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Shooting down a vertical shaft
Im on a job that requires me to be able to shoot down a 60m deep by 12m diameter round vertical shaft.
I have to shoot in an elevation to top of rebar that’s about 5m off the bottom and about 3-4m from the edge of the exterior walls.
I also need to shoot in forms for the shaft walls to measure plumbness and ovality.
The previous surveyor had 5 prisms on the top of the opening of the shaft that they tied in, the spun them 90 degrees down so they could do a resection from the bottom of the shaft. The down side of this is the terrible geometry. Does anyone have a better idea?
Im using a robot so I dont need to be able to see the targets but the angle is such that the instrument cannot just shoot in a control point on the shaft floor.
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5 Replies
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Nadir/Zenith plummet (the kind they use to plumb up floors for highrise construction) to plumb up from base install bracket at top from point at base and use steel tape to measure down. More precise than a robot at those vertical angles should think. Especially in a shaft that tight, even if you can get a diagonal eyepiece to observe such steep angles. Set multiple points at base and survey offsets at top and you have 3d control at base of shaft.
Assuming you have access to base. Could also use a heavy plumb bob to get setup points below approximately right if you put bracket at top in first.
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seems like this would be an awesome opportunity to use a scanner or a UAV with lidar for the big win, but maybe not. what is the precision and accuracy requirements?
a small platform on supported ibeams above the abyss and invert the TS and shoot away.
interesting dilemma for discussion.
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Is there any easily identifiable features or joints in the walls on the way down? You could shoot some of those reflectorless as control as well so you didn??t have as high of vertical angles on your resection in The bottom.
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Going a bit old school surveying here, have done quite a few of these in the construction and mining industry through the years, we fabricated/welded brackets at top of shaft with 1 or 2 inch metal tube stock and attached those brackets to top of shaft at 3 or 4 locations, we then dropped heavy plumb bobs (10-15lbs) attached with piano wire down the shaft into 5 gallon buckets of oil. The oil prevented the plumb bob and wire from swaying and kept it vertical. For horizontal control we would then precisely locate the piano wires at top of shaft and put it on whatever coordinate system it needed, we would then locate the wires very precisely at bottom of shaft through a Least Squares resection or Weisbach triangle method that was utilized for many years before LS became popular. You will find if done correctly that control at top of shaft will match the control at bottom of shaft within thousands. For vertical control it was simply a matter of dropping the tape down the shaft. I know there may be other ways of doing this as the technologies progress but this is a proven method that was done for years.
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I woud go with @JMS66‘s solution, but you could use as well lasers instead of the pianowires. If its not too dusty / high wind currents, then lasers will reach the bottom of the shaft without problems. to measure the shaft walls, you would need to either install yourself on a lowerable platform ( which would need to be in rails or be able to put in a stable position during the shots at different levels) or you could use (ab)use the old method of “light cuts) or try with SfM, using the lasers together with some reflective board as kind of “fix points at the different levels down the shaft. Good luck!
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