The second tunnel portal to be excavated

The shield machine excavating a tunnel 16 meters underground

      Core Tips
　　Walking down the over 1-meter-wide steel staircase to the shield machine working platform about 16 meters underground... a giant about 6 meters in diameter and 75 meters long appears before your eyes. At around 10 AM on December 8, the first shield machine for Nanning Metro Line 1 was launched at Nanhu Station, marking the first use of a shield machine for underground construction in Nanning.
The launched section is the right line from Nanhu Station to Jinhu Square Station, located on Minzu Avenue, arranged in an east-west direction, entirely underground. This "Ground Dragon" is tasked with the important mission of connecting Nanhu Station to Jinhu Square Station. According to reports, the section starts at the green space in front of the Guangxi Local Taxation Bureau, runs along the north side of Minzu Avenue from west to east, then diagonally crosses Minzu Avenue from north to south starting from the Minzu Jinzhou intersection, passes under the Jinhu Square underground shopping mall, and reaches the section endpoint at the Minzu Jinhu intersection, with a length of about 726 meters. The construction period for the right line section from Nanhu Station to Jinhu Square Station is planned for 5 months, with arrival at Jinhu Square Station scheduled for March 2014.
FirstLaunch Success
It is the "King" in the field of underground construction
　　"This Herrenknecht S540 shield machine was transferred from Harbin." At around 10:20 AM, He Xusheng, deputy director of the Engineering Department I of Nanning Rail Line 1, introduced that this "Ground Dragon" from Harbin has started working and the first launch was successful.
So how does the "Ground Dragon" drill? "A huge cutterhead is installed at the front of the shield machine." said He Xueke, chief engineer of the Nanning Rail Line 1 project department. The cutterhead is equipped with disc cutters, heavy-duty ripping teeth, and scraper blades. The disc cutters maintain the excavation diameter, while the other two types of cutters break the soil and perform the excavation.
When the shield machine starts, the cutterhead rotates rapidly, slowly drilling into the ground and advancing forward. The broken soil enters the soil chamber through the openings in the cutterhead, falls to the bottom of the chamber, and is then transported by a screw conveyor to a belt conveyor, and finally to muck cars on the tracks. The shield advances forward under the thrust of the propulsion cylinders. The shield shell provides temporary support for the excavated but unlined tunnel, withstands the earth pressure of the surrounding soil, the water pressure of groundwater, and keeps groundwater outside the shield shell. Operations such as excavation, soil removal, and lining are carried out under the protection of the shield shell.
"The shield machine is the safest method for underground construction." He Xusheng said that during construction, workers operate inside the inner shell of the shield machine, which protects them; in addition, shield construction can avoid underground pipelines, ensure construction quality, and produce low noise and minimal environmental pollution.
This shield machine is worth 50 million yuan and weighs about 500 tons.
      Not Fast
　　200-300 meters per month
　　This giant digs in all directions underground. Will there be errors?
"Errors are controlled within plus or minus 5 centimeters." He Xueke said that there is a measurement system at the front of the shield machine, and the survey data is connected to a display panel inside the shield machine. Workers can adjust at any time by checking the panel.
Currently, the construction contractor has arranged two shifts, with 28 people per shift, working 12-hour shifts, 24 hours a day.
On the morning of the 8th, after the shield machine was launched, the reporter saw workers pressing buttons to operate from time to time, and the "Ground Dragon" slowly advanced, but standing on the side of the shield machine, it was hard to feel the giant moving. Why?
"The shield machine advances about 5 centimeters per minute." He Xueke explained that because the advancement distance is small, it is difficult to visually detect its progress. Although it is hard to see the giant's capability with the naked eye, according to engineers, it can excavate 200-300 meters per month.
At the launch site of the shield machine, the reporter also saw tracks similar to train tracks below the shield machine, and the shield machine was slowly advancing between the two tracks. On both sides of the huge shield machine, arc-shaped objects made of concrete made it hard to guess their function.
"These are metro segment linings, which support the metro tunnel." He Xueke explained that after the metro construction advances a certain distance, a ring of segments can be automatically assembled to form a "circle" about 6 meters in diameter. The metro tunnel is supported by countless such "circles".

Workers further constructing inside the excavated tunnel

      Companion Appears
Second Shield Machine to Enter Site
　　He Xusheng introduced that metro construction is divided into three stages: first station construction, then section construction, and finally installation and commissioning. The use of the shield machine is for section construction.
"After the first launch, the second shield machine will also enter the site, with a gap of about 100 meters." He Xueke said that since the underground lanes are bidirectional, one shield machine excavates one lane, and the other lane requires a second shield machine. Now the second shield machine has begun assembly and commissioning and will soon be "on duty".
After connecting Nanhu Station to Jinhu Square Station, these two shield machines will continue forward to connect Jinhu Square Station and Nanning International Convention and Exhibition Center Station.
In addition, it was learned that of the 25 stations on Line 1, 23 more shield machines will be put into use successively, and a total of 41 kilometers of construction length on Line 1 will use shield machines.
Currently, except for Wanxiang City Station, the other 24 stations have completed underground diaphragm wall construction, 21 stations have started excavation and main structure construction, 5 stations will be capped by the end of this year, and most stations will be completed by early next year.
Challenges
Complex Geological Conditions in Nanning
　　"Nanning's geological conditions are complex; it can be said that this is the first time we have encountered such complexity." He Xueke said that there are certain difficulties in the construction of the "Ground Dragon". In the current construction section, the geology and groundwater are abundant, water pressure is high, and soil stability is poor. "Underground engineering fears water the most; after encountering water, ground subsidence and other issues may occur."
What methods will engineers use to ensure the safety of the "Ground Dragon"? He Xueke introduced that special treatment will be carried out. During construction, workers will fill in high-quality clay, namely bentonite, to facilitate the establishment of soil pressure and thus facilitate construction.