The TBM Method of Tunneling: An Overview and Case Studies from Encardio Rite’s Global Projects

Tunnel Boring Machines (TBM) offer efficient, reliable, and safe methods for boring through earth and rock. From subway systems in urban landscapes to water conduits beneath mountain ranges, TBMs are instrumental in modern infrastructure development.  
What is Tunnel Boring Machine (TBM) Technology? 


A Tunnel Boring Machine (TBM) is a complex system used to excavate tunnels with a circular cross-section through various types of geology. TBMs can bore through anything from hard rock to sand. They are preferred over traditional drilling and blasting methods for their ability to reduce the overall cost of tunneling, especially in urban areas, and their minimal disturbance to surrounding structures and the environment. 

A typical TBM consists of a rotating cutting wheel, followed by a series of trailers that house support equipment for systems such as material removal, hydraulic supports, and power supplies. The front section of the TBM is the rotating cutter head. As the cutter head rotates, it carves out the tunnel shape. Conveyor belts then remove the excavated materials. Behind the cutter head, hydraulic jacks press against the tunnel walls to propel the TBM forward. 


Read more: Tunneling Methods and Encardio Rite’s Contributions to Global Projects



Types of Tunnel Boring Machines 

TBMs are broadly classified based on the geology they are designed to tackle. Let's explore the various types of TBMs and their specific applications: 

1. Earth Pressure Balance Machines (EPB) 

  • Application: Best suited for soft ground such as clay, silt, and gravel. 
  • Mechanism: EPBs use a screw conveyor to control the pressure of the face of the tunnel, stabilizing the tunnel excavation face with excavated material as a support medium. These machines are equipped with a closed chamber that balances earth pressure to prevent settlement and are particularly effective in urban tunneling where ground stability is crucial. 

2. Slurry Shield TBMs 

  • Application: Ideal for areas with high groundwater content or unstable soils such as sands and gravels. 
  • Mechanism: These TBMs use a slurry separation plant to maintain face stability. The excavated material is mixed with slurry, which is then pumped to a separation plant where the slurry is treated and recycled. 

The use of slurry to support the tunnel face allows for efficient excavation in fluid soils and minimizes the risk of face collapse. 


3. Hard Rock TBMs 

  • Application: Designed for excavating through hard rock formations. 
  • Mechanism: These TBMs employ disc cutters to exert high pressure on the rock face, causing it to fracture and chip away. The broken rock is then transported from the face through a robust conveyor belt system. 

Hard Rock TBMs are characterized by their rugged construction and high-power output, which are essential for dealing with the high compressive strengths of rock. These machines can also be equipped with shield extensions to protect against rock bursts and falling debris. 


4. Mixed Shield TBMs 

  • Application: Used in geologies that transition between soft ground and hard rock. 
  • Mechanism: Mixed Shield TBMs combine the features of both EPB and Hard Rock TBMs, allowing them to adapt to changing ground conditions effectively. 

These TBMs can switch between different modes depending on the soil characteristics encountered, ensuring continuous progress and adapting to mixed geology without significant downtime for conversion. 

Read more: Cut-and-Cover Tunneling: Methodology and Case Studies from Encardio Rite’s Global Projects


Tunnel Boring Machines (TBMs) are highly regarded for their precision, which surpasses traditional tunneling methods, providing smoother tunnel walls. Their ability to operate continuously enhances the speed of the tunneling process, significantly reducing construction time. Additionally, TBMs improve safety by minimizing the risks associated with manual labor and reducing workers' exposure to hazardous conditions. They also have a lesser environmental impact, as they generate lower vibrations and cause minimal disturbances to nearby structures and ecosystems.  


However, the use of TBMs is not without challenges. The initial cost of TBMs is substantial, requiring a significant financial investment upfront. They are also subject to geological uncertainties; unexpected conditions underground can lead to delays and operational complications. Due to their complexity, TBMs demand skilled personnel for effective operation and ongoing maintenance, adding another layer of operational challenges. 


Encardio Rite’s Role in TBM Tunneling Projects 


Encardio Rite has been a part of significant infrastructure projects worldwide. With our data, technology and expertise we have ensured the success of many TBM tunneling projects.  
1. Kolkata Metro Project: The Kolkata Metro project, covering UG-1 and UG-2 packages, presented a challenging underground construction environment due to the dense urban setting of Kolkata. The project scope required monitoring to avoid impacts on existing buildings and infrastructure. Encardio Rite was tasked with monitoring responsibilities that included the installation of inclinometers and extensometers for assessing lateral and vertical movements within the soil and structures. Also, vibrating wire and Casagrande piezometers were deployed to monitor groundwater levels and pore pressures, crucial for managing the risk of subsidence and ensuring the stability of tunnel excavations.  
The integration of strain gauges and load cells provided data on structural loads, essential for real-time adjustments during construction. The systematic data collection enabled by manual and automatic monitoring, alongside robust programming and commissioning of automated systems, provided a continuous flow of critical data. This setup facilitated geotechnical risks management and adhered to safety standards by enabling immediate responses to potential structural and soil instabilities. 


2. Oslo Metro Fornebu Extension: For the Oslo Metro Fornebu extension, Encardio Rite's role was to provide precise hydrogeological assessments through the installation of around 70 piezometers. These instruments played a key role in the accurate monitoring of the hydrostatic conditions around the metro construction sites. These piezometers, complemented by portable smart readout units, were integral for the real-time monitoring and management of groundwater levels. This was to make sure that the tunneling work did not adversely affect the metro’s structural integrity or result in undesirable settlement of the surrounding ground.  



3. Doha Metro Gold Line Project: The Doha Metro Gold Line project required us to take a distinct approach to monitoring strategy due to its scale and the complexity of operating in an urban environment. Encardio Rite provided a full spectrum of monitoring solutions that included surface settlement points, inclinometers, piezometers, and strain gauges strategically placed to monitor every critical aspect of the construction process. The installation of multi-point borehole extensometers and automatic water level recorders was done for understanding the subsurface conditions and managing the impacts of tunneling on urban infrastructure. 
Also, our engineers deployed crack meters and load cells to help in monitoring the structural health of the underground stations and ensuring the integrity of diaphragm walls and other support structures. Data loggers and an online data management system facilitated the real-time monitoring and analysis of collected data, enhancing the project's ability to preemptively address potential issues, thus maintaining safety and construction efficiency. 


Read more: Micro-tunneling A Comprehensive Guide and Case Studies from Encardio Rite’s Global Experience


TBM in tunneling is a much-needed alternative to underground construction, offering enhanced safety, efficiency, and minimal environmental impact. The advanced capabilities of TBMs, ranging from precision in excavation to the safeguarding of surrounding environments, showcase a significant leap from traditional tunneling methods.  
Encardio Rite, through adoption of these new technologies, continuous monitoring and on-site support to the most challenging TBM tunneling projects worldwide, will continue to shape the subterranean landscapes of tomorrow's cities. 





Q1. What is a Tunnel Boring Machine (TBM)? 

Ans: A Tunnel Boring Machine (TBM) is a sophisticated mechanical system designed to bore circular tunnels through varied geologies, from soft soil to hard rock. It offers an efficient, precise, and safer alternative to traditional drilling and blasting methods.

Q2. How does a TBM differ from traditional tunneling methods? 

Ans: Unlike conventional methods, TBMs provide a continuous tunneling process, which enhances speed and reduces construction time. They are preferred for their precision in excavation, safety features, and minimal disturbance to surrounding environments.

Q3. What are the different types of TBMs? 

Ans: TBMs are categorized based on the geology they tackle: Earth Pressure Balance Machines for soft grounds, Slurry Shield TBMs for fluid soils, Hard Rock TBMs for hard rock formations, and Mixed Shield TBMs for geologies that transition between soft ground and hard rock.

Q4. What are the challenges associated with using TBMs? 

Ans: The initial investment for TBMs is significant, and they require skilled operation and maintenance. Geological uncertainties can also pose operational complications and delays.

Q5. How has Encardio Rite contributed to global TBM tunneling projects? 

Ans: Encardio Rite has played a crucial role in various international projects, providing advanced monitoring and real-time data analysis to manage geotechnical risks and ensure construction safety and efficiency.

Q6. Can TBMs be used for all types of tunneling projects? 

Ans: While TBMs are versatile and can handle a variety of soil conditions, their suitability depends on specific project requirements, including geology, tunnel length, and the surrounding environmental and urban settings.

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