Limits...
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.


Measured maximum surface settlements. a All settlements, b clay soils at tunnel roof, c sandy soils at tunnel roof
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC5121113&req=5

Fig17: Measured maximum surface settlements. a All settlements, b clay soils at tunnel roof, c sandy soils at tunnel roof

Mentions: During long distance driving of the shield, the scheme of simultaneously injecting foam and bentonite using the No. 1 formulation was usually adopted to control slumps of the excavated soils at approximately 160 mm. However, in difficult situations, such as passing large cross sections of sand soils or crucial risk sources, the injection scheme of the No. 2 formulation was used to increase the injection ratio of the foam. Compared with the No. 1 formulation, more foam was used in the No. 2 formulation. This led to the high earth chamber pressure, the less cutterhead torque and total thrust, and more importantly the small surface settlement. So the No. 2 formulation was used to get the small settlement in more difficult conditions. Generally, the above guidelines were executed in the whole tunnel construction, and the shield tunneling parameters were well regulated. The key parameters were as follows: earth pressure in the excavation chamber of approximately 0.12–0.18 MPa, total thrust of approximately 30,000–50,000 kN, cutterhead torque of approximately 30–45% of the rated torque, rotation of the cutterhead of approximately 0.4–0.7 rpm, and a shield advance rate of approximately 30–70 mm/min. While the active earth pressure balance was created at cutting face, the simultaneous backfilling of approximately 14.1–15.3 m3 per ring was exerted using the formulation from the field tests. The maximum ground surface settlements were well controlled within 10–40 mm, as shown in Fig. 17. The soils at the tunnel roof have a large influence on the surface settlement. When sand soils were encountered, the larger settlements of 20–40 mm were induced, as shown in Fig. 17c; when clay soils were encountered at the tunnel roof, the settlements were approximately 10–25 mm, as shown in Fig. 17b. As shown in Fig. 17a, most maximum surface settlements were 15.1–30 mm, accounting for 83.87% of the total. The percentage for each ground condition represents the distribution of the maximum surface settlement, which was the results of the shield tunneling operation (including the simultaneous backfilling grouting). A settlement of 40 mm was acceptable if no visible damages was done to the surroundings and structures. Surface settlements are also affected by many other factors, such as tunnel depth, shield operation parameters, injection pressure and volume (Banbendererde 1991; Bezuijen et al. 2004; Merritt and Mair 2008; Guo 2013).Fig. 17


Use of a 10.22   m diameter EPB shield: a case study in Beijing subway construction
Measured maximum surface settlements. a All settlements, b clay soils at tunnel roof, c sandy soils at tunnel roof
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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

Fig17: Measured maximum surface settlements. a All settlements, b clay soils at tunnel roof, c sandy soils at tunnel roof
Mentions: During long distance driving of the shield, the scheme of simultaneously injecting foam and bentonite using the No. 1 formulation was usually adopted to control slumps of the excavated soils at approximately 160 mm. However, in difficult situations, such as passing large cross sections of sand soils or crucial risk sources, the injection scheme of the No. 2 formulation was used to increase the injection ratio of the foam. Compared with the No. 1 formulation, more foam was used in the No. 2 formulation. This led to the high earth chamber pressure, the less cutterhead torque and total thrust, and more importantly the small surface settlement. So the No. 2 formulation was used to get the small settlement in more difficult conditions. Generally, the above guidelines were executed in the whole tunnel construction, and the shield tunneling parameters were well regulated. The key parameters were as follows: earth pressure in the excavation chamber of approximately 0.12–0.18 MPa, total thrust of approximately 30,000–50,000 kN, cutterhead torque of approximately 30–45% of the rated torque, rotation of the cutterhead of approximately 0.4–0.7 rpm, and a shield advance rate of approximately 30–70 mm/min. While the active earth pressure balance was created at cutting face, the simultaneous backfilling of approximately 14.1–15.3 m3 per ring was exerted using the formulation from the field tests. The maximum ground surface settlements were well controlled within 10–40 mm, as shown in Fig. 17. The soils at the tunnel roof have a large influence on the surface settlement. When sand soils were encountered, the larger settlements of 20–40 mm were induced, as shown in Fig. 17c; when clay soils were encountered at the tunnel roof, the settlements were approximately 10–25 mm, as shown in Fig. 17b. As shown in Fig. 17a, most maximum surface settlements were 15.1–30 mm, accounting for 83.87% of the total. The percentage for each ground condition represents the distribution of the maximum surface settlement, which was the results of the shield tunneling operation (including the simultaneous backfilling grouting). A settlement of 40 mm was acceptable if no visible damages was done to the surroundings and structures. Surface settlements are also affected by many other factors, such as tunnel depth, shield operation parameters, injection pressure and volume (Banbendererde 1991; Bezuijen et al. 2004; Merritt and Mair 2008; Guo 2013).Fig. 17

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.