Download - Engineering Surveyor

Tunnelling: around curvesOffice procedure on plan of tunnel, determine suitable chord(s) that will not be too close to tunnel wall along line of sight andcalculate deflection angle on plan, mark off ring width on inside radius of tunnel calculate offset from chord to centre of each ring calculate required lead per ring on outside radius (lead arises from greater arc length on outside ofcurves).Procedure in the tunnel with theodolite/Total Station at tangent point, sight back down tunnel on centre-mark and traverse throughdeflection angle (alternatively, set out using coordinates) set up two plumb lines on chord line in straight portion of tunnel set out square marks from tangent point as excavation proceeds extend chord line into curved portion and set up plumb lines check offsets from chord to centre of ring/shield when shield is clear of first chord point, set out this point accurately by bearing and distance set up on first chord point, sight on tangent point and traverse to set up second chord line set out square marks at first chord point check any deviation of the tunnel at the first chord point and distribute the error to bring tunnel back ontotrue centre by or before the second chord point repeat establishment of new chords as necessary.Site Information SheetA Site Information Sheet for the foreman/ganger should be prepared by the site engineer and include a table withheadings as below:Ring No. Offset (mm) Lead from markRing No. markedDistance (m)Tunnels are now commonly controlled by laser; similar principles apply.CIRIA C709117