How are tunnels built?
The first Alpine tunnel, the Buco di Viso or Tunnel de la Traversette, was built in 1480 to connect Italy and France and was 75 metres long. Almost four centuries later, on 10 January 1863, the world's first underground railway line opened in London. Since then, excavation techniques have continuously evolved thanks to increasingly in-depth geological studies and innovations in technology for the machines used, as well as the increase in professional expertise.
In this collection, let’s take a look at how major tunnels are built using traditional excavation methods and with Tunnel Boring Machines - TBMs, the giant mechanical ‘moles’ responsible for the construction of some of the world’s most advanced and longest tunnels, fully mechanising the excavation of tunnels and their lining.
What are Tunnels for?
Tunnels are structures excavated within mountains or underground environments with the purpose of connecting two locations. They might accommodate a road, a railway, a metro, but they are also executed for water projects, hydroelectric power plants, desalination plants or sewage treatment plants.
What are the main elements of a Tunnel?
Traditional methods or mechanized excavation?
The decision whether and how to build a tunnel depends mainly on the geological studies and the geotechnical characterization of the soils to be excavated. Whenever it’s possible, the preferred choice would be generally the use of mechanized excavation by Tunnel Boring Machine (TBM), which is safer, faster and more efficient, but when the ground conditions are unstable and do not allow the use of TBMs, or when the tunnels are too short to repay economically a mechanized approach, then traditional methods are preferable.
Traditional Tunnel Excavation (Sequential Excavation Method)
There are different methodologies for a Sequential Excavation Method (SEM), with different equipment and machinery depending on soils to be excavated and their short, medium and long-term stability.
When a SEM approach is adopted, the focus will be placed on how to reduce the various activities to a well controllable and repeatable industrial process, so that to achieve the higher safety standards and to control and optimize times and costs.
One of the most common SEMs for hard soils, called Drill & Blast, foresees the rock demolition by blasting with explosives. The length of a blast depends on the properties of the rock: the more compact the material is and the better its geomechanical properties, the greater the extension of the blast, which can span from 1 to 5.5 metres.
Drill & Blast (D&B) is a cyclical process subdivided into phases, involving a range of professionals and specialists that must be perfectly coordinated to achieve a constant progress rate.
Drill & Blast phases
In the first stage, the tunnel face is drilled, according to pre-arranged patterns and drilling sequences, to create the holes where the explosive charges shall be installed. Nowadays, this activity is performed by drilling rigs, installed on robotic booms mounted in a drilling equipment called a Drilling Jumbo which, in modern versions, can operate even three booms simultaneously.
The technology has achieved a remarkable level of innovation over the last few years: today, the entire drilling process is fully automated and georeferenced using a precise “blasting plan”, tailored to the type of rock and specifically designed for the type of blasting required.
Once the drilling pattern is complete, the explosive charges are installed inside the holes along with their detonators. As of today, NONEL shock wave detonators (or “non-electric shock tube detonators”) are largely utilized to improve the safety of the process by reducing the risk of unintentional ignition.
After completing the installation of explosive charges, detonators and ignition circuits, the blaster, a specialized and licensed operator, performs the blast: the explosive charges are not detonated simultaneously, but according to a specific sequence thanks to micro-delays, resulting in a ‘layered’ effect that begins in the centre of the tunnel face and progressively works outwards to the outer perimeter of the tunnel face. During the blasting, the miners leave the tunnel or recover themselves inside a special safe place, called a refuge chamber.
After blasting, dust and fumes caused by the explosion are removed by introducing fresh air through a ventilation circuit (fans and ventilation pipes). When the tunnel atmosphere is cleared, a safety-check is made to ensure that no unexploded charges are present.
The excavated material (spoil) is then removed using a loading shovel and enough trucks (dumpers) to ensure a constant disposal rate. After securing the tunnel face by removing unstable blocks of rock by the means of an excavator equipped with a hydraulic breaker (the jackhammer), the process moves on to the delicate profiling phase, which means shaping the outer edges of the tunnel, starting from the side walls and moving up to the vault.
After spoil removal and tunnel profiling, an initial shotcrete layer is sprayed through a compressed air lance to create a lining coat between 5 and 15 cm thick on the side walls and the tunnel face, reducing the risk of unstable material breaking away during the subsequent phases of work when workers will be present. This is another area where advancements in technology have played a relevant part, moving from manual lances held by operators (lancers) to the robotic systems commonly used in tunnels excavated by the Webuild Group.
At this point, a first-stage lining (“pre-lining”) is installed using either steel-ribs(arch-shaped metal profiles which are fitted close to the tunnel side walls) or by rock-bolting. When the installation of the pre-lining is completed, a new excavation cycle can begin.
The permanent lining of a tunnel is a concrete lining, sometimes steel-reinforced, which is poured using special formworks and always at pre-established distances from the tunnel face, according to the rock stability conditions. The tunnel lining construction usually foresees, in sequence, the casting of the tunnel invert (the arch-shaped structural element which ‘closes’ the lower part of the tunnel structural reinforcement ring), followed by the side walls and lastly the ‘vault’, which is the upper part, the ceiling, of the tunnel lining. Prior to casting the final lining, a waterproofing membrane is normally installed, which is designed to protect the concrete by increasing its durability and preventing water from seeping into the tunnel.
Sequential Excavation Method: most used equipment
A drilling rig equipped with robotic arms and pneumatic drills
A machine used to remove spoil from the excavation face and load it onto trucks that will transport it outside the tunnel.
A truck with a tipper body for transporting excavated materials.
This is the device used to spray the shotcrete onto the tunnel face and its side walls.
Excavators, normally equipped with an hydraulic breaker (the so-called 'jackhammer') or 'rippers', most suitable for soft-soils.
Sequential excavation method: roles involved
LOADING SHOVEL DRIVER
A specialist operator who drives the loader.
JUMBO DRIVER
A specialist operator who drives the Jumbo.
MINER
A worker involved in the excavation operations, ranging from the preparation and execution of the 'blasts' to the installation of the pre-lining on the excavation face.
LANCER
An operator who uses a lance to spray the shotcrete.
BLASTER
A specialist operator with the appropriate licence who oversees the preparation of the blast and carries out the blasting.
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