Download - Engineering Surveyor

Tunnelling: constructing shaftsSetting-out first ringMost shafts are circular and lined with pre-cast concrete or cast iron bolted segments. The first ring constructed isusually the third ring down.When ‘first’ ring has been set up with bolts hand-tight only: re-establish centre-point check concentricity of the segments check uniformity of level of all segments.The bolts can then be tightened, the rings above constructed and a concrete ‘collar’ placed for stability. If thesefirst rings are set and maintained true, subsequent adjustments for line and level need only be minimal.Plumbing down shaftCheck verticality of shaft as follows: establish four plumbing stations about 150 mm from shaft wall at ends of two orthogonal centre-lines suspend plumb lines from these stations (damp movement of bobs by suspending in oil or water) in deep shafts, where visibility is poor, check that plumb line is clear of shaft well by dropping snap washerdown plumb line check verticality by measuring offset at ‘first’ ring and last ring constructed.Alternatively, a suitable optical plumbing instrument or laser may be set up over each plumbing station.Establishing TBM in heading suspend weighted tape from timber frame at top of shaft set up level at surface, sight first on surface TBM, determine collimation level and read tape at A set up level in heading, read tape at B, and calculate collimation level determine reduced level of TBM on roof of heading.Note: Weight on tape should be calibration load less half the self weight of the suspended portion of the tape. Wherepossible, use full depth tape. If this is not practicable, fix suspension points at suitable intervals down shaft andtape between these. For very deep shafts, electronic/laser distance measurement equipment may beappropriate.Warning: Shot-fired pins are convenient for TBM and other levelling points in rock tunnels but check safety aspects,especially methane/gas risks.CIRIA C709111