Definition - What does Overcut mean?
Overcut can be defined as the difference between the diameter of excavation and diameter of the shield or pipe string. The overcut creates a gap that allows machinery to move forward freely.
Trenchless construction methods such as microtunneling, horizontal directional drilling (HDD) etc. create an overcut as the excavation proceeds. This prevents entrapment of machinery and allows easy installation of the pipeline.
Trenchlesspedia explains Overcut
In order to allow smooth steering of the tunnel boring machine (TBM) and lubrication of the pipe string, an overcut is essential. This overcut is achieved by excavating a hole slightly larger than the outside diameter (OD) of the product pipe. The overcut size depends on soil characteristics and ranges from a radius of 1/2-inch to 3/4-inch in soil to more than 1-inch in rock.
To prevent machine entrapment, factors such as overcut, shield diameter and length, torque, thrust force and rate of machine advancement have to be considered. Of these, the correct overcut is the most important factor to prevent jamming of the cutter shield. The appropriate overcut should be determined at the design stage and should be implemented correctly.