China’s cities on the Pearl River Delta sit atop one of the most complex geological formations in the world. Ground is layered with silt, sand, highly weathered granite, and hard rock with significant ground water. In addition, the region’s soft soils are typically back-fill, as many of the cities, including Guangzhou, were built upon once-submerged flood plains. As one official aptly put, “We like to call it our geological museum.”
A new metro project between the cities of Guangzhou and Foshan has required extensive testing of the conditions in advance of TBM tunnelling. Two EPBs, manufactured by The Robbins Company, are boring parallel 2.6 km long tunnels with high advance rates despite the tough conditions. The rates achieved by the machines point to distinct variables influencing the project outcome, including effective ground control and machine design.
China Embraces Light Rail
Since 2006, China has invested nearly USD $200 billion in rail infrastructure—a plan that promises to be one of the largest national railway expansions since that undertaken by the U.S. in the 19th century. The government hopes to stimulate the economy and speed efficiency of passenger and freight travel throughout the country by adding 17,000 km of new lines. Metros are planned or are underway in dozens of cities, including Zhengzhou, Xi’an, Chengdu, and Chongqing.
Guangzhou’s metro extension is part of the wide-ranging Pearl River Delta Inter-city Rapid Rail Project, a network that will connect key cities within the delta, including Shenzhen and Dongguan, by 2015.
The 32.2 km long Guang-Fo line is China’s first ever inter-city rail line, and will cut travel times between the cities of Guangzhou and Foshan to about 50 minutes by 2012. The project’s parallel rail tunnels and 21 stationswere awarded in 12 separate lots by owner Guangzhou Metro Company. Tunnelling is currently under way using 16 TBMs. A tight project schedule calls for the completion of the first section in Foshan, running from KuigiLu to Xilang station, before the start of the Asian Games in November 2010.
Lot 12, running between Jushu, Xilang, and Hedong stations, was awarded in 2007 to the China Communication Construction Corp., 2nd Navigation Engineering Bureau Ltd. (CCCC). The contractor selected two 6.3 m diameter Robbins EPBs for the parallel 2.6 km long rail tunnels. “There are many advantages to using TBMs in the project conditions—these include safety, faster advance rates, minimal effect on surface traffic, and simpler construction management,” said Mr. Ju Yicheng, Vice Chief Project Engineer for CCCC.
Cutterhead Design
Proper cutterhead design is critical for the variable ground present in EPB tunnels. Geology on the metro’s Lot 12 consists of acomplex layered profile, ranging from highly weathered to slightly weathered granite, coarse sand, and silt at pressures up to 4 bar. About 70%of the tunnelling is through a mixed face, with the alignment above the spring line in soft soils and the bottom half of the tunnel in rock of at least 50 MPa UCS. The remaining 30%consists of flowing sand with high water content. The resulting cutter head design had to address the sticky clay-like consistency of highly weathered rock, as well as the soft soils and harder rock conditions.
The Guangzhou machines have been designed with spoke-type cutter heads and a large opening ratio of 37%, allowing for a smooth flow of muck into the mixing chamber. Both carbide bits and 432 mm (17”) hard rock single disc cutters are used to combat both the clay-like and rocky conditions expected. In soft ground, the knife-edge bits scrape away at soils, while the disc cutters break away rock at the face by creating a crush zone through which fractures propagate.
Cutterhead rotation is kept low (around 1.5 rpm at maximum), in stark contrast to the higher speeds (around 3.0 rpm maximum) used in hard rock TBM tunnelling. In hard rock, high rpm results in fast advance, while in soft ground high rotational speed often results in ground disturbance and surface settlement of non-self-supporting geology. In soft ground, the same result of high advance rates can instead be achieved by increasing the cutterhead torque, which increases the instantaneous rate of penetration. Each EPB cutterhead is driven by ten variable frequency drives (VFD) electric motors, for a total cutterhead power of 900 kW. Ultimately, the type of motor used depends on the so-called power to volume ratio. Smaller diameter machines, less than about 5 m, must use hydraulic motors to achieve high power, in order to take up less space on the machine. Though less bulky, availability tends to be less for hydraulic systems—about 75%, compared to more than 90%average availability for electric drives.
Mixing the Muck
Four independent foam injection points on the cutterhead are used to further consolidate the flow of muck. The independent systems also prevent clogging of multiple ports on one side of the cutterhead, which is often the case when common lines are used. Clogged ports can lead to uneven wear of the cutterhead and cutter tools.
The type of additive used is based on a standardized curve comparing geology and particle distribution. Ground with less than 30%fines, or particles less than 0.2 mm in diameter, is difficult to fluidize. In this type of non-cohesive ground, bentonite is used for consolidation. Foam is being used on the Guangzhou tunnels, in part because it is less costly and because it reduces the required cutterhead torque. Though the ground is good quality and does not need an additive such as bentonite, neglecting to inject foam can still lead to increased thrust and required power, as well as higher cutter consumption. If water pressure is high and small particles are present, a polymer can be injected in addition to the foam to increase cohesiveness of the material.
Muck and additives are further mixed within the cutterhead in the mixing chamber. Two mixing bars fixed to the inside of the cutterhead and to the pressure bulkheadhomogenize the muck as much as possible before it exits via an 800 mm diameter shafttype screw conveyor. Shaft-type conveyors are typically used in ground where large boulders are not predicted, and where there is potential for significant water inflows. In ground where boulders are expected, a ribbon-type screw conveyor is installed, which allows for boulder removal through an opening down the center of the conveyor.
Articulation through Curves
Choice of machine articulation can be a major variable affecting project speed. Much of the transit twists beneath the city, with curve radii as small as 200 m. Active articulation is used in curves, which engages articulation cylinders between the front and rear shields to steer the machine independently of the thrust cylinders. 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, a common cause of project delays, occurs when the passive articulation system is used in curves. Passive articulation does not utilize articulation cylinders independent of thrust cylinders, so the machine reacts against sides of the segments unevenly in curves.
Geological Challenges and Surface Settlement
The two Robbins EPBs began boring in January and February 2009, and were launched from the cut and cover site of Jushu station in southern Guangzhou. Surface settlement is a concern throughout the tunnel route, which travels beneath rivers, research sites, roadways, and vulnerable building foundations. To mitigate the risk of subsidence, back-fill grout is used to fill the annulus between the 300 mm thick, pre-cast concrete segment rings and the surrounding soil. The backfill grout is injected where the completed rings exit the tail shield. The volume and the pressure of the backfill grout injection are continuously monitored and controlled to eliminate the risk of surface subsidence.
High-risk areas include the 80 m wide, 4 m deep Huadi River between Jushu and Xilang stations, a significant waterway dividing the city. Ground beneath the river consists of layers of fine silt and sand with highly weathered granite below. The machine passed through the section, expected to have water pressures up to 3 bar, with few problems. “While the underground water was quite significant, we carried out effective measures including increased polymer injection. This worked very well,” said Mr. Yicheng.
Near the waterway is another area for concern—the Pearl River Fisheries Research Institute, with numerous sensitive ponds. The ponds are used for research into high yield fish farming, and were monitored for signs of disturbance as the TBMs passed some 10 to15 m below. Other than a few small bubbles visible at the surface, no ground settlement was detected.
Monitoring is present throughout the tunnel route, as there are dozens of building foundations that the tunnel passes under. “We stabilized a number of the building sites using pile foundation underpinning. We’ve also controlled settlement using surface stabilization and mucking volume control to decrease the chance of voids,” said Mr. Yicheng.
Record-Setting Excavation
After seven months of tunneling, the machines have achieved more than 16 project records including a best month of 377 m—higher than any of the 16 TBMs that have worked or are working on the Guang-Fo Metro Project. Prior to the final breakthrough, the machines must emerge into the cut and cover site of Xilang station, where they are inspected before embarking on their last 664 mlong section to Hedong station.
Both 6.3 m (20.5 ft) diameter machines are more than a month early on their project schedules, with the first machine completing its initial breakthrough into Xilang station on August 15, 2009. The TBM excavated its first section of tunnel with few problems and minimal cutterhead wear. The second machine is on track for its first breakthrough in September 2009.
The two EPBs have operated at average 95% availability, in expected ground conditions requiring few cutter changes. As of August 2009, 66 disc cutters had been changed on the first machine and 46 on the second, while no carbide bits had been changed on either of the TBMs.
Mr. Yicheng of CCCC attributed the fast advance to two main factors. “We are quite satisfied with the excavation speed of these machines. We believe the high rates are a combination of fewer equipment failures and efficient project management.”
The two Robbins EPBs boring Lot 12 of the Guangzhou Metro are more than a month ahead of schedule Cutterhead design for the Robbins EPBs had to take into account hard rock, clay-like ground and coarse sand Generalized Illustration of the Power to Volume Ratio. More efficient electric drives can only be used on machines over approx. 5 m in diameter, as the available space on smaller machines allows for hydraulic motors only Ground settlement precautions were taken at the Pearl River Fisheries Research Institute, though no problems were detected The Robbins EPBs are utilizing active articulation to avoid segment deformation in tunnel curves The first 6.3 m diameter Robbins EPB broke through into the Xilang Station site in August 2009
