Optical plumbing becomes important with the increase in depth of internal shaft. The bob is suspended from a removable bracket attached to the surface side of the shaft. Oscillations of the bob can be controlled by suspending it in a pot with high viscosity oil. The vertical alignment can be done byĪ heavy plumb bob (5 to 10 kg) is suspended on either a wire of heavy twine. But vertical shafts are also sunk up to the required depth along the alignment of the tunnel at intermediate locations along the routes. If the stakes are set on the floor they should be offset into an area along the tunnel's edge.įor long tunnel excavation is carried inward from both parties. And here is sketch regarding how the tunneling work proceeds.Ĭentral line and grade stakes within the tunnel are usually set in the roof to avoid displacement and destruction by the constant flow of people and machinery as construction proceeds. The alignment is permanently referenced by a system of monuments within an area outside each tunnel portal. It is always advisable that the survey is based on the suitable local coordinate system. Based on the local conditions and proposed length of the tunnel the methods of working are adopted. A traverse connecting the portal points determines the azimuth, distance and differences in elevation of each end of the proposed tunnel. Surface survey connects points representing each portal of a tunnel. The survey work in connection with tunneling can be divided into:
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