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Tunnelling: from shaftsTransferring line down shallow shaftWith A and B defining the required heading direction: set up subsidiary stations C and D close to shaft from C and D, set out F and E respectively.Note: Check accuracy by repeating observations.Plumbing line down shaftTwo plumb lines are suspended down the shaft, as far apart as practicable. The two lines may define: given bearing or direction, or random bearing which is measured at the surface.‘Picking up’ bearingWhether at the surface or underground, the bearing defined by the plumb lines has to be ‘picked up’ by using a1-second theodolite/Total Station and the Weisbach triangle (see below). Underground, illuminate each plumb line(piano wire) with a lamp. A scale fixed behind the line will help in eliminating any residual movement by observing themean position.Use of the Weisbach triangle: set up instrument at C, about 3 to 4 m from B and offset by say 25–50 mm from the sine defined by AB measure the angle subtended by A and B, ϕ (average of a number of readings on both faces) measure the lengths AB and BC calculate θ = φBCABwhere θ and φ are in secondsθ (AB + BC)calculate offset CD =57.2958 × 3600 align on A and traverse through 180°− θ (clockwise as illustrated) to sight along line by calculated offset mark centre-line by taping from offset line (appropriate for small tunnels or tunnels with bends) or mark further line offset from centre-line by convenient amount up to, say, 1 m (appropriate for large tunnelwhere centre-line may be obscured by ventilating duct).Gyro-theodoliteA gyro-theodolite can provide an accurate azimuth where other methods would be impossible. It should be noted thatgyro-instruments track True North and not grid or magnetic north, hence a significant angular correction will benecessary. Setting up and operating a gyro-theodolite is a specialist operation and must not be undertaken byanyone who is not familiar with the concepts or competent to make the necessary adjustments.Controlling squarenessIn large tunnels where a primary lining is used, the control of line is related to the control of the ‘squareness’ of thelining. To check squareness: set up the instrument on (offset) centre-line turn through 90° (left and right) rotate telescope to 45° above and below horizontal and mark lining tape from the four marks to lining at face.CIRIA C709113