Heading is the digging of a smaller tunnel ahead of the main tunnel.
Heading during tunneling is carried out by excavation, support of excavated cavity and removal of excavated earth. Heading proceeds in small sections ranging between 0.5 to 1-m in soft ground. Heading length is important as the excavated portion needs to remain stable for up to 90-minutes until support is installed. Surface settlement is considerably reduced when the heading length is less.
Trenchlesspedia Explains Heading
There are different types of heading methods used for tunneling. Core heading consists of first excavating the sides and tops parts and supporting it before the central part or core is excavated.
Top heading consists of excavating the crown and temporarily supporting it with shotcrete before the bench is excavated.
Sidewall drift is a method of heading in which the side galleries are first excavated and supported after which the crown is excavated. Tunnel boring machines (TBM) also carry out excavation in incremental heading fashion because regular stops are needed for the maintenance of tools and supports such as bolting and wire meshing, especially in weak rocks.