It has been argued that the majority of good ground for tunneling in the world already has a tunnel going through it. While this might be an overstatement, it is true that more and more of today’s tunneling projects are being proposed in difficult, mixed ground conditions. Hybrid TBMs, specifically EPB/Slurry and EPB/Hard Rock machines, are increasingly becoming the best solution for these challenging conditions.
Hybrid machines have the potential to lower risk and make difficult excavations possible, as long as accurate geologic information is available. For example, a hybrid EPB/Open-Type machine can be optimized towards either end of the scale—depending on whether the majority of the drive is in soft soils or majority in hard rock—to produce the fastest possible advance rate over the entire project. If the tunnel is 80% soft ground and 20% hard rock, the overall machine design will be optimized towards EPB.
But what happens when the divisions are not so clear cut, or when the geology is 50% rock and 50% soft ground? A real example is the upcoming Sleemanabad Carrier Canal project, which will utilize a 10.0 m (32.8 ft) diameter Hybrid EPB/Hard Rock TBM. The 12 km (7.5 mi) long water transfer tunnel in Madhya Pradesh, India consists of shorts sections of hard rock and soft ground. Geology includes clay, gravel, marble, and hard rock up to 180 MPa (26,000 psi) UCS.
The solution at Sleemanabad is to provide three modes of tunneling: pressurized EPB tunneling, non-pressurized EPB tunneling, and open hard rock tunneling. The first half of the tunnel will be excavated using a specially designed screw conveyor, which can operate well in both the pressurized and non-pressurized environments. The oversized screw can be rotated faster to handle short sections of hard rock while minimizing any loss in efficiency.
When a longer section of rock is encountered, the screw conveyor is removed and switched out for the TBM belt conveyor. Additional welded steel is added to the cutterhead to close up the open soft ground design, and the machine operates as a Single Shield TBM.
In other conditions, such as sections of soft ground and rock mixed with sections of high water content, a Hybrid Slurry/EPB Type machine is desirable. The machines can similarly be optimized towards slurry or EPB depending on the amount of groundwater expected. An interesting example of this type of hybridization took place at the South Central Hillsborough Intertie Tunnel in Tampa Bay, Florida, USA. The machine excavated limestone with significant groundwater using both a screw conveyor and modified slurry removal system. See the case study here.
Hybrid machines are becoming more common, and are likely to gain in popularity over the near future. With greater experience will also come greater optimization of conversion times between modes—one of the biggest challenges during tunneling. While these machines do have some negatives, as all machine types do, the solution of Hybrid TBMs in mixed ground vastly outweighs any difficulties over the life of the project.
Settlement. It is one of the biggest risks in urban tunneling and the cause of sinkholes, cracked building foundations, and tunnel collapses. Surface subsidence and upheaval have vexed contractors and equipment suppliers worldwide, but there are some proven ways to minimize ground movement, even in low cover tunnels.
Tunneling at Mexico City Metro’s Line 12 provides a good, recent example of settlement prevention. The 7.7 km (4.8 mi) route is being excavated just 7.5 to 14 m (25 to 45 ft) below downtown Mexico City. The 10.2 m (33.5 ft) diameter Robbins EPB is surfacing into eight cut and cover station sites during excavation, all along major downtown streets.
The ground, which consists of watery clays, sand, and large boulders up to 800 mm (2.5 ft) in diameter, makes settlement a concern. Ismail Benamar, Tunnel Manager for contractor Ingenieros Civiles Asociados (ICA), spoke about the measures they are taking to control the ground: “We have an extensive monitoring program to detect displacements and pore pressure on the surface, underground, inside the tunnel, and in the most critical structures next to the tunneling line.” The risk of surface subsidence and vibration is also being controlled by regulating the rate of advance and controlling earth pressure at the front of the machine, as well as the back-fill grouting pressure.
In particularly low cover areas, Benamar cited a reduction in EPB cutterhead rotation as one of the primary means of avoiding ground disturbance. Cutterhead rotation is kept low (around 1.5 RPM) while torque is kept relatively high.
In addition, the machine’s two-component back-filling system is used to further stabilize the space between concrete segments and bored tunnel. The liquid mixture consists of water and bentonite cement plus an accelerant, which are combined in the tail shield and harden rapidly after injection. Because the two liquids are kept separate, high pressure concrete pumps, which can disrupt the surrounding geology, are not needed.
By May 21, 2010, the machine had broken through into Mexicaltzingo Station—the first of eight cut and cover station sites along the tunnel route. Little to no settlement issues were recorded during tunneling.
Taken together, the methods at Mexico City Metro show the best ways to minimize settlement in urban EPB tunneling. Settlement can be due to multiple factors, including over-excavation, improper ground conditioning, inadequate back-filling, or loss of face pressure. Despite the risks, proper machine design, continuous monitoring, and a knowledgeable crew can keep ground under control.
For more about the causes of settlement and tunnel collapse, visit an online forum at tunneltalk.com
Multiple Machines: The Challenges and Successes of Delivering Three TBMs for Malaysia’s Longest Water Tunnel
Simultaneous delivery of TBMs, some of the most complex machines in the construction industry, is no easy task. Robbins TBM assembly is most often a local process—one that requires skilled workers with knowledge of components designed and manufactured all over the world.
Robbins use of local or nearby shops can often cut down on delivery times. A partial video of the TBM assembly process in the Robbins China-Shanghai facility is available here:
The video depicts the recent assembly of one of three 5.2 m (17.2 ft) diameter Robbins Main Beam TBMs. The machines are being built simultaneously for Malaysia’s Pahang Selangor Raw Water Tunnels, and are a good example of the assembly process.
Each machine is built at a workshop in Shanghai from components manufactured in the U.S. and China. After assembly of each machine is complete, the TBM and continuous conveyor system are shipped from Shanghai to Port Kelang in Malaysia. Once at the port, the components are trucked to the jobsite.
“One of the most difficult processes is finding a single shop that can manage the volume of fabrications and machining. The delivery times for the Malaysian TBMs are one month apart, which means that at one point you are disassembling and shipping the first machine, testing the second machine and starting assembly of the third machine,” said Kolenich. On average, each TBM requires a skilled crew of 20-30 fabricators, welders, and technicians, but shorter delivery schedules can require up to 60 people in two shifts. “We have an experienced group in China that has been building machines for many years,” he added.
Assembly of TBM #3, the first machine to be built, was completed in early July 2010. Assembly of the machine began with core components, including the main bearing and back-up structures, and finished with installation of the electrical and hydraulics systems, as well as outer components including cutterhead pieces and ground support.
“We perform all of the same quality inspections in our shops, regardless of location,” adds Kolenich. “At the end of assembly there is a two to three week no-load testing period to ensure that each machine is set up and functioning properly per the contractual and engineering requirements.”
The two other Robbins Main Beams will be completed later in 2010, with the factory acceptance for TBM #1 scheduled in August and TBM #2 in mid-September. The three machines will be launched between December of 2010 and March 2011.
For more about the Pahang Selangor Raw Water Transfer tunnels, visit the Malaysian Government’s Ministry of Energy, Green Technology, and Water.
View a short video about the project here:
Conditions in high cover tunnels are often challenging. At the Olmos Trans-Andean tunnel, crews face 2,000 m (6,500 ft) of hard quartz porphyry, andesite, and other volcanic rocks under high stress. At two rail tunnels in China’s Gansu Province, open-type machines are boring beneath 1,400 m (4600 ft) of weak phyllite and sandstone.
Ground support, particularly in deep tunnel projects, is a highly debated topic. However, recent successes have pointed to several systems that work in a variety of situations. The Robbins Main Beam at Olmos was launched in 2007, and began to experience significant rock bursting and cathedralling under sections of high cover. “We encountered a 1,200 m (4,000 ft) section of highly fractured ground, with over 8,500 recorded rock bursts. Cavities also formed during tunneling that had to be injected with grout,” said John Simm, Robbins Field Service Superintendent.
To better contain the fractured rock, the machine’s roof shield fingers were removed and replaced with the McNally Support System, supplied by C&M McNally Engineering of Toronto, Ontario, Canada. The McNally support System provides the benefit of continuous support along the roof area of the tunnel, which protects workers from falling rock. The system works by replacing the curved finger shield plate for a curved assembly of pockets with rectangular cross-sections. Steel or wood slats are extruded from the pockets and bolted to the tunnel crown using a steel strap, effectively supporting and minimizing collapsing ground. As the machine advances, slats are reloaded into the pockets.
The below video shows the crew loading McNally slats into the pockets:
The system successfully contained numerous high-pressure rock bursts at Olmos, as seen in the below video courtesy of Dean Brox from Hatch Mott MacDonald:
For the machines at China’s West Qinling tunnels, the answer is a versatile ground support system that can be adapted depending on the ground conditions. The two 10.2 m (33.5 ft) diameter Main Beam TBMs are the first Robbins Main Beam machines built with specialized ground support and no finger shields. Instead of the roof fingers, the machines are designed with mesh pockets beneath a protective shield for workers to safely install mesh panels.
The machines can also use the McNally Support System if needed. Modified pockets can be bolted inside the mesh windows, allowing space for short slats of steel or wood.
In addition, streamlined materials handling and separate roof and probe drills allow for improved progress in high cover conditions. Redesigned ring beam installers allow for loose assembly and expansion against the tunnel walls, and can be easily converted to install steel straps rather than full rings.
Both machines are currently excavating 16.6 km (10.3 mi) long rail tunnels.
For more information on ground support in large diameter tunnels, check out this recent technical paper: Large Diameter TBMs_WTC2010
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