8. Conclusion

The wide range of tunnel-building techniques described in this article, from the most "elementary" to the most sophisticated, underlines both the need to adapt to the ever-increasing difficulty of projects such as tunnels in highly constrained urban areas or major mountain crossings, and the ingenuity of those involved in developing tools and methods to meet these technical, economic and safety challenges.

But in the end, the engineer is faced with questions that are often difficult to answer:

• in the design phase: how best to recognize the terrain to be crossed and assess its behavior during tunnel excavation, taking into account the constraints imposed by the environment (e.g. deformations of existing buildings, changes in hydrogeological regimes, etc.), and hence which construction technique will be best suited?...