Much has been made worldwide of the difference in performance between new and rebuilt TBMs. Worldwide, a bias exists that seems to favor new machines, but is the bias warranted? The reuse of machines can, if done to exacting standards, reduce costs and time to delivery while also reducing the carbon footprint. But guaranteeing the quality of TBM rebuilds is another issue—one that seems only minimally improved by the existence of international guidelines. This paper discusses the process of machine rebuilds and the use of rebuilt TBMs with performance examples from projects worldwide. It seeks to establish guidelines and recommendations based on real experiences of success in the shop and in the field.
Years of hard work and planning have paid off at the Bheri Babai Diversion Multipurpose Project. This 12 km tunnel is not only breaking through a historically difficult mountain range, but it has also managed to break down the notion, to the people of Nepal, that drill and blast is this only way to excavate the extreme conditions in the Himalayas. This paper highlights the first TBM in Nepal and how it is managing to bore at an exceptional advance rate of over 700 m per month, with a high of 1202 m in one month. It examines which design features of the Double Shield TBM are contributing to the great excavation rates, and how the crew’s operational methods have maximized these results.
EPB TBM tunneling in mixed face conditions—partially in both rock and soil—is inherently problematic for even the most experienced crews. Over-excavation, excessive damage to cutter tools and regular cutterhead interventions are major challenges when negotiating mixed face geology. This paper draws from real field experiences, including successful bores in abrasive rock and soil at India’s Chennai and Bangalore metro projects, to determine the optimal operational parameters for TBMs in such conditions. It also addresses reduction of air losses to facilitate cutterhead interventions under hyperbaric conditions when installation of safe-haven grout blocks is not an option due to surface structures.
In May 2018, a 7.93 m diameter open gripper (Main Beam) TBM completed the 24.3 km long Jilin Lot 3 tunnel under a maximum overburden of 272.9 m. The tunneling operation for the water transfer project, located in northeastern China, achieved a national record of 1,423.5 m in one month despite challenging conditions. This paper presents an improved, innovative shotcrete system for TBM preliminary lining, developed through experience on previous projects. The shotcrete system, along with other structural design elements and a properly developed ground support program, allowed the TBM to bore successfully in variable hard rock and fault zones. The paper discusses how the shotcrete system and structural design increased safety and improved performance in a cost-effective manner. It defines the variables that allowed the TBM to advance at rapid rates and makes recommendations for future types of projects that could benefit from the shotcrete system.
Vietnam’s Thuong Kon Tum Hydroelectric project is a 17.4 km headrace tunnel that will be the country’s longest once complete. A section of the tunnel was excavated by a 4.5 m diameter Main Beam TBM in granitic rock up to 250 MPa UCS. Started in 2012, the project’s original contractor left due to non-satisfactory performance. In 2016, the contract to refurbish the TBM and excavate the remaining 10.45 km of tunnel was awarded to a joint venture of Robbins and a local contractor. Robbins was fully responsible for the TBM operation, including supplying operational crews. The crew overcame massive granitic rock, fault zones gushing water at 600 l/s, and difficult conditions. In under two years, the TBM advanced from a standstill at 15 percent project completion to 85 percent complete. This paper addresses the refurbishment of the TBM in the tunnel, the work to streamline operation, and challenges faced.
Modern TBMs deliver high performance with availability rates that are beyond 90%. The TBM design concepts make the machines highly versatile for employment in varying soil and ground conditions. Machines can now withstand extreme loads and impacts in rough underground environments because of the components made for longtime use. Regular maintenance and planned service is the vital element in prolonging a machine’s life and for high performance and availability. A well-serviced machine provides excellent performance as well as active project safety. Proper operation in variable conditions is also key. For instance, a hard rock TBM may run into zones of swelling rock. The most appropriate method to overcoming the swelling rock is to keep going, avoiding any unnecessary stops. Worn disc cutters that have not been maintained in due course are a prominent example of such avoidable stops, which may result in long downtimes and severe damage to the machine. However with modern and advanced techniques to underground tunneling, rescuing and rebuilding TBMs is possible to save the project. This paper discusses methods and tools for modern TBM service and maintenance using present case studies about TBM rebuilds in extreme project conditions.
Managing water inflows is not new to TBM tunneling, but today there are an increasing number of methods and best practices to handle potentially high water inflows efficiently and safely. High volumes of water can be safely contained or managed in hard rock TBM tunneling, but this requires the proper foreknowledge and planning. This paper outlines how machines can be designed ahead of time for expected high water, and how risk can be mitigated during tunneling. It also covers the importance of pre-planning and includes a look into the future of water control methods. Case studies of hard rock tunneling with heavy water inflows are examined, with a focus on New York, USA’s Delaware Aqueduct Repair, the 3.8 km long bypass tunnel below the Hudson River requires excavation through limestone rock at water pressures of up to 20 bar. A unique 6.5 m diameter Single Shield TBM, sealable for high pressure excavation, is boring and lining the tunnel.
India’s Sleemanabad Carrier Canal is a prime example of just how challenging mixed face conditions can be, although other examples exist. The water transfer tunnel is being bored using a 10 m diameter hybrid-type rock/EPB TBM. However, in 6.5 years of tunneling the machine had only advanced 1,600 m. Commercial issues for the original contractor stalled the project frequently, while ground conditions turned out to be even more difficult than predicted. Low overburden of between 10 and 14 m, combined with mixed face conditions, transition zones and a high water table restricted advance rates. The TBM manufacturer mobilized a team to refurbish the TBM and within a period of 6 weeks a team of 180 people had been deployed to take over all aspects of tunneling and support activities. Production rates improved dramatically as the TBM advanced more than 400 m in four months. This paper discusses the problems faced and the methodology that enabled good advance rates in highly variable mixed face conditions.
- Rebuilding TBMs: Are Used TBMs as Good as New?
- Breaking Through Tough Ground in the Himalayas: Nepal's First TBM
- Tunneling in Mixed Face Conditions: An Enduring Challenge for EPB TBM Excavation
- 7.93 m Open TBM Shotcrete System Improvement and Innovation - Jilin Project, China
- Overcoming Extreme Tunneling Conditions in Vietnam's Longest Tunnel