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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.


A cross-section of the tunnel
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Fig4: A cross-section of the tunnel

Mentions: The selection between the EPB- and slurry-shields is influenced by three types of factors: economic, technical and geological, and a thorough and detailed discussion is often required. However, the use of an EPB shield in this project can be justified by two reasons: (1) the accumulated experience and skill with the use of the 6.15 m diameter EPB shields in Beijing area and (2) the lack of sufficient ground surface space for a sophisticated separation plant for the slurry shield. As presented in Fig. 3, the used machine is equipped with a spoke-type cutterhead, with an opening ratio of 65%. Important technical parameters of the shield are summarized in Table 2. In Table 2, the maximum torque coefficient is related to the installed cutterhead torque. The most widely used empirical formula for the installed cutterhead torque of soft ground shield machines is T = α D3, where T is cutterhead torque (kN m), and D is excavation diameter of the machine (m) and α is the maximum torque coefficient. A cross-section of the tunnel is presented in Fig. 4.Fig. 3


Use of a 10.22   m diameter EPB shield: a case study in Beijing subway construction
A cross-section of the tunnel
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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

Fig4: A cross-section of the tunnel
Mentions: The selection between the EPB- and slurry-shields is influenced by three types of factors: economic, technical and geological, and a thorough and detailed discussion is often required. However, the use of an EPB shield in this project can be justified by two reasons: (1) the accumulated experience and skill with the use of the 6.15 m diameter EPB shields in Beijing area and (2) the lack of sufficient ground surface space for a sophisticated separation plant for the slurry shield. As presented in Fig. 3, the used machine is equipped with a spoke-type cutterhead, with an opening ratio of 65%. Important technical parameters of the shield are summarized in Table 2. In Table 2, the maximum torque coefficient is related to the installed cutterhead torque. The most widely used empirical formula for the installed cutterhead torque of soft ground shield machines is T = α D3, where T is cutterhead torque (kN m), and D is excavation diameter of the machine (m) and α is the maximum torque coefficient. A cross-section of the tunnel is presented in Fig. 4.Fig. 3

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.