15-水下隧道主要施工方法简析(英)

The review of the main construction methods of the
underwater tunnel
Xu Hongfa, Ma Junqing, Tan Huanhuan
Engineering Institute, PLA University of Science and Technology, Nanjing, China, 210007,
Abstract This paper summarized the main construction methods of the underwater tunnel and introduced the concepts of mine tunneling method, TBM tunneling method, shield tunneling method and immersed tube method. Besides, it is concerned with the applying terms, construction work preface and merits and shortcomings of the main construction methods above. It also briefly introduces the open excavation method round the bank and the submerged floating tunnel. Finally, this essay compares immersed tube method with the mine tunneling method and shield tunneling method in detail.
Key words underwater tunnel; mine tunneling method; TBM tunneling method;Shield tunneling method; immersed tube method; open excavation method round the bank; submerged floating tunnel.
0 Introduction
Along with the development of the inland river and deep-sea shipping businesses, the tonnage and densities of ships in rivers or gulfs become more and more big, the requisitions of bridges clear height and span become more and more strict, which make the expenses and difficulties of bridge’s construction increase greatly. For this, we have to look for the new way to cross over rivers and straits, which is underwater tunnel [1].
Tunnels [2] which be used as a way to cross rivers and seas, are replacing ships. They are free from the influence of outside weather, rains and winds, waves, water currents, etc. and have no limit of navigation height, channel width etc. Saying for the main channel of the aquatic heavy traffic, they also have no worries that the bridges can not adapt to the development of ships and the increment of traffics in future.
In recent years, the development of science technique and economies promotes the tunnel construction technique consumedly. The development of the rock mechanics helps the establishment of the modern protect theories, and New Austrian Tunneling Method, shallow cut-and-cover method etc. are produced in this foundation. The Tunnel Boring Machine and the Shield Machine that are equipped with modern technique can adapt to certain geologic strata from strong and tough rock strata to the soft and hydrous soil, their credibility, usefulness, durability, flexibility and advance rates, make them widely apply in tunnels;In tunnel construction, the construction environment, excavation speed and drill-and-blast method are improved by the improvement of drill hammer, the emergence of full hydraulic pressure boring trolley and hydraulic pressure liner building trolley, the invention of the powerful equipments on muck loading and muck haulage, the manufacture of new blast devices and the development of blast technique and defense technique etc.[3]. At the same time, the development of immersed tube method provides a more valid way to build underwater tunnels.
Currently there are several main construction methods, such as mine tunneling method, TBM tunneling method, shield tunneling method, immersed tube method, open excavation method round the bank[1]. In addition, submerged floating tunnel is also an eligibility project when the sea is too deep or the rock bodies are hard to dig.
This essay summarizes the above-mentioned several construction methods and comments their merits and shortcomings.
1 Mine Tunneling Method (drill-and-blast method)
Since the Tollgate Railroad Tunnel was built with the mountain tunneling method in Japan in 1940s, many underwater tunnels for different purposes were built in the world. Among them, Aomori Hakodate Tunnel in Japan is the longest underwater tunnel in the world, which is 53.85Km and has a length of 23.30km undersea[4]. The mine tunneling method has been developing quickly for full and large cross section excavation by continuous renewal, perfect and the application of New Austrian Tunneling Method, shotcrete and rock bolt support and blast control technique. For this, it makes the machines of mountain tunneling method develop facing large scale and high-efficiency. With the application of Auxiliary methods such as grouting methods, freezing methods etc., the drill-and-blast technique is gradually perfected and the mine tunneling method that is similar to shield tunneling method and immersed tube method has been a characteristics method to build underwater tunnels [5,6].
The mine tunneling method [7] is often used to build tunnels. Wood or steel components are used as temporary supports and they will be gradually superseded by whole and thick liner buildings that are used as supports forever when the tunnel is excavated to have a shape.
The mine tunneling method is mainly used in hard rock tunnels, broken rock tunnels and short tunnels, but it is not ideal to build tunnels under rivers especially under rivers of weak soil[8].
According to the construction sequence of linings, the construction sequence of the mountain tunneling method is classified into wall-then-arch method and arch-then-wall method.
1) Wall-then-arch method
Usually concrete linings are cast from the bottom up after the tunnel is excavated to have a shape in wall-then-arch method. The construction speed of wall-then-arch method is quick, the mutual interference of each working procedure and each working face is small, the integrality of lining structures is well and the endurance of forces is strong.
2)Arch-then-wall method
The process of the arch-then-wall method is that: the tunnel is excavated under the cover of arch circles and linings are placed on border walls after the top of the tunnel has been excavated to have a shape and linings have been placed on arches. The speed of the arch-then-wall method is slow and it is difficult to construct at the top of the tunnel. But if the construction of arch is finished, other constructions are safe and fast. The integrality of lining structure and the endurance of forces are bad. And because the sedimentation of lining structure of arch is large, the angle of arch is big, the workload of excavation become difficult. The basic principles of the mine tunneling method [7] are that: less disturbance, earlier support, careful replacement, quick lining.
1) Less disturbance means that the times, intensity and duration of disturbance on rocks should be reduced as far as possible when the tunnel is excavated.
2) Earlier support means that temporary components should be built in time to support surrounding rocks and to prevent larger distortion and relaxation.
3) Careful replacement means that the collapse of surrounding rocks should be avoided during the process of replacement.
4) Quick lining means that permanent concrete lining should be built in time and should take part in work as early as possible while temporary supports are dismissed.
The advantages of the mine tunneling method: it can adapt to the different geology conditional variety and tunnel cross sections, it can excavate both hard rocks and soft rocks, and it is ascendant when the tunnel length is not over 3Km.
The disadvantages of the mine tunneling method: the working procedures are more and they can't be
constructed at the same time, the speed of excavation is slower, the disturbance of surrounding rocks is larger and the influence to environment is also larger.
2 TBM Tunneling Method
Since American Robbins company produced the first tunnel boring machine in 1952, the full cross section rock tunnel boring machines have already developed over 50 years and the technique has already matured. And TBMs are often used in tunnels especially in tunnels that the length is over 3Km all over the world [9]. Such as the Chunnel that connects England and France, the total length of three tunnels is 156Km, they were excavated with 11 TBMs and it took only three years and a half to finish.
The full cross section rock tunnel boring machine is a standard name that is named uniformly by the nation; it is also called tunnel boring machine and TBM for short. The TBM [9] is a kind of big engineering machine which can excavate tunnel cross section to have a shape at a time. It is driven by the electric motor and clamp down on the cutterhead via the circumrotation of principal axis, which makes the cutterhead come close to the surrounding rocks and break rocks by circular hobs. It is also one of the machines that have development potential in the excavation of rock tunnels currently. The full cross section tunnel boring machine includes two kinds: open TBM and shield TBM.
Along with the progress of design and fabrication, the development of construction technique and management, TBMs are used in construction from hard rocks to soft rocks.
The working process of full cross section TBMs: preparation work → excavation → outside tube lining or early shoring → advance → the disposal of grouting out side of tubes or lining 2nd time etc.
The advantages of TBM tunneling method [10~13]:
1) Safety. The cross sections excavated by TBMs are generally circular so their stability is good and they can endure heavier pressure. The possibility that surrounding rocks become flexible or break down is reduced with the method of cutting and moldings by machines.
2)Fast. According to the result of usage, in the homogeneous rocky terrains, advance rates can generally reach: 2m/h in soft rocks, 1m/h in medium hard rocks, and 0.5m/h in hard rocks. In the world's record of tunnels, advance rates of drill-and-blast method is 1237.8m/month and advance rates of TBM tunneling method is 2088m/month. It is generally thought that advance rates of TBM tunneling method can raise 2~2.5 times compared with drill-and-blast method.
3) Economy. The cross section excavated by TBMs is neat, the hole wall is smooth and do not have blast stress, so it do not need to build temporary shoring in hard rocks or it can be supported by anchors, circular steel beams and steel wires in soft rocks or medium hard rock. Besides, the unwanted excavation can control in several centimeters, so the workload and the concrete dosages can be reduced, and it is fit to grout concrete lining. Therefore, it is thought in foreign countries the total cost can lower 20%~30% when the tunnel is built by TBM tunneling method.
4) Transport TBM mucks easily. Most of TBM mucks are little and well-proportioned, so they can be transported outside directly by strap transporter.
5) Because of centralized control and operation, remote control and automation. are possible to come true.
The disadvantages of TBM tunneling method [10~13]:
1) The investment is large at a time, the size and weight of TBMs is big and the machine is complicated. The cycle to fabricate TBMs is long, it also takes more time, power and money to transfer TBMs and the expenses on depletion and repair are expensive.
2) Bad adaptability to rock strata. TBMs are effective in medium hard rocks, but there are also many difficulties in hard rocks and soft rocks. If in broken, uneven or changeful rock strata, advance rates will descend or TBMs can't work even.
3) Low efficiency. In tunnel construction, the work prefaces are many and they need to organize rigorously and cooperate in phase, so the rates of utilization is lower which is not over 50%.
4) The usage of TBMs is often subjected to the length and the diameter of tunnels. It is generally thought that the length of tunnels should be 600 times bigger than the diameter of tunnels and the proper diameter of tunnels is 4~8m. If the diameter of tunnels is too small, it is difficult to transport TBM mucks; if the diameter of tunnels is too big, it takes more power to excavate rocks and the advance rates will also descent.
3 Shield Tunneling Method
The shield tunneling method originates from Europe, because of technique problems in 1970s~1980s, it developed slowly. Along with the addition of shield supports, the settlement of assistance equipments and the cutting plate technique, the technical problems of construction in soft rocks, clay, sandy soil and sand layer, are resolved [14].When the shield tunneling method is applied in the tunnels that are in the bottom of seas or rivers, in fault or in the ground with high water levels, it is safe, credible and its advance rate is fast, so it develops quickly and is widely used in the tunnel construction in recent years.
The shield tunneling method [15] is a method to construct tunnels with shield machines but under the circumstances the surrounding rocks do not be destroyed. It controls cross sections and surrounding rocks in case that surrounding rocks collapse, and it excavates tunnels, transports mucks and patch up tubes to form linings in the shield machine simultaneously, then it sprays slurry on wall rocks. The famous Chunnel is built by shield tunneling method.
The shield tunneling method is applicable to alluvium and diluvium of 4th period, dust and sand surrounding rocks of 3rd period which are from very soft rocks to the rocks that can endure several thousand Pa stress. t is more fit to the tunnels that are built under the city ground, at the bottom of rivers or seas, cross the sand layer, the quicksand soil layer and under important building or building community [14,16], it is superior when the tunnels are built where the water table is higher than the tunnel, the rocks are broken and the rock layer is broken.
The process of main construction work preface [3]:
1) Set up a work well in the tunnel beginning and terminal.
2) Install the shield machine in a work well at the beginning of the tunnel.
3) Depend on the thrust of the shield jack to push the shield machine to advance from the bore of the start work well.
4) Advance along the design axes, transport mucks and install lining tubes at the same time.
5) Pour slurry to the clearance after liner building in time to prevent the geologic strata from moving and fix the position of liner building circles.
The characteristics of the shield tunneling method[17]:
1) Little influence to the normal function of the city and the surroundings environments.
2) The shield machine is designed, manufactured or altered according to the characteristics and the foundation circumstances of the tunnel.
3) High accuracy is requested in the construction.
4) The shield machine can't back off when the tunnel is under construction.
The advantage of the shield tunneling method[18,19]:
1) Safety
The shield tunneling method belongs to the underground dark construction; it is free from the influence of ground, traffic, river, shipping, tide, season, weather etc. It can ensure the safety of construction, excavation and liner building can be done safely under the protection of the shield machine.
2) High efficiency
The propulsion of the shield machine, the transport of mucks, installation, liner building etc. can put mechanization and automation into practice, the labor intensity of construction is low, so advance rate is quick.
3) Little influence
There is no noisiness and vibration during construction, so the shield tunneling method does not affect the surroundings environment and do not affect the normal shipping and the drainage of floods. .
4) Economy
It is superior in economy, technique, safe etc. when the tunnel is built in soft and hydrous strata, and the tunnel is very long or very deep.
5) Accuracy
Because the diameter of a shield machine is manufactured according to the size of liner building cross section, the size of a tunnel is accuracy and constant.
The disadvantages of the shield tunneling method [17]:
1) Long time to prepare
The design of a shield machine needs a great deal of inquisition and argument again and again, and it takes long time to manufacture and install a shield machine.
2) Difficult to repetitive usage
The shield machine is a kind of professional construction machine and its pertinence is very strong, the different tunnels need different shield machines that is manufactured, carried, installed and disconnected specially according to the engineering geology, hydrology geology condition, and the size of liner building cross section, the machine is more complicated. Therefore it can't repeat to use generally.
3) Complicated construction
The shield tunnel can be constructed smoothly only when each technique of construction cooperates closely, such as the equipments to supply air pressure, artificial precipitation through well, the prefabrication and waterproof of liner building tubes, the conveyance in the tunnel, construction measure, the place arrangement, and the installation and put together to pack and dismantlement of the work well etc.
4) Limited applicability
When the scale of a tunnel is smaller, especially the length of a tunnel is short in 750 Km, the cost is comparatively higher; It is difficult to construct a tunnel when the diameter of a tunnel is too small or the depth is a little shallow; The advance length of a shield machine is limited and the diameter of a shield machine is difficult to augment.
5) Difficult to control the transfiguration of earth's surface
When a tunnel is built by the shield tunneling method, it can not control the sedimentation of the earth's surface, within the certain scope of tunnel above, particularly in saturated, hydrous and soft soil layer; installation and liner building have a very high request to the whole waterproof technique.
4Immersed Tube Method
Since the first transportation tunnel constructed by immersed tube methods in the United States was the Michigan Central Railroad Tunnel under the Detroit River, completed in 1910, the immersed tube methods have been developing and perfecting continuously. Along with the settlement of the techniques of connection and foundation underwater, the application of sand-jetting system, sand-flow system and additive mixture method(外加混合剂法) etc, the techniques in prefabricating shells, launching tubes, disposing foundations, the connection and obstruction of tubes are more advanced and credible, so the immersed tube methods are applied more and more widely[20~23].
Immersed tube tunnels [24~26] are composed of prefabricated sections placed in trenches that have been
dredged in river or sea bottoms. The sections are usually constructed at some distance from the tunnel location and made watertight with temporary bulkheads. They are then floated into position over the trench, lowered into place, and joined together under water. The temporary bulkheads are removed and the trench is backfilled with earth to protect the tubes, such as Zhujiang Underwater Tunnel in Guangzhou and Yongjiang Underwater Tunnel in Ningbo.
Immersed tubes are often widely used for highway and rail crossings of lower reaches of a river and a gulf, especially of soft-bottomed, shallow estuaries and tidal rivers or canals in which trenches may be excavated with floating equipment[1][3].
The construction method of immersed tubes[27]:First at the season of low water, shipyards or dry docks are built at some distance from the tunnel location, in which the tubes may be fabricated, and they are made watertight with temporary bulkheads at the same time, when flood season comes, the tubes are moved by tugboats from the construction basin or outfitting site to the position over the trench, then they are placed in a predredged trench and then are joined, connected to form a whole tunnel, and they are protected by backfilling the excavation at last.
The advantages of the immersed tube method [28,29]:
1) It can ensure the construction quality
The tubes are fabricated in shipyards or dry docks which are concentrated at some distance from the tunnel location, so it is easy to carry on the all-weather and all-directions management, this promotes better quality of construction, particularly for control of water seepage.
2) The adaptability to the conditions of geology and hydrology is strong
The trench of a immersed tube tunnel is shallow, it is easy to dredge the trench and deal with the foundation, and immersed tubes are subjected to the buoyancy of water, so the rated loads are smaller. For this, the adaptability to the geology conditions is strong. Almost all of the construction is accomplished above the ground or water surface, so the immersed tube method can be used in deep seas, and it can adapt to the strong tides and currents.
3) The time limit for immersed tube tunnels is short
The tubes can be fabricated while the trench is dredging, most working procedures can be constructed at the same time and the time to ship and launch the tubes is also little, so under favorable conditions, the time limit for immersed tube method is littlest for any type of underwater tunnel crossing.
4) The price to build an immersed tube tunnel is lower
The quantity of earthwork is little during the dredging of the trench, and the cost of each stere is lower compared to the construction under ground ;The cost of holistic prefabrication is also lower; The joints between tubes is fewer, so it takes less money deal with them; The tunnel can be placed at the minimum possible depth. For an approach gradient fixed by operating criteria, this usually means a minimum tunnel length as well, so the total expense is also lower.
5) Control of water seepage is safe
Because the immersed tubes are very long and the joints between tubes are few, so it is more difficult for water to penetrate the tubes.
6) The immersed tube tunnel can be built for multilane in a big cross section
The cross section of a immersed tube tunnel can be made very large, because the tubes are fabricated in shipyards or dry docks. A big cross section can contain 4~8 lanes, so the utilization of a cross section is high.
The advantages of the immersed tube method [28,29]:
1) It is subjected to the local conditions of weather and hydrology.
2) It affects the environment, the drainage and shipping of the river greatly.
3) It is sensitive to the sedimentation of the groundsill.
4) It has a higher request to control of water seepage, so it needs to adopt the strict technique measure.
5 Open Excavation Method round the Bank
The open excavation method round the bank[1] is a kind of simpler construction methods and it is mainly used in rivers that are not deep or have low water seasons. The method[3] is first to dredge trench to the design elevation, then to place lining from the bottom up, to build the main structures and waterproof equipments, at last to backfill the soil and recover the road. The construction procedure of the open excavation method round the bank[3,30]:to predredge → to drive a pile→ to dredge →to establish support and dredge → to construct structures → to backfill the soil→ to pulls out piles and reco ver the road.
The advantages of the open excavation method round the bank: the tunnel is lower and shorter, the cost of illumination and ventilation is less, the expense of construction and maintain is lower and the construction method is easier etc; The disadvantages of the open excavation method round the bank: it is subjected to weather, it requests high construction terms and affects the environment largely.
6 Submerged Floating Tunnel
Because the depth of some straits is too deep or the water beds are hard to be dug, it is not possible to build conventional bridges or immersed tube tunnels, The Submerged Floating Tunnels (SFT) represents a new concept for crossing deep water.Over the past few years, the concept of SFT has garnered increasing attention particularly in places such as Norway, Italy, Holland and Japan. In theory, the technique of design and construction is already solved, if there are fit projects, SFT theory may be translated into practice [2,32].
Unlike conventional immersed tube tunnels, an SFT is not an embedded structure, but instead is suspended above the sea floor, anchored by a support system such as pontoons on the surface or by anchoring to the water bed. And the greatest difference between an SFT and a conventional immersed or bored tunnel is that the floating tunnel structure is surrounded by water; that is, the tunnel is neither placed within nor bored through the ground.
The SFTs contain two kinds of forms [32,33]: One is a tunnel that has negative buoyancy, namely its weight is enough to keep itself stable on the pontoons; another is a tunnel that has positive buoyancy, namely the tunnel is anchored by anchoring to the water bed. Different types of submerged floating tunnels are illustrated in Figures 1 through 6.[33]
Figure 1. Pantoons on the surface. Figure 2. fixed supports.
Figure 3. Tension anchoring to the bed. Figure 4. Combined horizontal and vertical support
Figure 5. Horizontal anchoring. Figure 6. Abutment anchoring.
Figure 1 through 6. Different types of SFTs, characterized by type of support system.
Although there are several structure patterns of SFTs presently,the structure that the ends of the tubes are anchored by anchoring to the stones to resist outside forces, can ensure the safety of SFTs and this technique is also more advanced[2].
The SFTs are used to transport the original materials currently, and it still needs to do a great deal of work to use the SFTs as traffic tunnels.
7 Conclusion
Because the applicability of open excavation method round the bank is often limited, the TBM tunneling method is mainly used in rock tunnels and the SFTs still need to be further perfected, we compare the immersed tube method, the shield tunneling method with the mine tunneling method only.
The conclusion of the comparison [23,28,34]:
1) The thickness of superstratum
In regard to the rectangle reinforced concrete immersed tunnels, the superstratum is only 0.5-1m or 0m, and the top of tunnels can be higher than river bed. The superstratum of a shield tunnel should be larger than the diameter of the tunnel at least. The superstratum of a mountain tunnel is thickest .Therefore, for an approach gradient fixed by operating criteria; this usually means a minimum tunnel length as well.
2) Watertightness
Seen from waterproof angle, the watertightness of reinforced concrete immersed tunnels is best. Seen from structures, the watertightness of a shield tunnel is worse compared to an immersed tunnel. The watertightness of a mountain tunnel is also worse than an immersed tunnel.
3) Multilane
Multilane can be built at a time in an immersed tunnel, but if to build multilane with the shield tunneling method or the mountain tunneling method, it has to build several parallel lanes.
4) Safety
Almost all of construction of a shield tunnel or a mountain tunnel is accomplished under the river bed, its safety and the work condition are worse, but almost all of construction of a immersed tunnel is accomplished from above the ground or water surface, so its safety is better.
5) Parallel construction
Most construction of a immersed tunnel can be done at the same time, but the parallel construction in a shield tunnel or a mountain tunnel is little, so it is flexible and superior to arrange time limit for a project.
6) The shape of tunnel cross section
The height and width of an immersed tunnel with a rectangular cross section can be changed, and the rate
of utilization is high, the size and shape of a mountain tunnel can be changed also, but the cross section of a shield is circular generally.
7) Seasonal
A shield tunnel and a mountain tunnel can be built at any time, but an immersed tunnel has a high request to the season.
8) Others
The influence to the environment is small under the construction of a shield tunnel, but the influence is great under the construction of an immersed tunnel or a mountain tunnel. The shield tunneling method has a high request on the factors of the construction machine and person, the immersed tube method has a high request on the ship technique, and the mountain tunneling method has a high request on the technique of drill-and-blast and placing concrete.
In a word, the methods of underwater tunneling construction have the particular applicability and adaptability. Each has its gains or losses and shows its merits. They will exist long in underwater tunneling construction. But the trend of development is in the direction of high efficiency, high quantity and mechanization. To select a better construction method, we must consider the geography, hydrology condition, economy, society, the environment performance and technical possibilities etc. We should compare each method again and again. The selection of tunneling construction methods is important for raising the work effect, shortening the work period, guaranteeing the safety and saving investment.
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