Hi guys. We're working on a municipal drainage project. Our control and all of our layout so far has been on the surface and we were told that we would not be asked to do anything underground. Well, of course, they just asked us to stake a couple of points at the bottom of the shaft which is 150' below ground. We don't usually do this kind of work. For those of you that do, how do you do it?
RNT,
Not sure what your accuracy requirements are but here are some tools that might help you get there:
https://paragoninstruments.com/surveying-and-construction/vertical-laser/
Best regards,
Terry
Well, of course, they just asked us to stake a couple of points at the bottom of the shaft which is 150′ below ground.
I'm curious, what kind of safety precautions will you be using?
That's a good question. I'm still stuck on the how do you do it. I don't think it will require any ropes or stuff like that. I'm pretty sure that we can access the tunnel at another spot and walk to our locus.
Thanks Terry, I can hear my boss's laughter already.
Out of curiosity, what is the diameter of the shaft and why do they want two points at the bottom?
First image I got from the OP is a storm manhole with an invert of 150 feet. 🙂
Set points on the face of the collar from the top, turn the prisms to face down and resect from the bottom.
3 glass at 120 degrees should be sufficient but I would set 4 or 6, depending on the diameter so that you have back up in case of points being destroyed and a check for sanity.
If you need to call in an expert the guys at Towill are really helpful.
We have recently done something like this but we were 300' below the ground surface.
The preceding surveyor used 8 fixed prisms at the opening of a 30' diameter shaft and performed multiple resections, using different prisms and rotating their tribrach after each setup. Those coordinates were then averaged and established. From there, multiple fixed prisms were set underground.
My firm did a resection under ground, using those fixed prisms and saw very good results.
T. Nelson - SAM
Assuming the shaft is more than 3' diameter: Wild ZNL Zenith Plummet at the bottom. Plexiglass panel at the top. Plumb underneath a mark on the plexiglass. Gyro-theodolite at the bottom to get an azimuth. If possible to setup on the top, use the ZNL to establish a point below. We have also used a right angle eyepice on a T2 from below.
We had to transfer position multiple times over several years inside a dam down two shafts, We got good results using the T2 with the right angle eyepiece. You should rotate it 90° all the way around which will give you a small circle at the top.
I have also done that in mine shafts from the bottom using the ZNL and an ARK2 gyro. To transfer elevation I used a steel tape weighted at the bottom and read the tape at the top and the bottom. Applied corrections for temperature and weight.
John has the right idea.
At one point in history the answer was a very large plumb bob + 150' feet of wire + a bucket of oil.
A Wild ZNL would be the better choice.
That said, if your contract did not include transferring control down there, you might want to reconsider if it is something you should do. The hardest part of heavy civil (IMHO) isn't the precision or the magnitude, etc. It is finding out what you are NOT supposed to be doing. If the owner is supposed to be providing control, make them do it. Don't put your neck out there on something that can burn you.
To answer a few questions: I found out that this doesn't end in the tunnel, it's actually an 82' dia x 150' deep pump shaft. You guys have given me the answers I need, thank you.