Shield method is widely used in subway, municipal and other projects because of its unique advantages in saving underground space resources, reducing the impact on the surrounding environment and reducing the project cost.
Taking the shield section from Guangzhou New Passenger Station to Shibi Station of Guangzhou Rail Transit Line 2 and Line 8 as the engineering background, this paper expounds the design points of shield section.
2. Project overview
The left line of Guangzhou Rail Transit from Guangzhou New Passenger Station to Shibi Station is 728.862m (including long chain 0.126); The total length of the right line is 728.41m; Shield construction is adopted for all sections, and the communication channel is set at Z(Y)CK 1+ 100; Minimum curve radius 1500m, maximum longitudinal slope 15.668‰, and maximum slope length of 530m.
Master Plan of Guangzhou New Railway Station-Shi Bi Station Section
Longitudinal section of Guangzhou New Railway Station-Shibi Station
3, the main design principles and standard structure design should meet the requirements of construction, operation, urban planning, civil air defense, waterproof, fire prevention and stray current prevention; The structure should have sufficient strength and durability to meet the needs of service life.
The design service life of the structure is 100 years; The structural safety level is Grade I; The waterproof grade of tunnel structure is Grade II; The clearance of shield circular tunnel is 5200mm.
4. Engineering Geology and Hydrogeology The soil layers crossed by shield construction in this project are: medium coarse sand, silty clay, hard plastic silty clay and strongly weathered zone of Cretaceous red bed. The buried depth of the tunnel is 5m~9m,
5. Segment structure design 1) Inner diameter of tunnel:
The circular tunnel of subway has a clearance of φ5200mm. The factors such as clearance, construction error, measurement error, line fitting error and uneven settlement should be considered comprehensively in determining the inner diameter of tunnel.
Combined with the successful experience of Guangzhou metro, the inner diameter of the tunnel is 5400 mm
2) Segment form and thickness:
According to the successful experience of shield tunnels in Guangzhou and Shanghai and similar projects abroad, it is reasonable to adopt single-layer flexible lining with certain stiffness. Lining deformation, joint opening and concrete crack development can all be controlled within the expected requirements, which fully meet the design requirements of subway tunnels. Single-layer lining is adopted, which has the advantages of simple construction technology, short project implementation period and saving investment. In view of the above reasons, the shield tunnel adopts single-layer assembled lining, and the commonly used flat reinforced concrete segment is used as the segment form. Considering the service life of the structure is 100 years, referring to the existing engineering example, the reinforced concrete lining thickness is 300mm, and C50 concrete segments are adopted.
3) Width and fragmentation of segment:
The greater the width of the lining ring, that is, the wider the segment width, the fewer joints of the lining ring, so the fewer leakage rings and bolts, the faster the construction speed and the lower the cost. However, the jack stroke of shield machine is long, and the construction difficulty is also improved to some extent. On the small radius curve, the design fitting error of 1.5m section is larger than that of 1.2m and 1.0m wide section, but the minimum curve radius of the shield tunnel in this project is 1500m, and the fitting error is very small.
Compared with the segment with ring width 1.2m, the segment with ring width 1.5m has the following advantages: on the one hand, it reduces the number of ring joints by 20%, reduces the probability of joint leakage and improves the waterproof quality of the tunnel; On the other hand, the use of joint water-stopping materials and connecting bolts is reduced; In addition, the assembly time can be reduced by 20%, and the construction speed is improved.
According to the current mechanical condition of shield machine in Guangzhou, considering the needs of segment production, transportation, assembly and curve construction, it is decided to adopt ring width1.5m.
The division of lining ring is mainly determined by factors such as segment making, waterproofing, transportation, assembly and structural mechanical properties. There are six and seven segments commonly used in subway tunnels. There is not much difference between six blocks and seven blocks in production, transportation and construction. In China, Shanghai Metro 1 Line and Guangzhou Metro 1, Line 2 and Line 3 all adopt six schemes for shield tunnels.
According to the practical experience of tunnel, considering the convenience of construction and the need of structural stress, the current sealing block generally tends to adopt the form of small sealing block. There are several assembly forms of the cover block, such as radial wedging and longitudinal insertion. The two side lines of the radial inserter must be internal splayed or parallel, and it is easy to slide down after loading, which is unfavorable. The gland block with longitudinal insertion is in good stress condition and is not easy to slide inward after loading. Its disadvantage is that it needs to lengthen the stroke of shield jack.
This design adopts six schemes, one capping block, two adjacent blocks and three standard blocks.
4) Ring, longitudinal seam and connection structure:
The connection structure of segment includes the design of gasket groove, caulking groove and concave-convex tenon, in which the former is a common construction method, and there are different understandings about whether to set concave-convex tenon in different periods and regions. The setting of concave-convex tenon is helpful to improve the joint stiffness, control uneven settlement, improve the waterproof performance of the joint, and facilitate the segment assembly in place, but at the same time, it increases the difficulty of segment fabrication and assembly, which is one of the factors leading to the cracking and late settlement of segment assembly, and objectively weakens the waterproof performance of segment.
According to the geological conditions of this section, considering reducing the construction difficulty, there is no concave-convex tenon in the longitudinal joint of the ring.
The outer side of the sector torus is provided with an elastic washer groove, and the inner side is provided with a caulking groove. The rings are connected by 10 M24 longitudinal bolts, which can not only adapt to certain deformation, but also control the longitudinal deformation of the tunnel within the scope of meeting the waterproof requirements. Using 12 M24 circumferential bolt connection between segments can effectively reduce the longitudinal joint opening and structural deformation.
The connection mode between segment and lining ring can be divided into flexible connection and rigid connection according to the mechanical characteristics. Practice has proved that rigid connection is not only troublesome and expensive to assemble, but also produces large secondary stress in the lining ring, which brings bad consequences. So at present, flexible connection is more common.
According to the shape of bolt connection, it can be divided into bending bolt connection, straight bolt connection, oblique bolt connection and tenon-mortise pin connection. The joints connected by bending bolts have a certain degree of freedom and are very convenient to install. Bending bolts are widely used in subway traffic engineering and domestic subways in Germany, France, Britain, Singapore, Denmark and other countries, and this joint system is very successful. Straight bolt and inclined bolt are segments connection forms developed in recent years, which have small hand hole volume and small segment strength loss, and are easy to realize rapid mechanized installation, but the installation is difficult and the construction error is small.
According to the successful experience of Guangzhou Metro 1 Line, Line 2 and Line 3, the connection between segments and rings in this design adopts the mature bent bolt connection in Guangzhou.
5) Structure of ribs and end ribs
When designing the hand hole form, in order to reduce the weakening of the hand hole, the length of the ring rib and the end rib is about180 mm.
6) Form and assembly method of lining ring
Lining ring form:
In order to meet the needs of shield tunnel deflection on curve and meandering rectification, wedge-shaped lining ring should be designed. At present, there are three kinds of lining rings commonly used in the world.
A) Combination of wedge-shaped liner ring and linear liner ring.
On the curve, the design of shield tunnel is to fit a smooth curve with several broken lines (the shortest broken line length is the width of a ring lining). The design and construction adopt the optimized combination of wedge-shaped liner and linear liner for line fitting. According to the turning direction of the line and the need of rectification in construction, the wedge-shaped lining ring and the left-right turning linear lining ring are designed. In the design, the lining ring of the whole line is arranged according to the line condition, and the fitting error of tunnel design is controlled within the allowable range. When the shield is advancing, the lining type of the next ring is determined according to the arrangement diagram and the current construction error. Because the type of lining ring used is not completely determined, it brings some difficulties to segment supply.
B) common parts.
At present, the general segment is popular in Europe. It only adopts a wedge-shaped lining ring, and the rotation angle of the lower ring is determined by the information given to the jack by the sensor in the inner ring of the shield machine during shield tunneling, so that the maximum wedging amount is arranged at the longest stroke of the jack, that is to say, the segment lining ring can rotate 360 degrees. Universal segment is adopted for the first time in Shenzhen Metro. Because it only needs one segment type, it can reduce the cost of segment mold, and it will not cause engineering quality problems because there is no segment type. However, it is very difficult to assemble the universal segment, which requires experienced shield machine operators.
C) Wedge-shaped lining rings are combined with each other.
At present, this segment combination is only used in Nanjing subway construction in China. It adopts several types of wedge-shaped lining rings, and the optimal combination of wedge-shaped lining rings is used for line fitting in design and construction. According to the deflection direction of the line and the need of construction rectification, a wedge-shaped lining ring with left turn and right turn is designed, and the left turn and right turn lining ring are combined into a straight line in a one-to-one correspondence in the straight line section. In the design, the lining ring of the whole line is arranged according to the line condition, and the fitting error of tunnel design is controlled within the allowable range. When the shield is advancing, the lining type of the next ring is determined according to the arrangement diagram and the current construction error. Because the type of lining ring used is not completely determined, it brings some difficulties to segment supply.
Combined with the actual situation in Guangzhou, three kinds of lining rings, namely standard ring, left turning ring and right turning ring, are adopted, among which the turning ring is used for tunnel rectification.
7) Segment assembly form:
There are two assembly forms of lining ring: staggered seam and straight seam. Staggered joint assembly can make the joint stiffness distribution of the ring tend to be uniform, reduce structural deformation and obtain better spatial stiffness. However, when the internal force of lining ring is greater than that of straight seam assembly, and the manufacturing accuracy is not enough, the segment is easy to crack or even break in the process of propulsion. The construction of straight joint assembly is less difficult, and the internal force of lining ring is less than that of staggered joint lining ring, which can reduce the reinforcement of segment, but the space stiffness of lining is slightly poor.
According to the successful experience of Guangzhou Metro 1, 2 and 3 lines and the accuracy level of existing segment fabrication, staggered segment assembly is determined for this project.
8) Segmentation marking
Each cycle segment is divided into six blocks, namely, three standard blocks (A 1, A2, A3), two adjacent blocks (B, C) and a capping block (K). There are three kinds of liner rings: standard ring (P), left-turn ring (L) and right-turn wedge ring (R).
Road marking is divided into permanent marking and temporary marking. Permanent marks are mirror-cast on the steel mold during manufacture, which mainly reflect segment ring type (standard ring, left turning ring and right turning ring), block type (standard block, adjacent block and sealing block), segment end face butt joint mark and bolt hole butt joint mark. After the segment is demoulded, it is temporarily marked as spraying, and the segment number and production date are mainly marked.
9) Special sectional design
The emergency evacuation passage and the waste water pump room passage are connected with the main tunnel.
At segment is designed as a special segment ring, which can remove local segments in the main tunnel.
When steel pipe segments are used in special segments, temporary steel pipe segments are easy to disassemble, but the processing is difficult and the cost is high. Although the reinforced concrete segment is difficult to disassemble, it has low cost and simple manufacture. The development of reinforced concrete cutting and bar planting technology is very common, so it is suggested to adopt special reinforced concrete segments.
6, segment structure design
6. 1 calculation principle
1) Water and soil pressure is calculated separately in sandy soil and economically in cohesive soil;
2) The weight of overlying soil should be considered for vertical load. Stratum reaction is balanced with vertical water and soil pressure, lining self-weight and ground overload;
3) The lateral load is calculated according to the lateral pressure coefficient of the stratum or C and φ angles; Ground overload 20KPa；;
4) The combination of earthquake action and main load is used to check the structure, which improves the overall seismic capacity of the joint;
5) The influences of joint stiffness, assembly stress and shield jack force are considered in the lining calculation.
6) Anti-floating safety factor of structure: ≥ 1. 15 when considering friction, ≥ 1.05 when not considering friction.
7) Segment crack width ≤ 0.2mm. 。
6.2 Calculation of load The structural design of shield tunnel mainly considers the following loads:
Ground overload (generally calculated by 20kpa)
Structural self-weight g
Vertical and horizontal earth pressures Q 1, E 1-E2.
hydraulic/water pressure
Lateral formation resistance
reaction of formation
Construction load (shield jack force)
Internal load of structure
Special load (earthquake load, civil air defense load)
In the structural design, the structural strength, stiffness and crack width are checked respectively for the most unfavorable load combination that may occur in the construction stage and normal operation stage. But in the special load stage, only one special load is combined at a time, and the comprehensive adjustment coefficient of material strength is considered (there is no need to check the crack width).
6.3 The section at YCK 1+400 in the calculation diagram has a buried depth of about 9m, and the calculated load adopts the standard value. The calculation results are shown in Figure 4. 1-2:
Figure 1 calculation model
6.4 analysis
Reinforcement (per ring):
Circumferential reinforcement:12φ18; Longitudinal reinforcement: 18 ~ 10. See the figure below for specific reinforcement:
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