Tunnels are simultaneously one of the simplest and most complex architectural construction projects, depending on whether one focuses on the result or the process.
Tunnels are simultaneously one of the simplest and most complex architectural construction projects, depending on whether one focuses on the result or the process.
The result should be a simple, straightforward structure that can be used safely for decades without substantial tunnel repair, but the process in order to get there can be incredibly complex and requires extensive surveys and expensive equipment.
Typically, when building a tunnel, there are three phases to the construction process:
- Planning and Design
- Geotechnical Investigation
- Construction
All can be undertaken in a range of different ways depending on the type of ground the tunnel is set to be built in and the purpose of the tunnel itself.
Planning And Design
The first step towards any tunnel works will be identifying the need and the benefits of such a substantial piece of infrastructure. Because of the costs involved, this often becomes a matter of government discussion.
For example, the currently in construction High Speed 2 took 12 years from its original proposition to the start of its construction, in no small part due to the tunnels that all needed to be approved in order for the route to function.
As well as this, the land that needs to be excavated needs to be selected as well as the machinery used as well as a breakdown of all of the work required.
Geotechnical Investigation
For a major tunnel project to be successful, there needs to be an investigation of the ground into which the tunnel will be bored. This involves using boreholes and advanced surveying technology to determine the constitution and properties of the ground.
The aim is to find where the groundwater is to avoid it or ensure the construction bears it in mind, how long the newly born cave will hold itself without added supports before collapsing, and the shape of the tunnel’s cross-section and how this will affect the design and stability of the project.
Construction
Once the project has been approved and every aspect of the ground has been factored in, it is time to dig the tunnel, and there are typically three main types of tunnel construction that can be adapted to fit a range of different factors.
Cut-And-Cover
The simplest and most common method of tunnel construction, cut-and-cover tunnels are where a trench is excavated, the tunnel is constructed and then once it is completed the ground is restored to the way it was.
The main issue with this method is that it necessarily causes a great deal of disruption across the entire length of the tunnel until it has been completed, which in the early days of underground construction could bring whole cities to a standstill.
Tunnel Boring Machines
An alternative to the process of digging up, building the tunnel’s structural foundations and then re-laying the ground on top is to create a machine that can automate the tunnelling process. This what was led to the creation of tunnel boring machines (TBMs).
Currently being used as part of HS2’s construction, TBMs are huge dedicated machines that create a circular tunnel cross-section as well as build parts of the tunnel support.
TBMs are highly efficient, although because of their substantial upfront cost. They often need to be specially constructed for each tunnel and can be difficult to transport owing to their sheer size, but the longer the tunnel they are used, the more cost-effective they are.
Underwater Tunnels
Building tunnels underwater can be done using TBMS, but a more common method is the immersed tube technique, where a tunnel is built in segments and then shipped to the tunnel site, sunk into place and linked together under the sea.