In territories with predominantly developed mountainous Road and railway infrastructures are often built through the construction of engineering works capable of “to pass through” the mountains and allow the continuity of the road flow. We are talking about the tunnels, or tunnel, that is, real engineering works capable of supporting the mountains above them and preventing them from collapsing inside them, thus allowing the use of the road or railway passage and usually guaranteeing a time saving (as well as minimizing environmental pollution) in reaching a specific destination. But how do they not collapse? The answer is linked to both the so-called bow effectwhich involves the adoption of some engineering solutions, including ribs, coverings And ground consolidation.
Stability of the excavation face
We have to think of a tunnel as a hole (more or less large) inside an object with weight – in our case the mass of soil. In the absence of the void, the weight of the soil is transmitted to the deep layers (therefore downwards) continuously, counting on the presence of the material present, which also has the capacity to support this column of material weighing on it.
When you go to make a hole, the weight of the part of the ground above the hole no longer finds support lower and must therefore find an alternative way to go downwards, that is, to be transferred into the deeper layers of soil. If this does not happen, we may witness the collapse of the part of the ground without a support. In addition to the weight of the ground above the tunnel, there may be other forces that affect the safety conditions. In particular, there may be loads that represent the weight of buildings built on the surface in a period prior to the construction of the hole. Other forces may come into play, but in essence the concept presented does not change: we could consider all these loads as if they were free-surface ground, but with a hole deeper than the previous one.
These actions are counteracted by the mechanical resistance of the ground. There are equations that, depending on the type of ground in which the hole is made, provide what geotechnical engineers call stability factor: when this factor exceeds a certain value, the collapse condition is certain and the tunnel requires some intermediate support already during the excavation phase to ensure safety. Therefore, there may be conditions in which the hole is stable only thanks to its mechanical characteristics. In part, this is also due to the curved shape given to the hole, which allows the exploitation of the arch effect to transmit actions from the superficial to the deep layers.
Support techniques
As anticipated, it is not always possible to find terrains that can support themselves. Therefore, it becomes necessary to have alternative methods that allow to overcome the resistance problems to build the tunnel safely. The design of the support system is done by estimating the force that must be applied from inside the hole to ensure safety. There are several construction methods and techniques that ensure the support of the excavation face. The main ones are illustrated below.
Use of the ribs
The structure of the hole can be kept stable thanks to the use of ribs, which represent real prefabricated support structures. Usually, they are made of steel and follow the geometric development of the hole. They are positioned at regular intervals and create a real support skeleton around the hole.
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Coatings
The coverings can be of various types. We can deal with real prefabricated concrete segmentspositioned ad hoc along the cap as the excavation progresses. In other cases, a lining can be artificially created on site. This occurs in two phases: first, a steel mesh or steel fibres are positioned, then concrete is sprayed over the entire cap. This technique is known as Shotcrete. If the cladding alone does not meet the stability requirement, then the ribs can be used to improve its performance.
Soil consolidation
In some cases, it may be useful “to dope” the land around the hole to ensure the same high mechanical performance: by artificially increasing the mechanical characteristics of the soil, the cavity is induced to be stable to allow safe working. This occurs through cement injection spread over the entire shell or, in other cases, even using some techniques for freezing water present in the soil. Yes, because when the equilibrium of the ground is disturbed, the water present wants to move away from the initial position. This generates a rearrangement of the matrix of solid elements, possibly with a configuration no longer capable of withstanding the applied forces. However, if the water is frozen, it cannot move and in the short term it maintains its position regardless of the effects produced by the creation of the hole.
Final coatings
The coatings that we are used to seeing when we go through a tunnel are called second phase, or definitive. They can reach thicknesses of up to one metre if the depths of the tunnel become significant, or if the mechanical conditions of the complex are particularly poor.
