The Trench Method: Cut-and-Cover Tunneling Explained

What is Cut-and-Cover Tunneling?

Cut-and-Cover is a simple but disruptive method of tunnel construction used for shallow tunnels. As the name implies, the process involves two main stages: “Cutting” (excavating a large trench from the surface) and “Covering” (building a roof over the tunnel structure and restoring the surface).

Unlike deep tubes created by Tunnel Boring Machines (TBMs) that work silently underground, cut-and-cover creates a massive open canyon in the city streets. It is the standard method for constructing underground metro stations, underpasses, and shallow subway lines.

Two Distinct Techniques

Depending on the urban environment and soil stability, engineers choose between two approaches:

• Bottom-Up Method (Traditional): A trench is dug first, with the sides supported by struts. The tunnel floor, walls, and roof are built inside the trench from the bottom up. Finally, the trench is backfilled. This is simpler but keeps the surface open for a long time.
• Top-Down Method (Urban): Side walls (diaphragm walls) and the roof slab are built first at ground level. The surface is then restored immediately (allowing traffic to resume), and excavation continues underneath the roof slab. This minimizes disruption to city life.

Comparison: Cut-and-Cover vs. Bored Tunnel (TBM)

Why tear up a street when you could bore underneath it? It comes down to cost and depth.

The Urban Challenge

The biggest challenge of cut-and-cover is not engineering, but logistics. Excavating a city street requires relocating a maze of utilities—sewers, water pipes, electricity cables, and gas lines—before digging can even begin. This is often why metro station construction takes years to complete.