southern end. These faults remained active in the Cenozoic period, especially the NW trending Haikou-Yunlong Fault, which is still active and is related to the occurrence of several moderate earthquakes near Haikou. The southern end of the tunnel line is located in the Haikou seismic zone.

4.2 Line Ⅱ

The undersea section of the tunnel line is located near the eastern side of the railway ferry route. The plan is a straight line. The northern end is in Sitang of Xuwen and the southern end in the Tianwei Cape of Haikou. The strait here is 18.8 km wide, basically the narrowest part of the strait. There is a deep trough in the south of the strait with a maximum water depth of 90 m. 7 km of the trough is 80 m or deeper, and the northern part is relatively shallow, most of which is 30-45 m deep.

The seabed strata along Line Ⅱ are basically the same as Line Ⅰ, mainly the Quaternary and the Neogene strata.

The main faults near Line Ⅱ include the NW trending Nadan-Fangshenling Fault in the northern end, the approximately EW trending Qiongzhou Strait Fault in the middle, the approximately EW trending Changliu-Haikou Fault in the southern end and the NW trending Changliu-Xiangou Fault. These faults have been active since the Cenozoic period, especially the NW trending Nadan-Fangshenling Fault and the Changliu-Xiangou Fault, which are the main seismogenic structures. About 5 km west of the line is the seismic zone of Qiongzhou Strait Fault, and the northern end is the Sitang seismic zone.

The entrance and exit of the tunnel line are close to the existing railway line, so the connection is very convenient and the engineering quantity is the minimum.

4.3 Line Ⅲ

The southern end of the undersea section of the tunnel line is near Macun in Haikou, and the northern end near Xindi of Xuwen. The plan is basically straight but is curved at both ends near the shore. The sea surface is 34.2 km wide. The water depth in the middle is 100 m, and the northern and southern sections are relatively shallow, most of which are less than 30 m deep.

The seabed strata along Line Ⅲ are basically the same as Line I, mainly the Quaternary and the Neogene strata.

The main faults near the line include the NE trending Zhangchoucun-Xinliao Fault in the northern end, the approximately EW trending Qiongzhou Strait Fault in the middle, the approximately EW trending Maxiao-Puqian Fault in the southern end and the NW trending Yanchunling-Daoya Fault. These faults have been active since the Cenozoic period, especially the NW rending Yanchunling-Daoya Fault. The middle section of the line passes through the Qiongzhou Strait seismic zone, and the southern end is near the old town seismic zone.

Due to great strait width and depth, this tunnel scheme is only used for comparison.

4.4 LineⅣ

The line is located in the western narrow of the strait that is on the flat terrain outside the deep basin. The plan is basically straight except for a curved section in the north. The line is arranged along the outer edge of the deep basin. The northern end is located at Dengloujiao of Xuwen Jiaowei Village, the southern end at the Leigong Island of Hainan. The undersea section is about 30.8 km long and the seabed topography is relatively flat except for a scarp on each end with water depth of 40-45 m.

The main seabed strata are the Quaternary (Q) and the Neogene (N2) strata. The lithology of the Quaternary stratum are mainly medium coarse sand, silty clay and weathered basalt, with a maximum thickness up to 25 m. In addition, a medium coarse sand layer is evenly distributed at the seabed. The lithology of the Neogene stratum is mainly clay and interbeded clay-sand layer, and the maximum thickness is up to 200 m.

The main faults near the line include the NE trending Dengjiaolou Fault and the approximately EW trending Qiongzhou Strait Fault in the undersea section, and the approximately EW trending Maxiao-Puqian Fault at the southern end. The Qiongzhou Strait Fault and Maxiao-Puqian Fault are the main earthquake-controlling structures. They are still active and the fault activity in the east is stronger than the west. The line is located at the western end of the fault and is far from the epicenter, so it is in a relatively stable zone.

With relatively flat seabed, shallow water and favorable geological conditions, Line Ⅳ is suitable for immersed tunnel or shallow buried shield tunnel.

4.5 Scheme comparison

By comparison, it can be seen that, among the four lines, the undersea part of Line Ⅲ is the longest, the water depth is the greatest above the line and the seabed is not gentle. Therefore, Line Ⅲ is not a good option and shall be eliminated first. As for Line Ⅰ and Line Ⅱ, the maximum water depth and length of undersea section are the same. However, there are two deep troughs along Line Ⅰ, which is unfavorable for geological exploration and tunnel construction. Furthermore, fault structures and seismic stability along Line Ⅰ are worse than Line Ⅱ. Besides, connection between Line Ⅱ and the existing railways is more convenient and economic, compared to Line Ⅰ. Therefore, Line Ⅰ can also be eliminated. Line Ⅱ and Line Ⅳ each has their own merits and shortcomings. However, Line Ⅳ is longer and farther from Haikou. Therefore, Line Ⅱ is better on the whole. The tunnel alignment will be further adjusted after detailed investigation on natural conditions, especially geological conditions along Line Ⅱ.

5 Longitudinal profile and cross section of the Qiongzhou Strait railway tunnel

The following study of the longitudinal profile and the cross section of the Qiongzhou Strait railway tunnel is carried out base on undersea tunnel construction technology in China and abroad[3-10].

5.1 Longitudinal profile

The maximum water depth along the recommended line (Line Ⅱ) is 90 m, and the width is 18.8 km. According to the railway line design specifications, the maximum longitudinal slope gradient for passenger transit line is 20‰ and for the freight transit line is 16.8‰. The longitudinal profiles of the tunnel are then designed and compared as follows:

Scheme Ⅰ: The limiting gradient is 20‰, and the total tunnel length is 25.42 km. The line passes through the deep trough near Haikou. The minimum overburden depth is 12.3 m;

Scheme Ⅱ: The limiting gradient is 16.8‰, and the total tunnel length is 26.41 km. The line could not be connected directly to the existing Haikou Station. Instead, the station needs to be extended by 650 m toward the south. The tunnel passes through the deep trough near Haikou. The minimum overburden depth is 11.9 m;

SchemeⅢ: The limiting gradient is 16.8‰, and the line could be directly connected to the Haikou Station. The minimum overburden depth of 5.3 m can be used to derive the longitudinal tunnel alignment. The tunnel section with overburden depth less than 1D(Drepresents the tunnel span) is 821 m, and the section with overburden depth less than 2D/3 is 376 m.

After preliminary comparison, Scheme Ⅰ is recommended. The longitudinal profile of the tunnel, u-shaped, is shown in Fig. 6.

Fig. 6 Longitudinal profile of the Qiongzhou Strait tunnel

5.2 Cross section

At present, the cross sectional forms of double-track railway tunnels mainly include double-hole and single-track, single-hole and double-track, single-hole and double-track with a partition wall, double-hole and single-track with service tunnel. For the double-hole and single-track scheme, trains operate in separate tunnels. It has higher security and better passenger evacuation ability because of its transverse evacuation plan. However, no service tunnel can be used for prediction of geological conditions and reinforcement of adverse geological bodies. Hence, the construction risk is relatively higher. For the single-hole and double-track scheme, multiple trains operate in one tunnel at the same time. If an accident happens, secondary disasters for the other line may be triggered. Besides its large section and average evacuation ability, no service tunnel can be used for geological forecasting and advanced reinforcement. Therefore, its construction difficulty and risk are the greatest. For the single-hole and double-track tunnel with a partition wall, passengers can be evacuated quickly with its transverse and longitudinal evacuation plans. However, its large cross section leads to higher construction difficulty and risk. For the double-hole and single-track tunnel with service tunnel, it has two traffic lanes on both sides, and one service tunnel in the middle. The service tunnel can be constructed first and used to explore the geological conditions along the line. On one hand, the service tunnel can be used as a shelter during construction of the main tunnel. On the other hand, it is favorable for ventilation (using roadway jet ventilation) and safety during tunnel construction. Based on the above analysis, the double-hole and single-track scheme with service tunnel is recommended.

The tunnel cross section is shown in Fig. 7, consisting of two main tunnels and a service tunnel in between. The center-to-center distance between the service tunnel and each main tunnel is 20 m and the size and structural form of the tunnels are the same. The outer diameter of the main tunnel is 11.1 m. The segmental lining is 600 mm thick and the secondary lining is 250 mm thick. The utilization rate of the cross section is 48.4% and the width of the evacuation passageway is 1.5 m. The effective clearance area of a single tunnel is 69 m2. The disaster prevention and rescue ability is strong with transverse passenger evacuation to the service tunnel through the passageway and longitudinal passenger evacuation by rescue vehicles. The service tunnel is divided into three floors, with the upper floor used for smoke exhaust duct, the area of which is 17.3 m2. The middle floor is used as a two-lane traffic rescue channel, and the lower floor is a pipe gallery reserved for electricity and water supply pipelines.

6 Construction methods for the Qiongzhou Strait railway tunnel

Base on reference [5], [6] and [10] and according to the specific geological conditions and construction scale, both the main tunnels and the service tunnel will be constructed by the shield tunneling method. The tunnel is divided into 4 sections for construction. The first section, from the tunnel entrance to the shore shaft in Guangdong, is to be constructed by the open cut method; the second section, from the shore shaft in Guangdong to the middle of the strait, is to be constructed by the shield tunneling method; the third section, from the middle of the strait to the shore shaft in Haikou, is to be constructed by the shield tunneling method; the fourth section, from the shore shaft to the tunnel entrance in Haikou, is to be constructed by the open cut method. A total of 6 slurry balance shield machines will be used with 2 for each tunnel launched from both ends simultaneously.

Fig. 7 Cross section of Qiongzhou Strait railway tunnel (unit: mm)

7 Conclusions

(1) The construction of the Qiongzhou Strait railway tunnel is of great significance for safeguarding the sovereignty of territory, airspace and waters of South China Sea and promoting the economic prosperity in the region. The proposed railway tunnel will become an important component of coastal railway.

(2) The geological condition of the Qiongzhou Strait is very complicated. A comprehensive in-depth investigation on the engineering geological problems, including the stability of rock and soil, active faults and earthquakes etc., needs to be performed and the effects on the tunnel stability need to be estimated for selection of the tunnel route.

(3) Based on the study of the meteorological, hydro-logical and geological data of the Qiongzhou Strait, four undersea tunnel lines are discussed in this article. Line Ⅱ is recommended because of its favorable location and suitability for shield tunneling. It is also suggested that further geological exploration along Line Ⅱ needs to be performed before the tunnel layout can be finalized.

(4) The longitudinal profile of the tunnel is U-shaped with the limiting gradient of 20‰. The cross section of the tunnel is composed of two main railway tunnels and one service tunnel and all the tunnels have the same size and structural form.

(5) All the three tunnels will be constructed by the shield tunneling method, which is feasible with the current tunnel construction technology in China.

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