1 Introduction
In modern urban construction, the development and utilization of underground space has become an important part. Because of its advanced construction technology and continuous improvement of construction technology, shield tunnel has achieved great success in the development of urban underground space, and is increasingly used in the construction of urban subways, sewers and underground tunnels. In major cities of China, such as Shanghai, Beijing, Shenzhen, Guangzhou and Nanjing, most subway tunnels that have been built or are under construction are constructed by shield method. However, on the one hand, in order to solve the traffic bottleneck problem that restricts urban economic development, major cities have great demand for the development of underground rail transit. On the other hand, the tunnel constructed by shield method is very expensive in engineering cost, which restricts the development and utilization of urban underground space to some extent.
Therefore, how to reasonably control the construction cost of shield tunnel and reduce the project cost has become a subject that must be seriously studied in the development of underground space. At present, this research work has achieved phased results. For example, the Ministry of Construction of Japan has formulated the Guiding Principles for Reducing the Cost of Civil Engineering [1], which requires five measures to be taken from the design stage, such as reducing the structural section, simplifying the structural shape, prefabricating components, standardizing and standardizing materials, and standardizing construction technology. Guan [1] summarized the main factors affecting the subway cost, and pointed out that reducing the construction cost should mainly start from the following three aspects: reducing the purchase cost of vehicles and other equipment, reducing the operation and management fees, and reducing the cost of civil engineering as infrastructure. Zhang Fengxiang [2] and others put forward the development direction of shield technology in China in the future according to the present situation and characteristics of shield technology development in China: reducing cost, improving quality, speeding up construction and prolonging service life, that is, through mechanization, intelligence, informationization, design standardization and adopting new materials and technologies. Y tayagaki [3] and others put forward measures to increase the segment width of shield tunnel, improve economic benefits and reduce costs. Recently, both Japan and China [4] and [5] are carrying out research on the application of prestressed high-strength segments. Because the joint bolt and secondary lining are omitted in this section, the outer diameter of shield tunnel is reduced, thus reducing the total cost. On the basis of analyzing the cost composition of shield tunnel, this paper summarizes many construction achievements at home and abroad, and combining with the existing technical level, discusses how to reduce the cost from the perspective of design and construction technology, so as to achieve the purpose of reducing the engineering cost.
2 cost composition of shield tunnel
Table 1 is the composition analysis of shield tunnel engineering cost between China and Japan [7], from which we can see the proportion of each major project in the whole tunnel engineering cost. Moreover, it can be found that the main components of the cost mainly include these items: segment lining, mechanical equipment, transportation and treatment of waste soil, and protection cost of shaft construction. Therefore, the focus of this paper to reduce costs is also here.
Study on reducing the cost of shield tunnel
3. 1 Cost reduction of segments and lining
3. 1. 1 rational design method
The design of shield tunnel is mainly aimed at the design of segment and lining, but the usual design method is widely used at present. The conventional design method is to regard the segment joint as a ring with uniform stiffness, without considering the reduction of stiffness. It is assumed that the soil will produce foundation reaction with the deformation of segment ring. This design method does not consider the reduction of segment joint stiffness, so the calculation result is relatively high. Beam-spring model method [6] simulates the segment ring as a beam frame (straight beam or curved beam), respectively simulates the segment joint and the indirect head of the ring with rotary spring and shear spring, analyzes the elastic performance of the segment ring with finite element method, and calculates the section force. It is considered that this calculation method is an effective method to explain the bearing mechanism of segment ring.
In the design of an expressway tunnel, the segment thickness calculated by beam-spring model method is 40 ~ 45 cm, while the segment thickness calculated by conventional design method is 50 ~ 65 cm. It can be seen that the former can improve the calculation accuracy and reduce the segment thickness, on the one hand, reduce the tunnel section, on the other hand, reduce the manufacturing cost.
3. 1.2 omit the secondary lining
The secondary lining of shield method has the following functions: anti-corrosion, drainage, fire prevention, smooth inner surface of tunnel, serpentine correction of segment assembly and tunnel lining reinforcement. Under the condition of ensuring the lining strength and structural safety, the advantages of omitting the secondary lining mainly include the following points:
(1) As the secondary lining is omitted, the cost is directly reduced;
(2) Because the secondary lining is omitted, the construction period is shortened;
(3) Due to the reduction of excavation section, the discharged waste soil will be reduced, thus reducing the scale of mechanical equipment, starting and arriving shafts, etc.
3. 1.3 Increase the line segment width
By increasing the width of segment, the number of segment joints along the longitudinal direction of tunnel can be reduced, thus reducing the production cost of segment; Under the condition that the tunnel length is constant, increasing the segment width will reduce the number of assembly times, increase the daily advance and shorten the construction period; Increasing the segment width reduces the number of circumferential joints in the tunnel, which not only improves the waterproof conditions of the tunnel, but also reduces the investment of joint waterproof materials and connectors. However, the increase of segment width may lead to the problem that the segment bending stress increases due to the shear force generated at the segment ring connecting bolt, and it is mainly concentrated at the edge of segment surface. In this regard, Y.TAYAGAKI[3] proposed to use staggered joints to widen the segment, so that it has high strength to ensure the structural safety of the tunnel; In addition, the stress concentration at the joint can be well eliminated by adopting measures such as high-strength joint, reinforcing steel bar at the edge of segment and uniformly arranging segment.
3. 1.4 Use of prestressed high-strength segment
This is a new type of shield segment. The method is to assemble the factory-made concrete segment into a ring behind the shield machine, and insert the prestressed steel strand into the sleeve embedded in the segment for tensioning and anchoring to form the prestressed segment ring, which has the characteristics of no crack, good roundness, no water leakage and durability.
The advantages of using this structure are:
(1) Due to omitting the secondary lining, the thickness of components and the outer diameter of shield tunnel are reduced, which can reduce the total cost.
(2) Because the connecting bolts between rings are omitted, the constructability is improved and the construction period is shortened. In addition, metal objects such as bolts are omitted in the joint part, which simplifies the configuration of segmental reinforcement.
(3) Metal objects are not exposed on the surface, which not only improves the water sealing performance, but also makes the inner surface smoother, and is also very suitable for omitting secondary lining.
3.2 Reduce the cost of mechanical equipment
3.2. 1 Reasonable selection of shield machine
The selection of shield machine mainly includes: the selection of shield type, such as muddy water type or earth pressure type; Selection of specific structure of shield machine, such as cutter head form, cutter head configuration, opening position and opening rate, propulsion stroke of propulsion jack, etc. The selection of shield machine is not only directly related to the purchase cost of equipment, but also related to the rationality of the cost. Unreasonable selection, on the one hand, will lead to excessive equipment reserves and low utilization rate, which will lead to a high proportion of equipment purchase expenses in the whole project cost and cause unnecessary waste; On the other hand, if the selected shield does not have good stratum adaptability, it will not only cause high energy consumption and low output, but also delay the construction period, which will eventually lead to a sharp rise in the project cost. Therefore, reasonable and scientific shield selection should be determined after comprehensive analysis of functions, total length, buried depth, geological conditions, environmental conditions such as ground buildings, underground structures and pipelines along the line, and surface deformation control requirements, so that the selected shield can produce the maximum cost-effectiveness ratio.
3.2.2 Use of special shield machine [7]
(1) shield machine suitable for long-distance excavation
On the one hand, long-distance shield tunneling helps to reduce the number of shield machines under construction at the same time, on the other hand, it also helps to reduce the number of intermediate connecting wells and the number of stratum improvement when entering and leaving the tunnel, thus achieving the purpose of reducing the project cost.
(2) The shield machine adapts to the change of section shape.
Subway tunnels are mostly circular. In the process of subway construction, two tunnels with different cross-sectional shapes are often combined into one underground (such as subway stations). Usually, the shaft is built in the place where the cross section changes, and shield machines with different cross section shapes are used for construction. Whether using shield machines with different cross-section shapes or constructing intermediate connecting shafts, the whole construction cost will inevitably rise. Therefore, the countermeasure is to adopt a special shield machine with telescopic section shape. For example, when a subway tunnel is connected to a station, a circular shield is used for the tunnel between adjacent stations, and when it arrives at the station, its wings are unfolded into three circles of shields for station excavation; When the cross-section diameter suddenly changes from large to small, it is appropriate to use the mother-child shield machine to realize the separation of the mother-child shield machine at the changing diameter. In all these cases, only one shield is used for construction, and the middle connecting shaft is omitted to reduce the cost.
3.2.3 Use of high-efficiency and high-energy cutting tools
In order to adapt to long-distance excavation, the requirements for the selected shield machine and its supporting equipment are as follows:
(1) Minimize the replacement times of worn materials.
Here mainly refers to the replacement of tools and sealing materials, reducing their replacement times will avoid more downtime delays. At the same time, in order to solve the problem of tool replacement during long-distance propulsion, some manufacturers began to develop shield machines that can safely and quickly replace tools at any time under normal pressure. Recently, Japan's Mitsubishi Heavy Industries and Ishikawa Island Broadcasting and Grinding Heavy Industries have jointly developed a new type of shield machine. Its cutter head adopts "sphere" technology, which can rotate 180, and then the cutter is replaced at atmospheric pressure.
(2) Improve the durability of the cutter.
The improvement of durability mainly depends on the improvement of cutter head and cutter material (for example, the wear of cutter head is obviously reduced by embedding superhard alloy cutter head on the cutter); Reasonable selection of cutter head and cutter shape; And the reasonable arrangement of various cutting tips. In addition, enlarging the cutter head is also one of the necessary means to improve the wear resistance.
(3) Effective transportation of building materials and excavated soil and sand.
Long-distance excavation, because the number of intermediate shafts is reduced, the transportation distance is relatively extended. Therefore, new requirements are put forward for material conveying equipment, such as large equipment capacity and high transportation efficiency.
3.3 shaft construction cost reduction
Generally speaking, the longer the total length of subway tunnel, the more underground combined shafts are needed. Therefore, the cost of shaft construction and stratum improvement at the entrance and exit of shield machine will be higher. Therefore, the reasonable selection of the number of shafts and their structural forms will directly affect the total cost. In order to reduce the shaft construction cost as much as possible, effective measures can be taken, including: long-distance shield tunneling, reducing the number of intermediate shafts; Using special shield, the shaft at bifurcation and diameter change can be omitted on the ground; On the premise of ensuring the operating space, the building area of the shaft should be reduced as much as possible. In addition, it is also important to choose reasonable construction methods (underground continuous wall, SMW method, caisson method) and shaft structure (rectangle and circle), which requires detailed technical and economic comparison.
3.4 High-speed construction
High-speed construction can obviously shorten the construction period, help to reduce equipment maintenance costs and labor costs, and thus help to reduce the total construction cost. In order to achieve the purpose of high-speed construction, the measures that can be taken are:
(1) Improvement of driving speed: adopting high-power and large-capacity equipment;
(2) Efficient segment assembly: increasing segment width and reducing the number of joints; Simplify the joint form, such as changing bolt joint into plug-in joint;
(3) segment assembly and shield tunneling are carried out at the same time;
(4) High-speed transportation: including the improvement of large-capacity transportation equipment and transportation speed.
4 conclusion
There are many factors that affect the construction cost of shield tunnel, such as tunnel length, tunnel buried depth, tunnel section shape, tunnel linear conditions, geological conditions of shield crossing stratum, environmental conditions along the tunnel, obstacles and so on. Therefore, the research on reducing the cost of shield tunnel should start from many aspects. Based on the analysis of many engineering examples at home and abroad, the cost composition of shield tunnel is studied, and how to reduce the cost is discussed from four main aspects: segment and lining, shield machinery and equipment, shaft construction and high-speed construction, so as to achieve the purpose of reducing the engineering cost. In addition, the development of new technologies, the application of new materials and new processes are increasingly becoming the main countermeasures to reduce construction costs.
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd