Xi’an, China, is a hodge-podge of the ultra-modern and the historical. Vast contemporary shopping malls compete for attention with ancient bell towers and 7th century pagodas. Once the capital of China, the 3,100-year-old city also served as the eastern terminus of the Silk Road. Despite the balance of old and new throughout the downtown area, Xi’an’s transportation infrastructure has, until recently, been decidedly outdated. More than eight million people rely on surface transportation such as buses and taxis for travel within the city. That will all change in September 2011, when trial operations are scheduled to begin on Xi’an’s first new metro route, the 26.6km Line Two. The city’s 26.4km metro Line One is currently under construction, and will run from north to south through the downtown area. Ten TBMs, including two 6.2m diameter Robbins EPBMs, are currently excavating the new rail route, which travels directly below some of Xi’an’s most sensitive heritage sites.
Unique geologic conditions
The new metro Line One will travel below the famed Bell Tower of Xi’an, built in 1384 during the Ming Dynasty. Ancient Xi’an was a walled city, and that wall still remains, dividing the area between ‘Old Xi’an’ and ‘New Xi’an’. This wall, built in 1370, sits directly above the rail route’s bore path. Geology includes everything from sand and clay to abrasive pebbles up to 80mm in diameter—conditions that can be harsh on TBMs. “Line One is very difficult; the most difficult section in the city,” says Steven Zhou, Robbins project engineer. “The most critical issue is the mixture of pebbles with sand and soil. This can result in very serious wear to the cutterhead and cutters.” Extensive studies of the Lot 12 site where the Robbins EPBMs are boring indicated layers of highly to moderately compressed loess in bands up to 10.1m thick. Below the loess layers and the water table is a 30m-thick layer of silty clay with moderately abrasive sand. Abrasive pebbles are found throughout the mix.
Tunnelling is made even more complex by a city-wide ordinance limiting settlement to ± 15mm; significantly less than the 25mm limit that is typical of most Chinese tunnelling projects. The strict settlement guidelines, implemented due to the ancient structures, required specialized designs for the EPBMs.
The two machines are excavating Lot 12 of Line One for the 11th Bureau of China Railway Construction Corporation (CRCC). The parallel 3.6km tunnels pass through four cut and cover station sites—
Changlepo, Wanshou Road, Kangfu Road, and Jinhua Road—under shallow cover ranging from eight to 22m. “The biggest concern for us was the control of settlement,” says Zhai Xianxi, mechanical and electrical engineer for CRCC Bureau 11. “We wanted to ensure that the machines could keep the right earth pressure and control the output of the muck, and reduce the malfunction time. This would allow the machine to pass through any difficult layers as soon as possible.”
Maintaining face pressure through machine design
The Xi’an EPBMs were engineered to smoothly maintain face pressure in variable ground conditions and to maintain tunnel stability, from cutterhead design to active articulation.
Cutterhead design
Each machine was supplied with a spoke-type cutterhead, which utilises a large opening ratio to ensure a smooth flow of muck into the mixing chamber. The overall design of the spoke-type cutterhead allows for less abrasive wear due to the smaller surface area exposed to the face, compared with breasting plate cutterheads.
Once spoils have been scraped from the face, muck and additives are further mixed within the cutterhead, inside the mixing chamber. Two mixing bars are fixed in the machine—one on the inside of the cutterhead and one on the pressure bulkhead—to homogenise the muck as much as possible before it exits via the shaft-type screw conveyor. The uniform muck is better able to maintain pressure and hold the excavation face, while the shaft-type screw design ensures water tightness.
Foam/grout injection
Independent foam injection nozzles were also used on the cutterheads, to inject foam as well as bentonite and water for ground conditioning, depending on the geology present. The independent foam ports are used to consolidate the flow of muck and prevent clogging on one side of the cutterhead, which can lead to uneven wear.
To date foam has been predominantly used over bentonite and water because of its abilities to reduce the required cutterhead torque as well as overall machine wear. Insufficient foam injection has been associated with increased thrust and required power, as well as higher cutter consumption. A programmable logic controller (PLC) continuously regulates the variables of the additive system to prevent surface subsidence.
Active articulation
Multiple curves along the tunnel alignment range from 3,000m radii to a low of 1,000m, requiring active articulation to better stabilise segments. The setup engages articulation cylinders between the front and rear shields to steer the machine independently of the thrust cylinders. Flat joints in the articulation cylinders allow for two to three degree curve adjustments over the length of each segment or stroke.
The process allows the thrust cylinders to react evenly against all sides of the segment ring during a TBM stroke in a curve. Segment deformation, or racking, is a common cause of project delays that
occurs when the passive articulation system is used in curves. Passive articulation does not utilise articulation cylinders independent of the machine’s thrust cylinders, allowing the TBM to react against sides of the segments unevenly in a curved alignment.
Machine launch under challenging conditions
In June and July 2010, commencement of Xi’an’s Metro Line One began with the launch of the two Robbins machines from Changlepo towards Wanshou Road Station—the first TBMs tunneling on the project. Eight refurbished Komatsu EPBMs were launched at later dates on the remaining lots.
The Lot 12 section was located in a densely urban area, with the tunnels travelling below a college, a hospital, and a marketplace. “The machines were launched in sandy soils with cobbles more than 200mm in diameter—a type of ground with high risk of settlement,” says Jason Xiao, Robbins project manager. Crews continuously monitored the excavation rate and overall muck removal volume, by adjusting the thrust force, advance rate, and screw conveyor speed, while keeping the cutterhead speed low, at one rotation per minute. Bentonite was also used to improve soil conditions at the tunnel face.
Sections of collapsible, water bearing soils were also present in early sections of the tunnel alignment. Crews approached these sections with similar measures, including strict earth pressure control in the mixing chamber, paired with injection of bentonite and water for soil conditioning.
Minimising settlement
As of August 2011, one machine had recently completed its second section of tunnelling between Wanshou Road and Jinhua Road, and the other was on its last section between Jinhua Road and Kangfu Road in layered loess conditions. “The machine is currently boring about 10m below a viaduct structure,” says Xiao. “So the demand on settlement control is even less than 15mm here. We are working to keep the appropriate earth pressure and control the output of the muck, while performing regular maintenance to reduce downtime through this section.”
The last intermediate breakthrough, at Jinhua Road station, occurred on 28 July. Advance rates for the Left Line EPBM have been good—up to 579m (386 rings) per month and 36m (24 rings) per day. Planned maintenance was performed while in the cut and cover area, such as changing of cutters and tail seal brushes as well as checkups of the hydraulic system and electrical system. Semi-segment rings consisting of the invert and two segments were installed to allow the machine to ‘walk’ through the 140 m long station site.
Advance rates for the Right Line machine have been similarly high—up to 453m (302 rings) per month and 39m (26 rings) per day. Settlement has been kept below 15 mm with an average settlement of 5mm. “We have been able to control settlement, and we feel we are mastering the EPBM more and more,” said Xianxi.
Tunnelling should be complete in November 2011 for the left line machine, and in December for the right line.
Once online in 2013, Line One will reduce traffic times across the city from well over one hour to 39 minutes. More lines are planned for Xi’an—up to six new routes totaling more than 250km by 2020. The new Line Three broke ground in May 2011, following extensive feasibility studies of the loess layers and fissures along the alignment. The route, expected to be complete in 2015, will run from southwest to northeast through high-traffic areas.
Two 6.2m diameter EPBMs are excavating at monthly rates of up to 579m Figure 1, Xi’an in Shaanxi Province, China Figure 2, metro lines of Xi’an. Planned lines dotted. Construction of Lot 12 of Line One in solid red
