The first phase of line 17 of chengdu metro is completed

On the morning of April 9, the right tunnel between huangshi station and shiwu hospital station area of the first phase of chengdu rail transit line 17 was completed at the same time, and the right tunnel between mingguang station and no.1 air shaft became the first two sections of line 17 to be completed.
     Reporters learned that the chengdu rail transit line 17 is a series of central chengdu city and wenjiang, shuangliu peripheral areas of the important express line, the design speed of up to 140 km. The total length of the line is about 26.1 km, with 9 stations, including 2 elevated stations and 7 underground stations.
     The tunnel on the right line between huangshi station and shiwu hospital station is about 1.5 kilometers long, and the tunnel on the right line between mingguang station and no.1 air shaft is about 1.8 kilometers long. At present, there are 18 shield tunneling machines with a diameter of 8.6 meters rushing underground.

It is the first time to successfully challenge the construction of water-rich sand and pebble formation with "hard bone" large-diameter shield tunneling machine
          
       In the underground area with a total length of about 20.8 kilometers, at the peak, there were 20 combined earth pressure balancing shield tunneling machines with a cutter head diameter of 8.6 meters at the same time. This is the first time that the maximum diameter shield tunneling machine is applied in the excavation of water-rich sand and pebble strata in the field of chengdu rail transit construction.
     Chengdu area are mainly composed of sand gravel stratum, the pebble grain size increases gradually from east to west, pebble intensity gradually increased, especially the salween region pebble content was 50% ~ 80%, individual particle size of more than 90 cm, with local silty sand layer, and salween near the minjiang river, the water system is relatively developed, formation of impermeable layer, and the permeability coefficient of 25 to 28 m/day.
      Chengdu subway construction by shield method has more than 10 years, with the increase of shield driving mileage, water rich sand pebble formation of shield tunneling technology breakthrough, but due to the particularity of water rich sand pebble formation, middle earth pressure balance shield construction of large diameter process is difficult to control, the stability is poor, spewing and tunnel segment assembling machine, series of problems such as broken fault may be larger. Therefore, the tunneling construction of large-diameter shield tunneling machine is also described as "drilling holes in flowing sand and rocks".

Winning the 'safe relay' of the dragon

       It is understood that the two tunnels have successfully crossed the rain sewage pipe, fengxi river and chengwen qiong highway. Among them, the chengwenqiong expressway is a very significant source of risk. The large-diameter shield tunneling machine is used to cross the expressway in the medium-dense sand and gravel stratum with shallow burial depth, which is also the first time in the shield tunneling construction in chengdu area. Especially, the top of the tunnel is only 11.8 meters away from the highway surface in vertical direction.
      In order to ensure the smooth crossing of the shield, radar detection and flatness detection are carried out on the expressway to assess whether the pavement condition grade and indexes of the expressway meet the standard requirements. Then aiming at the stratum difficulty, the project adopts the way of strengthening the super long pipe shed to minimize the safety risk. In addition, the project team and cheng wenqiong highway company and related units to form an emergency rescue team, and emergency personnel for professional training, in advance of the organization of emergency drill, for the safety of the smooth through cheng wenqiong highway run "safety first."
     As the knife plate slowly rotating, the shield machine through the chengwen qiong highway "second rod" officially started. Although before crossing project has strengthened with super long fill tent, but shallow buried depth through Micronesia sandy pebble formation coefficient of safety risk is extremely high, the project decided to increase the synchronous grouting quantity in the process of excavation, make secondary grouting, timely to ensure the tunnel segment and formation compaction, and reduce the torque and thrust of shield machine, striving to security risks to a minimum. The project surveyors and construction monitors acted as the "eyes" during the shield tunneling, and carried out high-frequency settlement observation on the expressway 24 hours a day in shifts. The dense monitoring data became the biggest basis to ensure the safe crossing.
       The reporter learned that, with the close cooperation of all parties, the shield tunneling achieved the objectives of effective settlement control, smooth tunneling speed, safe and controllable quality, successfully completed the "safe relay race", and laid a solid foundation for the completion of the tunnel.