In the world of underground construction, the excavation of hard rock tunnels poses significant engineering challenges. Engineers and project managers are often faced with the decision of selecting the most suitable method for tunnel excavation. Two widely used methods in hard rock tunnelling are Tunnel Boring Machine (TBM) tunnelling and the Drill and Blast method. Each method has its advantages and disadvantages, and understanding their differences is crucial in making informed decisions for successful tunnelling projects.
Tunnel Boring Machine (TBM) Tunnelling:
Tunnel Boring Machines, often referred to as TBMs, are advanced mechanical devices designed for the excavation of tunnels with precision and efficiency. TBMs are typically shielded machines that bore through rock using a rotating cutter head equipped with cutting discs or tools. As the TBM advances, it removes the excavated rock from the tunnel face and lines the tunnel walls with concrete segments or pre-cast rings.
Advantages of TBM Tunnelling:
Speed and Efficiency: TBMs are renowned for their high-speed excavation capabilities, which result in faster project completion compared to other methods. The continuous excavation process reduces downtime, leading to significant time savings.
Safety: TBM tunnelling offers a safer working environment for tunnel construction workers since most of the excavation takes place within the TBM itself. This minimises the risks associated with drilling, blasting, and rock fragmentation.
Minimal Surface Disruption: TBMs are particularly suitable for urban environments where surface disruptions must be minimised. Their underground operation ensures that there is limited impact on the surface above, reducing disturbances to traffic and nearby structures.
Consistency in Tunnel Diameter: TBMs create uniform tunnel diameters, allowing for precise tunnel design and easy integration with other tunnel elements.
Drill and Blast Method:
The Drill and Blast method is a conventional tunnelling technique that involves the cyclic process of drilling holes into the rock face and subsequently blasting the rock into fragments using explosives. After blasting, the exca
vated material is removed, and the tunnel is supported using rock bolts, shotcrete, or other support systems.
Advantages of Drill and Blast Method:
Versatility: The Drill and Blast metho
d can adapt to varying geological conditions, making it suitable for tunnels in different types of hard rock formations.
Cost-Effectiveness: For shorter tunnel lengths and smaller projects, the Drill and Blast method can be more cost-effective than using large TBMs.
Lower initial investment: Unlike TBM tunnelling, which requires a higher initial investment to procure the TBM before the start of construction, the Drill and Blast method requires a lower investment at the start of the project.
Flexibility in tunnel cross-section and alignment: Unlike TBM, which bore the tunnel in a circular cross-section, the tunnel can be excavated in any shape using the Drill and Blast method. Changing the tunnel profile is also feasible when using the Drill and Blast method. When TBMs are used, sharp bends in the tunnel alignment must be avoided, but the drill and blast method does not have this restriction.
Comparison between TBM Tunnelling and Drill and Blast Method:
1. Efficiency and Speed
When it comes to speed and efficiency, TBMs have a clear advantage over the Drill and Blast method. TBMs can excavate at a constant rate,
while the Drill and Blast method's progress depends on the effectiveness of the drilling
and blasting process, which can be subject to variations in rock conditions.
2. Safety and Environmental Impact:
TBMs offer a safer working environment for tunnelling crews since most of the excavation takes place within the TBM itself. In contrast, the Drill and Blast method involves more hazardous activities related to drilling and explosives handling. Additionally, the Drill and Blast method may cause more surface disturbances and vibrations, potentially affecting nearby structures and the environment.
3. Geotechnical Conditions:
The choice between TBM tunnelling and the Drill and Blast method often depends on the geotechnical conditions of the project site. TBMs work best in stable rock formations, while the Drill and Blast method can be more adaptable to varying geological conditions.
4. Project Size and Budget:
The size of the tunnelling project and budget considerations play a crucial role in the selection of the tunnelling method. TBMs are better suited for long tunnels with a consistent cross-section, as the initial investment in a TBM may be significant. For shorter tunnels or projects with budget constraints, the Drill and Blast method can be a cost-effective solution.
In conclusion, both TBM tunnelling and the Drill and Blast method have their merits in hard rock tunnel excavation. TBMs excel in speed, safety, and minimal surface disruption, making them ideal for larger projects in stable rock formations. On the other hand, the Drill and Blast method offers versatility and cost-effectiveness, making it a viable option for smaller projects or tunnels with complex geological conditions.
Ultimately, the selection of the appropriate tunnelling method hinges on a thorough analysis of the project requirements, geological factors, budget constraints, and safety considerations. With careful planning and execution, either method can lead to the successful completion of hard rock tunnels, meeting the demands of modern infrastructure development.