Why choose a tunnel? Why not a bridge?
Societies invest in infrastructure for the common good. Infrastructure should not only support the economy in the region, but also protect social groups, the environment and the climate. Simultaneously!
For major infrastructure projects, there is often a balance to strike between the planned construction cost and the identified benefits (or drawbacks). Other solutions constructed at ground level, such as a bridge, could often serve the same function and are typically less costly. However, tunnels are often selected for the long-term benefit of freeing up space at the surface.
When planning new infrastructure, solutions where the most benefits are identified at the lowest possible cost, MUST be considered. However, the solution with the lowest cost is not always the solution with the most benefits. Therefore, logical and systematic decision making is required to identify a sustainable solution with an acceptable balance of pros and cons. The detailed solutions will differ depending on several factors - the location, for instance.
Subsea tunnels are constructed as fixed links across rivers, inlets, straits, fjords and harbours, where tunnels are sometimes an excellent alternative to a bridge. For example, in a harbour where a tunnel could have less impact on shipping traffic. Or where a tall bridge could require long approach spans or influence the flight paths to and from a nearby airport. In general, where there is an opportunity to protect the existing natural landscape, or maintain the identity of a place, a tunnel may be an excellent alternative to a bridge.
Why does location matter?
Tunnels are constructed to pass natural barriers like mountains or water or manmade obstacles in rural areas; sometimes, the ground condition itself drives the shape of the infrastructure – for instance, in excavated tunnels where favourable ground conditions will often define tunnel alignment. The most important factors to bear in mind when considering the benefit and construction of a tunnel are the people getting the benefit from the tunnel and the natural settings - geology, environment, climate, vegetation, access to resources, energy, and water.
Societies that would benefit from a tunnel would still depend on the natural settings they live in and settings they have nearby. One should ask, how good is access to natural resources? What are the local needs for energy? What are the existing physical connections and barriers between places, and how will this new infrastructure impact the people living there? Are these connections and barriers governed by mountains, extensive forests, urban settings, water, or deserts?
The natural setting shaped how people lived in the past - how they got their resources, connected with their neighbours, who they traded with, and how technology was introduced in new locations. It is seen in the historical development of a given society. Tunnel construction is highly influenced by the local natural settings. But we also know that there are many elements to tunnel design that can be used universally around the world when the local conditions are understood. This is the case, especially for unique and record-breaking tunnelling projects.