China’s vast variety of EPB tunnelling projects, in a wide spectrum of geologies, has been instrumental in developing the next generation of soft ground TBMs. The lessons learned in difficult ground on recent projects have guided the design of new EPBMs, from modular drives to highpowered screw conveyors.
Field experiences range from tunneling in layered ground at Guangzhou Metro to achieving record advances in permeable alluvium at Chengdu Metro. “There were multiple challenges that we got through successfully,” said Jason Xiao, Robbins field service manager, of tunneling on the Guangzhou Metro, Guang-Fo Line in 2009. “There were sensitive building structures and research sites, permeable ground, and sticky clay that all had to be dealt with.” The drives at Guangzhou would begin a string of engineering improvements from project to project, with often recordbreaking results.
Soil complexity in urban surroundings: Guangzhou Metro
Two 6.3m diameter EPBMs began boring Lot 12 of China’s Guangzhou Metro, Guang-Fo line in January and February 2009, through the country’s characteristically complex soils. Ground consisted of layers of highly weathered to slightly weathered granite, coarse sand, and silt at earth pressures up to four bar.
The machines were designed for mixed face, variable ground using spoke-type cutterheads with a 37 per cent opening ratio. Both carbide bits and 17in hard rock single disc cutters were used to combat both the clay-like and rocky conditions.
Each EPBM cutterhead was driven by ten variable frequency drive (VFD) electric motors, for a total cutterhead power of 900kW. Cutterhead rotation was kept low (around 1.5 rpm at maximum), in order to reduce ground disturbance and surface settlement in non-self supporting geology. At the same time, the machines were designed with high torque to increase the rate of instantaneous penetration in soft ground.
Sticky ground
The machines were launched from cut and cover station sites in a densely urban area of Guangzhou. “From the start, the viscosity of a red, clayey soil layer was higher than expected, making the mud cake easy to form, but also often blocking the central area of the cutterhead,” said Xiao. In order to pass through the sticky layer, foam was added to reduce viscosity, while reserve water injection holes inside the man-lock were used to flush any clogged soil from the central cutterhead area. The situation was aided by a system of four independent foam injection points on the cutterhead, which prevented clogging of multiple ports on one side of the cutterhead, an occurrence that can lead to uneven wear.
Urban complexity
During tunnelling, the urban area and low tunnel cover (10 to 15m) required strict monitoring of surface settlement. An artificial lake, with a lake bed composed of oyster shell and sandy soil, sat directly overhead the tunnel alignment, creating a high risk for water inflows. Crews utilised the polymer injection system to inject polymer into the layer and absorb water, improving soil conditions for a smooth discharge of muck.
The 4m deep Huadi River and its surroundings were another area for concern—the Pearl River Fisheries Research Institute sits nearby the river, with numerous research ponds directly above the tunnel route. Because of the sensitive nature of the research, there was no way to probe the pond to determine the geology and its resulting risks. Polymers were again injected in the area below the ponds to keep settlement to a minimum. As the TBMs passed, the ponds were visually monitored for signs of disturbance. Other than a few small bubbles visible at the surface, no ground settlement was detected.
As the machines passed below the Scientific & Development Building, part of the research institute, more careful measures were needed. The building, built in the 1980’s, was determined to have a very low tolerance to vibration and ground disturbance. “We needed to strictly control the earth pressure to about two bar below this section, in order to allow the machine to move forward at an even rate of speed. Cutterhead speed was controlled to one rotation/min in order to achieve this, and get through the section safely,” said Xiao.
Other methods were used in concert with the TBM tunnelling measures. “We stabilised a number of the building sites using pile foundation underpinning. We also controlled settlement using surface stabilisation and mucking volume control to decrease the chance of voids,” said Ju Yicheng, vice chief project engineer for the contractor, China Communication Construction Corp., 2nd Navigation Engineering Bureau (CCCC).
Despite the challenges, the machines achieved more than 16 project records including a best month of 377m—higher than any of the 16 TBMs that worked on the Guangzhou Metro, Guang-Fo Line Project. Both EPBs completed tunneling more than a month early, in September and October 2009.
Stemming settlement: Xi’an Metro
In June and July 2010, commencement of Xian’s Metro Line 1 began with two 6.3m diameter TBMs. A combination of sand, pebble, and clay geology and ancient architecture including a 12th century bell tower and city wall translated to extensive measures for preventing settlement. Due to the sensitive area in one of China’s most ancient cities, regulations stipulated a maximum of ± 15mm surface extrusion or settlement for any tunnels.
Spoke-type cutterheads and 800mm diameter shaft-type screw conveyors were used to aid in efficient advance while maintaining a water-tight seal and balanced pressure.
The high-powered machines were also designed with 3600 tonnes of thrust force to guarantee a fast advance in the ground conditions. The high thrust force additionally required a robust reaction frame.
“The shield machines launched in sandy soils with cobbles more than 200mm in diameter—a type of ground with high risk of settlement,” said Xiao. Crews carefully 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 are also present along the tunnel alignment.
Crews are tackling these sections with similar measures, including strict earth pressure control in the mixing chamber, paired with injection of bentonite and water for soil conditioning. Tunnelling at Xi’an Metro is ongoing, with Line 1 scheduled to open at the end of 2011.
Managing permeability: Chengdu Metro Line 2
China’s ChengduMetro Line 2 travels 31.6kmbelow the city, and will open to rail traffic by the end of 2011. Itsmost recent tunnelling breakthrough, at Lot 18, underscored the usefulness of EPB tunneling in permeable ground. The 6.3mdiameter Robbins EPB was designed to excavate a complexmatrix of alluvial geology, found nowhere else in China. Geotechnical surveys of the tunnel area indicated highly permeable ground, with lenses and layers of glacial boulders up to 250mmin diameter, along with sand and stiff clay.
Effective machine design
To excavate in the conditions, a mixed ground cutterhead was provided, mounted with carbide bits and seven 17in diameter disc cutters around the gauge. The large cutterhead opening ratio allowed for a smooth flow of muck into the mixing chamber. The cutterhead was powered by ten 90kW variable frequency drives.
The machine was designed with active articulation for curves with radii as small as 500m. The active configuration was utilised, because it engages articulation cylinders between the front and rear shields in curves 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. The setup eliminates segment deformation, or racking—a common cause of project delays when passive articulation is used through curves.
A foam injection system was used to stabilise any running ground, with each cubic meter of foam mixture able to stabilize about 40 rings of advance. “The TBM allowed very good control of foam and other additives into the ground at the face,” said Li Quanshe, project manager for contractor China Railway Construction Corporation (CRCC), Bureau 23. Four independent foam injection points on the cutterhead were used, in a similar configuration to those on the Guangzhou Metro.
Riverbed challenge
The TBM excavated mainly through stiff red clays under about two bar earth pressure, but had previously experienced a short mixed face section consisting of rock and earth while tunnelling 3.5m below Donggan River. “Only three cutter changes were required, while passing through this section—two in the left tunnel and one in the right tunnel,” said Xiao. One cutter change required 1.5 bar of air pressure, while the other two were done in free air because the ground was very stable.
The machine completed tunnelling in December 2010, having achieved project records of 180m in one week, and 459.5m during its best month—higher rates than 11 other machines working on Line 2.
The next generation of EPB design: Chengdu Metro West Extension
A new generation of EPBs is currently being built, from the lessons learned at the above projects and others, including India’s New Delhi Metro, the Upper Northwest Interceptor Sewer in California, USA, and Mexico City Metro Line 12. These next generation EPBs are optimised for fast excavation, particularly in China’s harder ground, while minimising machine wear. The first two EPBs to utilise the designs will be a pair of 6.3m diameter machines for the West Extension of Chengdu Metro Line 2. “The machines are designed to give higher power and higher torque, which is ideal as long as the ground is stable. These features also make the machines adaptable for a variety of tunnel lengths and geologies,” said Shinichi Konda, Robbins chief EPB engineer.
One way to achieve the higher power, for a variety of diameters, is with a modular cutterhead drive system. The cutterhead drive is standardised, while the number of motors can vary. “For example, up to eight motors can be installed on a 6m diameter, metro size TBM,” said Konda. Within days, the number of motors can be changed on a machine by removing or adding pinion housings, making the machines flexible and easily customisable. The design also has the added benefit of allowing the man-lock to be moved from the center of the cutterhead to the top, increasing worker safety when the cutterhead mixing chamber is partially filled with muck.
To match muck removal to the faster advance rates, high powered screw conveyors are being designed specifically for the harder, less fluidised ground seen in many Chinese EPB projects and in other countries. The designs are capable of up to 230kNm of torque, and offer greater flexibility as well as added abrasion protection.
The main seals are also being redesigned to cope with the extra heat generated during fast tunneling. Many commonly used main seals in the industry consist of a hard urethane rubber with low heat resistance, so that water must be added during fast tunneling to cool the rubber surface. New EPBs will utilise an NBR rubber for the main seals, which is softer and offers more heat resistance. The seals are also being reinforced with hard facing for added abrasion resistance.
Engineers are currently overseeing the assembly of the newly designed EPBs at the China South Locomotive & Rolling Stock Corporation (CSR) manufacturing facility in Chengdu. The machines will be launched to bore parallel 3.2 km long drives for China Engineering Railway Company (CERC) in late spring 2011.
The next generation of Robbins EPBs, seen here as a cross-sectional view of a machine from the back, include modular VFD motors and man-locks oriented at the top of the machine The Robbins EPBs at Guangzhou Metro broke through ahead of schedule, despite facing challenges including low tunnel cover and layered soft ground with rock. The Robbins EPBs at Guangzhou Metro faced a particular challenge under the Pearl River Fisheries Institute, requiring polymer injection and steady control of advance rates The 6.3m diameter EPB at China’s Chengdu Metro Line 2 achieved a project record of 180m in one week while excavating through the area’s permeable alluvium The Robbins EPB at Chengdu Metro excavated an area of harder ground and glacial boulders below a river, requiring only three cutter changes. The machine broke through in December 2010 Two Robbins EPBs at Xi’an’s Metro Line 1 are excavating below ancient structures including a 12th century bell tower and city wall Ground conditioning equipment including bentonite soil injection was used to stabilise sandy soils with cobbles below the ancient city of Xi’an
