18/02/2025
As one delves deeper beneath the Earth's surface, the pressure increases due to the cumulative weight of these layers.
This geostatic pressure can significantly compress the subsurface materials, influencing the stability and structural integrity of the tunnel.
It is essential to account for this pressure gradient in tunnel design to ensure adequate support and prevent collapse.
With this in mind, when a horizontal passageway is built through this material, you interrupt the flow of these compressive forces.
Think of it this way - if you remove a column from a building, you weaken the structure.
When excavating a passageway, you're removing support from the material above.
Therefore, when building a tunnel, it's essential to replace this support.
Common ways to achieve this include lining the tunnel with steel or concrete. This provides support to the tunnel walls.
These passageways use strong materials like concrete, steel, or iron.
They must carry the weight of people and vehicles traveling through them.
Engineers will also add extra support to more extensive tunnels for better stability.
Metal rock bolts in conjunction with resin are used to secure tunnels, and there's drilling done underneath rocks to prevent collapses.
This process is very costly but integral to ground support and overall safety.
⛏️ Do Tunnels Ever Collapse?
The drain pipe collapsed.
Philld at en.wikipedia, CC BY 2.0, via Wikimedia Commons
Like any structure built by man, there's a risk of collapse.
However, many measures are put in place to prevent tunnels from collapsing from the initial support to the final liner support. This makes collapsing highly unlikely.
Causes of collapse may include floods, earthquakes, or corroding infrastructure.
For instance, water leaking into the vertical shaft will cause the excavation to become unstable. This increases the risk of collapse.
⛏️ Ancient Tunneling
Ancient tunnels in Africa
Tunnels have been used for years to mine ores, carry coal, conduct water and sewage, among other uses.
Underground chambers were often used to connect passageways. They continue to be used for power plants, pumping stations, storage of oil, and more.
Ancient tunnels were likely constructed by prehistoric people that wanted to make their caves bigger.
All major ancient civilizations had their own methods of building beneath the surface.
For instance, in Babylonia, they were used for irrigation. Around 2180 BC, pedestrian tunnels were built under the Euphrates River to connect the royal palace with the temple.
Brick tunnel lining was used to create this 3,000 feet long tunnel. This type of tunneling was achieved by diverting the river during the dry season.
Egyptian techniques included cutting soft rocks with copper saws or hollow reed drills surrounded by an abrasive.
This technique was first used for quarrying stone blocks and later to excavate temple rooms within rock cliffs.
Eventually, very elaborate temples were being excavated within solid rock in India and Ethiopia.
⛏️ Modern Tunnel Construction
Underground TBM
TBM Underground being prepped
It's interesting to learn how construction for tunnels has evolved from ancient times to the Middle Ages and into the present day.
From soft soil to hard rock, tunnels are dug in different materials.
Tunneling methods depend on several factors such as surface conditions, length of the tunnel drive, tunnel depth, and more.
Three main methods of building tunnels include cut-and-cover, bored, and immersed tunnels.
Cut and Cover
Cut and Cover Tunnel
Cut-and-cover tunnels are excavated in a shallow trench.
The overhead support needs to be strong enough to hold the load of what will be built above the tunnel, for example, buildings.
Cut-and-cover tunneling includes two methods:
Bottom-up method. For this technique, a trench is excavated, and the tunnel is constructed within it. It can be held in place by varying materials like concrete, precast arches, or corrugated steel arches. In ancient times, brickwork was used.
Top-down method. For this method, the side support walls and the capping beams are built from the ground level. The tunnel roof is then constructed using precast beams or concrete. The surface is then placed back on except for access openings.
Bored
TBM overcomes obstacles
TBM breaking through a subway station under construction
Bored tunnels are made in situ (in position) without removing the ground surface.
This is possible through trenchless technologies like the New Austrian Tunnel Method (NATM), pipe jacking or microtunnelling.
A tunnel boring machine or TBM is used to automate the tunneling process.
Thanks to underground space technology like TBMs, tunneling costs can be reduced significantly.
The downfall of this construction method is the large size of tunnel boring machines.
It's often difficult to transport a TBM to the site of construction. It's also very expensive to assemble the TBM on-site.
There are many TBM designs for operation in a variety of conditions (hard rock, soft ground, etc.).
Some types even have pressurized compartments at the front that allow them to be used in water conditions below the water table. This helps pressurize the ground and balance the water pressure.
TBMs are the preferred method nowadays over older tunneling methods. The old method of tunneling in compressed air requires operators to work you
