In mixed face conditions, any combination of rock and soil may be present in the tunnel cross section, but the classic scenario is the top half of the tunnel in soil and the bottom half in rock. In decades past such tunnels would be conventionally mined using timbers to stop material from moving, while breaking out the sections of rock. When the tunnel alignment cannot be raised or lowered, the process for EPB TBM tunnelling has generally been to adjust boring parameters and use additives to condition the soil. But tunnelling in mixed face conditions is inherently challenging, says Robbins projects manager for India, Jim Clark: “Everything becomes more difficult in mixed face conditions. Experienced engineers or supervisors should be on hand to prevent over-excavation. An operator can be easily fooled by earth pressure readings, because settlement at or near the surface can move down and keep face pressure constant. It is absolutely necessary to monitor how much material you’re taking out.” So can efficient excavation proceed smoothly in such variable conditions?
WHERE ROCK MEETS SOIL
A commitment to measurement of the muck volume removed is key, continues Clark. That means using belt scales to monitor the volume of muck and visually monitoring muck cars or muck removal. Muck cars should then be weighed by crane if possible, when they are taken out of a shaft. “An engineer needs to be onsite to check out muck samples and the specific gravity of the material, as well as to record amounts of foams and additives injected into the face.” The engineer can then determine how much material is being excavated.
Measuring muck is just one part of the complexity of boring in mixed face conditions however. Interventions must happen regularly in order to check how the geology is changing and to assess wear. “During every intervention a competent geologist needs to go in and map the face, and look at the percentage of rock vs soil. It is also important to assess if muck is flowing through the cutterhead, as it is common for rock fragments and pieces of boulder to get jammed in the cutterhead openings.” If advance rates are decreasing, then the cutterhead could be partially or fully blocked.
Machine design is another key part of the process. Deciding between an EPB and a crossover machine such as the XRE (cross between rock and EPB) is largely dependent on geology. At Bangalore, India’s Namma Metro, two non-Robbins EPBs were challenged by tunnels that shifted from residual soils to mixed face conditions with rock to a full face of rock.
“We had to monitor vibration in the full face of rock. On one section there were dwellings directly above the tunnels so we had to reduce cutterhead RPM to keep the vibration down, The vibration levels were well within limits but we were dealing with the human response to vibration, which is a common issue on drill and blast projects,” explains Clark, who managed the field service team operating the TBMs. Crossover machines would likely have had better advance rates, he added. “Had we been using crossover machines, significantly higher production rates could have been achieved in the rock sections due to the huge advantage of the torque-shift system.”
TBMs with the torque-shift system are equipped with multi-speed gearboxes, enabling efficient excavation in both soft ground and hard rock conditions.
EPBs themselves can be designed for mixed conditions, with abrasion-resistant plating on high-wear surfaces such as the cutterhead, screw conveyor, and screw conveyor casing. Screw conveyor flights can be made replaceable once they reach a certain wear limit. Mixed ground cutterheads allow for interchangeable cutting tools and disc cutters, while a disc cutters, while a robust steel construction can hold up to higher vibration levels in rocky conditions. However in full face rock conditions their efficiency will be lower than a crossover design.
INCREDIBLE INDIAN GEOLOGY
The complex interface of rock and soil is present throughout the world, but some of the best contemporary examples of tunnelling in these conditions come from India. At the Chennai Metro Rail Tunnels, a Robbins 6.65m diameter EPB tackled conditions that included rock in one section of the face, soft ground in another section and a long transition zone consisting of a mixed face of rock and soils. While the EPB was designed for mixed ground, the conditions were harsh on the cutting tools in particular.
“When the cutters rotated from softer ground into rock the impact with the rock caused damage, such as chipping and radial cracks to the discs. Machine downtime due to frequent hyperbaric interventions for cutter replacement was substantial,” said Clark.
The ground conditions on one stretch contained a host of obstacles: a high percentage of rock, low percentage of fines and soil and high water ingress, which meant maintaining a plug in the screw was an issue. Additives were pumped in but water inflows often resulted in spillage from the screw conveyor, which needed frequent cleaning
The key came when skilled field service personnel came to the project, assessed the geological conditions and adjusted the boring parameters of the TBM accordingly: “The main change was reducing the cutterhead RPM, and although the advance rate of the machine decreased slightly we were able to reduce cutter consumption by about 50 per cent, which resulted in an increase in production rates by 50 to 60 pe cent,” said Clark. With the adjusted parameters the number of interventions decreased significantly, and the machine was able to finish its tunnel.
The crew applied those lessons learned to subsequent tunnels on the Chennai Metro using the same machine—each successive tunnel was completed faster than the last.
The TBM’s third bore, a 1.8km-long section of tunnel completed in October 2017, saw penetration rates of 80mm per minute despite difficult mixed face conditions. WHEN VARIABILITY IS A CONSTANT
The lessons learned on Indian metro projects proved invaluable for an even more challenging project using a crossover TBM. In a remote area of Madhya Pradesh, India the 12km-long Sleemanabad Carrier Canal has presented one of the most consistently difficult projects in the world of TBM tunnelling. The project is part of the larger Bargi Diversion Project for the Narmada Valley Development Authority, a division of the Madhya Pradesh Government. The major trans-valley canal will stretch 194km (120 mi) from the existing Bargi Dam on the Narmada River to arid areas. Once complete, the Bargi Diversion Project will transfer 152 cubic meters (40,000 gallons) of water per second to Katni, Satna, Panna, and Jabalpur districts, irrigating over 100,000 hectares (250,000 acres).
A 10m diameter crossover machine was launched in 2011 following onsite first time assembly at the jobsite. The unique TBM is designed to bore in long sections of 180 MPa (26,000 psi) UCS jointed rock and marble, interspersed with clay and gravel. In sections of soft ground, the TBM can run as a standard, pressurised EPB with an abrasionresistant, shaft-type screw conveyor.
When short sections of rock or mixed ground are encountered, the machine can be run in a non-pressurised open mode. In longer sections of rock, the machine can be converted to a hard rock single shield setup by switching out the screw conveyor with a TBM belt conveyor.
The contractor crew began excavation in full EPB mode, as the first 3km of tunnel was expected to consist of mainly soft ground. After the initial section, the machine would then be operated as a non-pressurised EPB or as a hard rock single shield TBM, depending on the ground conditions.
However over the next six years, the TBM would complete only 1,600m of boring. Commercial issues for the original contractor stopped the project frequently, while ground conditions turned out to be even more difficult than predicted. A low overburden of between 10 and 14m of ground above the machine (barely over one machine diameter), combined with ground conditions of residual soils, slate, marble, dolomite, and transition zones stymied the contractor’s crew. The water table, which sat above the machine for the length of the drive and was recharged each monsoon season, sent water gushing into the tunnel. Alluvial deposits with boulders for the first 2.7km of the tunnel length acted as conduits for water to enter the tunnel and for it to accumulate in the ground. Much of the time, the machine sat virtually inactive. In September 2017, Robbins agreed to operate the TBM for the project’s senior JV partners, Patel/SEW, and by November 2017 a team of 170 people had been mobilised to take over all aspects of tunnelling and support activities, both inside the tunnel and at the surface. Patel/SEW would provide the segments for tunnel lining, but all other operations fell within the Robbins scope. “We came with the knowledge and experience of operating a TBM of this size and with direct access to major TBM spares, cutters and wear parts. Most of the local Robbins India team had recent experience in a similar task at Bangalore Metro, where we recovered two TBMs and completed the stalled Namma line. We are aware of what to expect and have the know-how to recover the situation and to deal with the challenges ahead,” said John Simm, Robbins field service project manager at Sleemanabad. Simm continued, “We inherited a site that had been unmaintained for several years. The site camp had to be renovated before personnel could start to arrive. The plant and external site equipment was old and unmaintained, requiring us to source spare parts and make improvements as we went. We are still making changes and as we advance further into the project we will bring better equipment to the site to enable us to sustain production rates.”
Production rates have certainly improved—by 1,000 per cent over the previous six years. Since taking over in November and beginning a full month of operation in December 2017, the Robbins Field Service team has excavated more than 400m, surpassing the 2km mark in less than three months.
Achieving this kind of turnaround wasn’t easy, but the field service team has a proven process. “We mobilised project management and technical personnel with in-depth experience. We fine-tuned the operating parameters of the machine, such as cutterhead thrust, as well as the foams and polymers used for ground conditioning.” Currently the crew is using a mixture of conditioning foams and synthetic polymers to condition the ground and excavate more smoothly. Logistical operations were also improved so that delays in waiting for segments, rails, grout, and other materials were minimised.
Improving the process of interventions was key to the whole tunnelling operation, as regular interventions had to be conducted to identify cutter wear and prevent catastrophic cutting tool failure. “We pumped a weak mix grout solution through the cutterhead to prevent air losses during hyperbaric interventions. This negated the need for constructing safe haven grout blocks from the surface,” says Clark. The team also employed a geologist to conduct face mapping during each intervention in order to better understand the geology and assist in determining the correct operating parameters for the machine. Ground conditions are certain to keep the project challenging throughout its drive, as they are consistently changing. “The ground conditions now are very difficult as there is a mixed face; the bottom 50 per cent is fresh hard marble, and the top 50 cent is weathered rock. We are also just below the natural water table which is giving high inflows of ground water during the boring cycle and slowing down the spoil removal process. We are finding that the geology is very changeable,” says Simm.
IN CONCLUSION
So, can a tunnel in mixed face conditions be bored efficiently and smoothly? For Clark, it all depends on rigorous monitoring, face mapping, and equipment upkeep. Above all, he says, the right personnel in such conditions is key:
“A common adage is that there is no substitute for experience and in underground construction this is especially the case.” Employing experienced personnel and specialised equipment may cost more up front, but can save significantly more money over time due to increased efficiency and advance rates.”
