Sometimes, roads, train routes, and other passages can be routed around or over mountains, hills, buildings, or water.

In most circumstances, the best option isn’t to go around or over, but under or through obstructions, and when that’s the case, doing so requires building a tunnel.

Building tunnels poses some unique risks to workers due to the risk of cave-ins, but there are ways to ensure everyone’s safety.

Read on to find out about five methods of tunnel construction and how to make sure workers and, eventually, users will be able to pass through the structures safely.

1. The Open-Cut Method
   In some cases, the simplest way to build a tunnel is to start from the ground up. The process of creating an open-cut tunnel involves first digging a large open trench, then building the structure into it using a Tunnel Formwork System and appropriate materials.
   The exact methodology varies based on soil conditions and ground form. In some cases, the walls can be constructed before workers dig the trench. In others, trenching must be completed first before the sidewalls and ceiling can be created and installed.
2. The Drill and Blast Method
   The drill and blast method is usually reserved for areas where hard rock makes digging a tunnel impossible. These days, engineers typically use tunnel boring machines (TBM) instead of breaking out the dynamite, but drilling and blasting is still routine practice in some parts of the world. It involves the controlled use of either explosives or pressure blasting to break up large rocks for easier excavation.
   The most common applications for the drill and blast method are mining and quarrying. However, some civil engineers still employ it to build tunnels, especially in remote areas. The best way to ensure worker safety is to make sure everyone is aware of the blasting schedule and to check the blast zone before each application.
3. Using Road Headers
   Road headers are excavating machines that feature boom-mounted cutting heads. They’re designed to travel along tracks, cutting through hard rock as they go. Also referred to as header machines, these complex pieces of equipment offer a greater level of precision than the drill and blast method and are often more cost-effective. Since they are continuous-use machines, road headers can also cut back on
   project completion time.
4. The Cut and Cover Method
   The cut and cover method is a simple concept. It’s also the oldest known method for tunnel creation. Workers start by digging a trench, then construct the tunnel inside the trench before returning the surface
   material to its original state.

The problem with cut-and-cover is that it is a very disruptive process. In cities, it can interfere with utilities, foot and vehicle traffic, and many other urban activities. In rural areas, it disrupts ecosystems. With proper planning and sufficient formwork to ensure stability, engineers can offset the effects of these potential drawbacks and help to keep everyone safe.

5. Immersed-Tube Tunneling
   Immersed-tube tunnels consist of multiple tunnel segments joined together at the ends to form a continuous route. These types of tunnels are only appropriate for underwater applications. The ends of the tunnel that extend beyond the water’s edges are usually constructed using the cut and cover method.
   The Bottom Line
   There are many ways to build a tunnel. The most important things for engineers to consider are worker safety and structural integrity. Having the right formwork in place and providing everyone on the job site with adequate training and safety gear is crucial no matter what tunnel construction method engineers
   choose.