soft-ground tunneling

Soft-ground tunneling involves constructing tunnels in soil or very weak rock, which presents unique challenges compared to hard rock excavation. A primary concern is managing water, as tunnels below the water table risk collapse from soil and water inflow. This can be mitigated by lowering the water table through pumping or by using compressed air within the tunnel to counteract external water pressure.1

Modern techniques often employ specialized machines called "moles" or "shields." Shields provide support and can have pockets for workers to excavate ahead, or they can be pushed forward to displace the soil. Pipe jacking is another method used for smaller tunnels, where concrete pipes are jacked into place.23

A critical aspect of soft-ground tunneling is minimizing "lost ground," which is soil that moves into the tunnel beyond its actual volume. Excessive lost ground can cause settlement damage to nearby buildings, especially those with shallow foundations. Therefore, careful construction methods are essential to keep lost ground to acceptable levels, typically around 1 percent.4