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Use of a 10.22   m diameter EPB shield: a case study in Beijing subway construction

View Article: PubMed Central - PubMed

ABSTRACT

Introduction: Beijing subway line 14 includes four stations and approximately 2.8 km of tunnels between the Dongfengbeiqiao and Jingshunlu areas of the city. Due to the surface and underground space limitations of this section, a double-track running tunnel instead of two single-track running tunnels was adopted to connect the two stations. The double-track tunnels were excavated by a 10.22 m diameter earth pressure balance (EPB) shield. It was the first time that an EPB shield more than 10 m in diameter was used in Beijing subway construction.

Case description: The shield, which passes underneath densely built-up areas of the city and is equipped with a spoke-type cutterhead, with balance between the ground pressure and the earth chamber pressure at the tunnel face, is of great importance. Referring to experiences gained in the EPB shield tunneling, attention was paid to the function of soil conditioning and simultaneous backfilling grouting of the shield, and some special designs were considered in manufacturing the machine.

Discussion and evaluation: In addition to the agitating rods welded to the cutterhead, two independently driven agitators were added to fully mix everything in the earth chamber. Independent pipelines were arranged for injecting different conditioning agents. Indoor tests in combination with field tests were conducted to find suitable additives and injection ratios of the additives, and determine the mix ratio of the two-component grout for simultaneous backfilling grouting. A scheme was employed for simultaneously injecting the bentonite slurry at 8% concentration and the foam liquid at 5% concentration to condition the excavated soil. The cement–sodium silicate grout was adopted to fill the tail void and the injection volume per ring was 14.1–15.3 m3.

Conclusions: The performance of the shield and evaluation of the corresponding tunneling technologies are introduced in terms of the shield tunneling induced ground surface settlements. The success of the project is of great significance to Beijing subway construction and underground space utilization. The findings serve as a useful reference for similar projects.

No MeSH data available.


Soils at the test sections
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Fig11: Soils at the test sections

Mentions: For construction of the shield tunnels in Beijing, which were a little more than 6 m in outer diameter, slumps of approximately 130 mm of the conditioned soils were sufficient. In the initial construction stage of the project, it was attempted to use the existing conditioning scheme in the shield driving. However, operation of the 10.22 m diameter shield was difficult. The earth chamber pressure, cutterhead torque and total thrust fluctuated widely, and large clods of earth existed, resulting in obvious ground surface settlement and a wider settlement trough. It was absolutely necessary to perform field tests to further optimize the conditioning scheme based on the indoor experiments. Considering the ease of operation and engineering economy, the scheme of simultaneously injecting 8% bentonite slurry and 5% foam liquid was adopted in the field tests based on the above findings. Through the field tests, the differences in conditions between the indoor tests and the construction were justified and the actual effects of the conditioning agent and the injection ratios were further verified. The test sections is given in Fig. 11 and properties of the soils are presented in Table 10.Fig. 11


Use of a 10.22   m diameter EPB shield: a case study in Beijing subway construction
Soils at the test sections
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5121113&req=5

Fig11: Soils at the test sections
Mentions: For construction of the shield tunnels in Beijing, which were a little more than 6 m in outer diameter, slumps of approximately 130 mm of the conditioned soils were sufficient. In the initial construction stage of the project, it was attempted to use the existing conditioning scheme in the shield driving. However, operation of the 10.22 m diameter shield was difficult. The earth chamber pressure, cutterhead torque and total thrust fluctuated widely, and large clods of earth existed, resulting in obvious ground surface settlement and a wider settlement trough. It was absolutely necessary to perform field tests to further optimize the conditioning scheme based on the indoor experiments. Considering the ease of operation and engineering economy, the scheme of simultaneously injecting 8% bentonite slurry and 5% foam liquid was adopted in the field tests based on the above findings. Through the field tests, the differences in conditions between the indoor tests and the construction were justified and the actual effects of the conditioning agent and the injection ratios were further verified. The test sections is given in Fig. 11 and properties of the soils are presented in Table 10.Fig. 11

View Article: PubMed Central - PubMed

ABSTRACT

Introduction: Beijing subway line 14 includes four stations and approximately 2.8 km of tunnels between the Dongfengbeiqiao and Jingshunlu areas of the city. Due to the surface and underground space limitations of this section, a double-track running tunnel instead of two single-track running tunnels was adopted to connect the two stations. The double-track tunnels were excavated by a 10.22 m diameter earth pressure balance (EPB) shield. It was the first time that an EPB shield more than 10 m in diameter was used in Beijing subway construction.

Case description: The shield, which passes underneath densely built-up areas of the city and is equipped with a spoke-type cutterhead, with balance between the ground pressure and the earth chamber pressure at the tunnel face, is of great importance. Referring to experiences gained in the EPB shield tunneling, attention was paid to the function of soil conditioning and simultaneous backfilling grouting of the shield, and some special designs were considered in manufacturing the machine.

Discussion and evaluation: In addition to the agitating rods welded to the cutterhead, two independently driven agitators were added to fully mix everything in the earth chamber. Independent pipelines were arranged for injecting different conditioning agents. Indoor tests in combination with field tests were conducted to find suitable additives and injection ratios of the additives, and determine the mix ratio of the two-component grout for simultaneous backfilling grouting. A scheme was employed for simultaneously injecting the bentonite slurry at 8% concentration and the foam liquid at 5% concentration to condition the excavated soil. The cement–sodium silicate grout was adopted to fill the tail void and the injection volume per ring was 14.1–15.3 m3.

Conclusions: The performance of the shield and evaluation of the corresponding tunneling technologies are introduced in terms of the shield tunneling induced ground surface settlements. The success of the project is of great significance to Beijing subway construction and underground space utilization. The findings serve as a useful reference for similar projects.

No MeSH data available.