Any suggestions on how to set an alignment for a TBM 25 metres down a shaft? I need two points at the bottom for the alignment but can not see down into the hole. The dimensions of the hole are 3 metres wide by 6 metres long (usable space at the bottom). the bore is 300 metres long so any error in the setout of the points at the bottom will make a great big ops at the end.
This Content Originally Published by a land surveyor to Land Surveyors United Network
This sounds like time for gryo station like this one,
Tough problem. No idea where to start.
I have used a contruction line laser that we set up at night in the dark. I have laser glasses that help me see the laser when I use artificial lighting to see what we are working on and lining into above ground. I line the laser up above ground and it will also shoot down into the hole in the ground. We have then set up a second laser underground and aligned it with the laser beam that is coming from our laser above ground. This process worked well and we did not need to rent a gyrostation... we hit our mark as they bored straight down from the far site and it was just over 900 linear feet from end to end.
If you dont want to invest more a heavy plumb bob should do the trick....but then it has to be suspended and air movement has to be accounted for.... else a plumb line laser should do the job but its costlier.
Well, it's VERY old school, but also very precise. We used piano wire for a plumb-bob string (actually used 6 lbs of lead) lowered into 55-gal. drums filled with oil. (Could use smaller barrels) Determine a precise position on the tops of the plumb wires, go into the tunnel and line the instrument on the bottom wires. Voila! I'm sure you can figure out the rest.
(sometimes the "old Pharts" still know a thing or two.
This method is tested for accuracy and practical. No need for sophisticated tools.
This is what I would do:
If you can, set up four or five retro targets around the top of the shaft and resect to all of them (using a 90 degree eyepiece - maybe rent one because they're pricey). You might need someone to hold up a target perpendicular to each so you're not looking at them on too oblique an angle. Then just use ref line. As the tunnel progresses always set up in the same place and do the same resection, you shouldn't be too far out. Don't expect to be spot on with any method, as you know, an error of even 0.5mm in your line will make you 150mm out at the end. And your resection from the original control is unlikely to be without error. This is not a good construction methodology if your looking for perfection. Hopefully the tunnel diameter has allowed for some error.