Monolithic Cut-and-Cover Tunnels

Monolithic Cut-and-Cover Tunnels
Airformed Concrete Tunnels or Culverts

By David B. South

Cut-and-cover underground tunnels have historically been the domain of the standard rectangular concrete form of construction. Cut-and-cover describes a process whereby a slot is cut in the ground. The tunnel is then built and backfilled. These are used in many places, but we are particularly interested in underground tunnels, under a fertilizer or coal storage or under streets to carry conveyors. In general, we are talking tunnels 8' to 12' wide. They are great as large culverts.

One of the more recent developments is to Airform these tunnels. This is accomplished simply by pouring a concrete slab floor, attaching an Airform to the floor, inflating the Airform and constructing by using Monolithic Construction Technology, similar to the EcoShell II Construction Method.

There are great savings in the speed with which a tunnel can be formed using an Airform. More importantly, there are enormous savings by using a round or curved tunnel as compared to a rectangular tunnel. In general, there needs to be less than half as much rebar and concrete in curved tunnels.

These tunnels generally cost about 30 to 35 percent less than a rectangular tunnel. The Airform tunnel is a very rapid construction process, saving calendar time for the project. A typical tunnel is ten feet wide and eight feet tall and of any length. It can be up to 50ft wide and 40ft high and can traverse for hundreds of feet or allow a stream or river to pass beneath a narrow roadway.

Monolithic Cut-and-Cover tunnels can be built of many shapes.

Cut-and-cover tunnels are open to a wide range of uses. They can be sewer pipes six feet in diameter or larger, water flumes and protective tubes for cabling, such as fiber optics and electrical. Also, the cut-and-cover tunnel provides a passageway for conveyors to carry coal, grain, gravel or timber great distances.

Whether the need is to keep traffic moving, keep water flowing or simply create a passageway, Monolithic cut-and-cover tunnels offer a more affordable and longer lasting solution when compared to traditional cut and fill methods.

This is a diagram overlay of a typical 8' square tunnel as compared to a 10' wide by 8' high air-formed tunnel. It is easy to see that the round tunnel is biggest (2' wider) where it is needed. Any engineer can verify that the round shape is much stronger than the square. The round tunnel walls are 8" thick as opposed to 12" to 16" for the square. Even though it is wider the total amount of wall and roof is about 15% less. Approximately 35% less concrete and reinforcing bars are needed in the construction.

The tunnel sides are perfect for pipe chases.

The Airformed tunnels are faster to construct and save on materials.

The tunnel can easily change directions. It can start above ground and dive under a building.

The tunnel can also have other elements "molded" into it. These include drop chutes for material above to reach the tunnel conveyors.

Monolithic Cut-and-Cover Tunnels Airformed Concrete Tunnels or Culverts

Monolithic Cut-and-Cover Tunnels
Airformed Concrete Tunnels or Culverts