Shield interval structure design?

The shield construction method has been widely used in subway, municipal and other engineering construction because of its unique advantages in saving underground space resources, reducing the impact on the surrounding environment and reducing the project cost.

this paper mainly takes the shield section from Guangzhou New Passenger Station to shibi station of Guangzhou Rail Transit Line 2 and Line 8 as the engineering background, and expounds the key points of shield section design.

2. General situation of the project

The total length of the left line of Guangzhou rail transit from Guangzhou New Passenger Station to shibi station is 728.862m (including long chain of .126); The total length of the right line is 728.41m; All sections are constructed by shield, in which a communication channel is set at Z(Y)CK1+1; The minimum curve radius is 15m, the maximum longitudinal slope is 15.668‰, and the maximum slope length is 53m.

General plan of Guangzhou New Railway Station-shibi station section

Longitudinal section of Guangzhou New Railway Station-shibi station section

3. Main design principles and standard structural design should meet the requirements of construction, operation, urban planning, civil air defense, waterproof, fire prevention and stray flow prevention; The structure should have sufficient strength and durability to meet the needs of service life.

the design service life of the structure is 1 years; The safety level of the structure is Grade I; The waterproof grade of the tunnel structure is Grade II; The clearance of the shield circular tunnel is 52mm.

4. Engineering geology and hydrogeology The soil layers that the shield construction of this project passes through 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 the tunnel: < P > The clearance of the subway circular tunnel is φ52mm. The determination of tunnel inner diameter should comprehensively consider the factors such as clearance, construction error, measurement error, line fitting error and uneven settlement.

According to the successful experience of Guangzhou Metro, the inner diameter of the tunnel is set at 54mm.

2) Segment form and thickness:

According to the successful experience of shield tunnel in Guangzhou, Shanghai and other places and similar projects abroad, it shows that it is reasonable to adopt single-layer flexible lining with certain rigidity. The lining deformation, joint opening and concrete crack development can be controlled within the expected requirements, which can fully meet the design requirements of subway tunnels. And a single-layer lining is used, so that the construction process is simple, the project implementation period is short, and the investment is saved. In view of the above reasons, the shield tunnel adopts single-layer assembled lining, and the commonly used flat reinforced concrete segment is selected as the segment form. Considering the 1-year service life of the structure and referring to the existing engineering examples, the thickness of reinforced concrete lining is 3mm, and C5 concrete segments are used.

3) Segment width and segmentation:

The wider the ring width of the lining, that is, the wider the segment width, the fewer the joints of the lining links, so the less the number of leaking links and bolts, the faster the construction speed and the less the cost. However, the stroke of shield machine jack is longer, and the construction difficulty is also improved to some extent. On the curve with small radius, the fitting error of 1.5m segment is larger than that of 1.2m and 1.m wide segments, but the minimum curve radius of shield tunnel in this project is 15m, and the fitting error is very small.

compared with the segment with ring width of 1.2m, the segment with ring width of 1.5m has the following advantages: on the one hand, the number of circumferential joints is reduced by 2%, the probability of joint leakage is reduced, and the waterproof quality of tunnel is improved; On the other hand, the usage of joint water-stopping materials and connecting bolts is reduced; In addition, the assembly time can be reduced by 2%, and the construction speed is improved.

according to the current mechanical situation of shield machine in Guangzhou, considering the needs of segment fabrication, transportation, assembly and curve construction, it is decided to adopt a ring width of 1.5m ..

the partition of the lining ring is mainly determined by factors such as segment making, waterproofing, transportation, assembly and structural mechanical performance. The number of segments commonly used in subway tunnels is six and seven. There is no big difference in production, transportation and construction between six blocks and seven blocks. In China, the shield tunnel of Shanghai Metro Line 1 and Guangzhou Metro Line 1, 2 and 3 all adopt six schemes.

according to the practical experience of tunnels, considering the convenience of construction and the need of structural stress, the current capping blocks generally tend to adopt the form of small capping blocks. There are several assembling forms of capping blocks, such as radial wedging and longitudinal insertion. The two side lines of the radial inserter must be internal splayed or parallel, and they tend to slide downward after being loaded, which is unfavorable. The capping block in the form of longitudinal insertion is in good stress condition, and it is not easy to slide inward after loading. Its disadvantage is that the stroke of shield jack needs to be lengthened.

this design is determined to adopt six blocks, one block for capping, two adjacent blocks and three standard blocks.

4) Ring, longitudinal seam and connection structure:

The joint structure of segment includes the design of gasket groove, caulking groove and concave-convex tenon, of which the former is a general construction method, and whether the concave-convex tenon is set or not has different understandings in different periods and regions. The setting of concave-convex tenon is helpful to improve joint stiffness, control uneven settlement and improve joint waterproof performance, and it is also beneficial to segment assembly in place, but at the same time it increases the difficulty of segment fabrication and assembly, which is one of the factors that lead to segment cracking during assembly and later settlement, and objectively weakens segment waterproof performance.

according to the geological conditions of this section, the construction difficulty is reduced, and there is no concave-convex tenon in the longitudinal joints of the ring.

the outer side of segment torus is provided with elastic gasket groove, and the inner side is provided with caulking groove. The rings are connected by 1 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. The segments are connected by 12 M24 circumferential bolts, which can effectively reduce the longitudinal seam opening and structural deformation.

From the mechanical characteristics, the connection modes between segments and lining rings can be divided into flexible connection and rigid connection. Practice has proved that rigid connection is not only troublesome to assemble and expensive, but also produces large secondary stress in the lining ring, which brings bad consequences. Therefore, at present, flexible connection is more common.

according to the bolt connection shape, it can be divided into bent bolt connection, straight bolt connection, inclined bolt connection and mortise and pin shaft connection. The joint connected by bent bolts has a certain degree of freedom and is very convenient to install. Bending bolts are widely used in subway transportation projects in Germany, France, Britain, Singapore, Denmark and other countries and in domestic subways, and this joint system is very successful. Straight bolt and inclined bolt are the connection forms of segments developed in recent years, which have small hand hole volume, little segment strength loss and easy to realize rapid mechanical installation, but the installation is more difficult and the construction error is less.

According to the successful experience of Guangzhou Metro Line 1, Line 2 and Line 3, the segment-to-segment, ring-to-ring connections in this design are connected by bent bolts which are relatively mature in Guangzhou.

5) Structure of ribs and end ribs

In order to reduce the weakening of hand holes, the length of ring ribs and end ribs is about 18 mm in consideration of the design needs of ring ribs and end ribs.

6) Lining ring form and assembling method

Lining ring form:

In order to meet the needs of shield tunnel deflection on curves and meandering rectification. At present, there are three types of lining rings commonly used in the world.

A) combination of wedge-shaped lining ring and linear lining ring.

On the curve, the shield tunnel is designed to fit the smooth curve with several broken lines (the shortest broken line length is the width of a ring lining). The design and construction adopt the optimization and combination of wedge-shaped lining ring and linear lining ring for line fitting. According to the turning direction of the line and the needs of rectification in construction, wedge-shaped lining rings and linear lining rings with left and right turns are designed. In the design, the lining rings of the whole line are arranged according to the line conditions, so as to control the fitting error of tunnel design 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) universal segment.

At present, universal segment is popular in Europe. It only adopts one type of wedge-shaped lining ring, and the rotation angle of the lower ring is determined by the information of the sensor of the inner ring of the shield machine to the jack during shield tunneling, so that the maximum wedge amount is disposed at the longest stroke of the jack, that is to say, the segment lining ring can rotate 36. Universal segment is adopted for the first time in Shenzhen Metro. Because it only needs one segment type, it can reduce the cost of the segment mold and will not cause engineering quality problems because the segment type is not available. However, it is difficult to assemble the universal segment, which requires experienced shield machine operators.

C) The wedge-shaped lining rings are combined with each other.

At present, this segment combination form is only used in Nanjing subway construction in China. It adopts several types of wedge-shaped lining rings, and the design and construction adopt the optimization and combination of wedge-shaped lining rings for line fitting. According to the deflection direction of the line and the needs of construction rectification, wedge-shaped lining rings with left turn and right turn are designed, and a straight line is formed by one-to-one correspondence combination of left turn and right turn lining rings in the straight line section. In the design, the lining rings of the whole line are arranged according to the line conditions, so as to control the fitting error of tunnel design 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.

according to the actual situation in Guangzhou, three types of lining rings are adopted, namely, standard ring, left turning ring and right turning ring, in which the turning ring is used to rectify the deviation of the tunnel.

7) Segment assembling form:

There are two assembling forms of lining ring: staggered joint and straight joint. Staggered joint assembly can make the joint stiffness distribution of circular ring tend to be uniform, reduce structural deformation, and obtain better spatial stiffness. However, the internal force of lining ring is larger than that of straight joint assembly, and the segment is easy to be cracked or even broken in the process of propulsion when the manufacturing accuracy is not enough. The construction of straight joint assembly is less difficult, and the internal force of lining ring is smaller than that of staggered joint lining ring, which can reduce the reinforcement amount of segment, but the space stiffness of lining is slightly worse.

according to the successful experience of Guangzhou metro lines 1, 2 and 3 and the existing level of segment fabrication accuracy, staggered joint assembly is determined for segment assembly in this project.

8) segment marking

each ring segment is divided into six blocks, namely three standard blocks (A1, A2, A3), two adjacent blocks (B,C) and one capping block (k). There are three kinds of lining rings: standard ring (P), left turn (L) and right turn wedge ring (R).

segment marking is divided into permanent marking and temporary marking. The permanent mark is cast on the steel mold in mirror image at the time of manufacturing, which mainly reflects the segment ring type (standard ring, left turning ring and right turning ring), block type (standard block, adjacent block and capping block), segment end face butt joint mark and bolt hole butt joint mark. Temporarily marked as sprayed after segment demoulding, mainly marked with segment serial number and production date.

9) Special segment design

The segment where the emergency evacuation communication channel and the waste water pump room channel meet the main tunnel

is designed as a special segment ring that can remove local segments inside the main tunnel.

When special segments are made of steel pipes, the temporary steel pipes are easy to disassemble, but the processing is difficult and the cost is high. Although it is difficult to disassemble the reinforced concrete segment, the cost is low and the production is simple. The development of reinforced concrete cutting and bar planting technology has been very common, so it is suggested to adopt reinforced concrete special segments.

6. Segment structure design

6.1 Calculation principle

1) Water and soil pressure is calculated by water and soil separation in sandy soil, and water and soil pressure is calculated by water and soil cost-effectiveness in cohesive soil;

2) The weight of overlying soil is considered for vertical load. The stratum reaction is in balance with vertical water and soil pressure, self-weight of lining and ground overload;

3) The lateral load is calculated according to the lateral pressure coefficient of the stratum or the angle C and φ; The ground overload is 2KPa；;

4) the combination of earthquake action and main load is used to check the structure, and the overall seismic capacity of the joint is improved;

5) The influence of joint stiffness, assembly stress and shield jack force are considered in lining calculation.

6) Anti-floating safety factor of structure: ≥1.15 when considering friction, When the frictional resistance is not considered, it is ≥1.5

7) The segment crack width is ≤.2mm.

6.2 The structural design of shield tunnel mainly considers the following loads:

ground overload (generally calculated as 2kpa)

structural dead weight g

vertical and horizontal earth pressure Q1. E1-E2

-water pressure

-lateral stratum resistance

-stratum reaction

-construction load (shield jack force)

-structural internal load

-special load (earthquake load, shield jack force). However, 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 calculation sketch The calculation section adopts YCK1+4 section, the buried depth is about 9m, and the calculated load adopts the standard value. See Figure 4.1-2:

Figure 1 calculation model

6.4 analysis

reinforcement (per ring):

circumferential reinforcement: 12 φ 18; Longitudinal reinforcement: 18 ~ 1. See the figure below for the specific reinforcement:

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