Definition - What does Tail Void mean?
The tail void, in the shield method, is the space between the outer diameter of the shield and the outer diameter of the primary lining, often a pipe or tubular lining, usually steel or PVC. It not only prevents hole collapse but also prevents surface- or groundwater contamination of the borehole.
This annular space is filled with grout. The grout properties determine the ground loading on the borehole along its longitudinal axis and the degree of movement of the primary lining.
Trenchlesspedia explains Tail Void
The tail void, and filling the tail void, are important considerations in tunnel boring operations, as well as other horizontal directional drilling efforts, such as the installation of conduits, where the conduit forms the primary lining.
The final call belongs to the engineers. In tunneling operations, it falls to the grout properties, because the grout properties determine the load the soil places on the tunnel's primary lining. The engineers choose the grout based on geotechnical information (which may vary from tunnel segment to tunnel segment throughout the length of the tunnel) and information regarding the properties of individual grout mixes.