The process of transferring the centerline from the ground surface to the tunnel interior through a deep vertical shaft is typically done using
- AA standard magnetic compass.
- BSimple visual sighting from the top of the shaft.
- CA GPS receiver placed at the bottom of the shaft.
- DTwo or more heavy plumb bobs suspended in a bucket of oil or water.Correct
Solution
Concept
Tunnel Shaft Plumbing
Formula
N/A
Step-by-step solution
1. Read the question about transferring a centerline down a deep vertical shaft in tunnel surveying. 2. Recall that using heavy plumb bobs suspended in liquid dampens oscillations to accurately project the centerline. 3. Compare with the given options and select Option (d) as the correct choice.
Explanation
To transfer a precise centerline down a deep vertical shaft into a tunnel, heavy plumb bobs attached to fine wires are lowered from the surface. Immersing the bobs in a liquid like oil or water dampens oscillations caused by air currents or vibrations, keeping the plumb lines perfectly stationary so surveyors can accurately project the centerline underground.
Shortcut
To easily remember the method for tunnel shaft plumbing, use the rule "Heavy Plumb + Liquid Dampening": - Heavy Plumb Bobs: Gravity pulls them straight down to project a precise vertical line. - Bucket of Oil/Water: Liquid dampens swinging/oscillations caused by air currents in the shaft. - Why not GPS/Compass?: GPS signals cannot penetrate underground rock/soil, and magnetic compasses are unreliable inside steel-framed shafts.
Common mistake
A common mistake is selecting option C by assuming GPS receivers can function deep underground inside a tunnel shaft; GPS signals require a clear line of sight to satellites and cannot penetrate rock or soil. Another mistake is choosing option A, confusing surface navigation tools with precise underground vertical transfer methods.
Correct answer: D) Two or more heavy plumb bobs suspended in a bucket of oil or water.