Soft Ground Tunnel Boring Machines

Soft ground Tunnel Boring Machines (TBMs) have evolved over the past thirty years to a point where it is possible to construct tunnels in virtually any type of soft ground.

For the purposes of this paper Soft Ground is defined as ground which is not self-supporting and which requires immediate support to prevent deformation and collapse. Thus soft ground ranges from stiff clays and soft waterbearing chalks through to silts, soft clays and waterbearing sands and gravels.

Prior to the advent of the modern tunnelling machine, tunnels were excavated and lined using bolted cast-iron and concrete segments built behind a tunnel shield. These shields can be split into two main categories.