Definition - What does Annulus mean?
Annulus refers to the region or space between the borehole or tunnel wall and the surrounding ground. An annular gap is created by the shield of the tunnel boring machine (TBM) or the drill bit of a drilling machine during the boring procedure. Trenchless construction methods such as horizontal directional drilling (HDD), horizontal auger boring (HAB) and microtunneling leave annular gaps between the borehole wall and the installed pipe, which is later filled with grout.
Trenchlesspedia explains Annulus
When boring with TBMs, prefabricated segments are installed as tunnel linings or in case of HDD rigs, pipes are pulled in after the reaming process. In both cases, a gap is left between the lining or pipe and the surrounding ground. The gap is caused by an overcut left by the shield of the TBM and by design requirements of the seal (or reamer) of a drill rig, and is essential in order to reduce friction and enable installation of the pipe or lining. The annular gap is filled with grout and is an important procedure for a stable tunnel as it helps transmit forces to the surrounding ground and prevents settlement. The material of the grout should be strong enough to do this and grout gap should be filled evenly.