Soil subsidence, also known as settlement, is defined as a downward vertical movement of the ground, generally caused by changes in stresses within the soil. Subsidence is often described as a sinking or caving of the ground surface. Excessive vertical ground movement can result in the distortion of nearby structures, eventually leading to the development of cracks and, in some cases, total failure or collapse of the structure.
Subsidence can either be instantaneous, occurring almost immediately after soil disturbance, or it can take place over several years.
Subsidence is especially an issue during trenchless construction activities, such as tunneling. As the tunnel boring machine (TBM) advances along the intended bore path, it removes the surrounding soil. This activity loosens the ground while leaving soil layers directly above the bore path unsupported. As a result, the strength and stability of the soil matrix are affected, causing the earth to shift in a downward direction.
TBMs control ground subsidence by employing a construction technique known as the Sequential Excavation Method (SEM). During SEM, the tunnel is divided, excavated, and supported in several relatively small segments. As tunnel construction moves forward, the cutting head at the face of the tunnel boring machine loosens the surrounding soil. The cuttings are then removed by a system of screw and belt conveyors to the entrance pit location.
Once the cutting head has advanced the tunnel to the required distance, precast concrete liner segments are fed to the TBMs rotating ring erector system. This mechanism uses a vacuum to lift each precast section into position for installation. The liner segments are installed one by one around the circumference until the tunnel section is complete.
This completed tunnel segment possesses strength properties which allow it to support the layers of soil above the bore path.
The hydraulic jack system then pushes the cutting head forward, and the lining process is repeated. Each excavated segment is carefully monitored using specialized equipment to ensure that soil movements do not exceed allowable limits. TBMs and SEM techniques allow long and complex tunnels to be constructed without inducing subsidence caused by excessive soil disturbance.