Understanding the Differences Between Tunnel Boring Method and Cut-and-Cover Method

When constructing subways and other underground transit systems, constructors have to chose between using the tunnel boring method or the cut-and-cover method. Both methods serve different functions depending on the materials and space constraints. Here, we delve into the technical considerations, costs, and potential impacts of each technique.

Introduction to Tunnel Boring Method and Cut-and-Cover Method

Tunnels for transit systems are dug through a variety of materials, from soft clay to hard rock. There are two common methods for constructing these tunnels: the cut-and-cover method and the tunnel boring method. The cut-and-cover method involves digging the tunnel in a shallow trench, while the tunnel boring method involves drilling into the ground in situ without disturbing the surface.

Techniques for Subway Construction

Both methods are crucial for expanding subway and light rail transit systems. The practice of using these methods has varied among different metro areas, including the one where I reside, which has expanded over the last 12 years with both techniques.

Technique 1: Cut-and-Cover Method

The cut-and-cover method, also known as 'open-cut' construction, is characterized by the following steps:

Excavation: A large trench is dug on the surface. Laying the Subway Guideway and Track: The subway tracks and other infrastructure are installed in the trench. Covering: Once the subway is in place, the trench is filled with concrete and other earth materials to create a temporary surface layer that covers the infrastructure.

This method is typically used when the tunnel needs to be constructed under roadways. It can be effective in allowing for the integration of the transit system with existing infrastructure.

Technique 2: Tunnel Boring Method

The tunnel boring method, often referred to as 'full-face' tunneling, is as follows:

Drilling: A machine digs directly into the ground to create the tunnel. Reinforcement: As the tunnel is drilled, the walls are reinforced with concrete to prevent collapse. Maintenance and Safety: While drilling, it is essential to ensure that the section behind the drillhead is reinforced to prevent cave-ins. This requires continuous monitoring and maintenance.

The tunnel boring method is particularly useful when the terrain is dense with rock or when it's necessary to build tunnels that do not require disturbing the surface. This method is less disruptive to surface operations but is more costly and requires specialized machinery.

Comparison of Techniques

Both the cut-and-cover and tunnel boring methods have their merits and drawbacks in terms of cost, environmental impact, and operational efficiency.

Cost Comparison

The cut-and-cover method is generally less expensive compared to the tunnel boring method. The main reason for this is that the cut-and-cover method does not require the specialized machinery that is necessary for tunnel boring. However, the cost of materials and labor can still be high.

The tunnel boring method involves higher initial capital costs due to the necessity of purchasing and operating tunnel boring machines. However, this method minimizes surface disturbance and can be more efficient for long tunnel stretches.

Environmental and Social Impacts

The cut-and-cover method can lead to significant disruption to the surface environment. When the trench is dug, traffic and public access can be affected, potentially leading to local business closures and lawsuits if the disruption is prolonged.

The tunnel boring method, while more expensive, is less disruptive to the surface and can minimize the impact on businesses and traffic. However, there is a risk of ground instability and potential cave-ins if appropriate safety measures are not taken.

Conclusion

Choosing between the cut-and-cover and tunnel boring methods depends on a variety of factors, including the type of material to be excavated, the depth, the presence of surface structures, and the budget. Each method has its own set of advantages and disadvantages that should be carefully considered before beginning any construction project. Whether it's the cut-and-cover method or the tunnel boring method, both have significantly advanced the capability and speed of underground construction projects, enhancing urban mobility and infrastructure development.