Trying to find a good way of doing this. We have an opportunity to get inside an intake tunnel that is generally filled with water running at 3200 CFS. The tunnel is concrete and natural stone construction. I'm trying to come up with ways that we can install a few control points that have any chance at all of lasting under these conditions; topo in this tunnel has been done before, but all of the control set was of the temporary variety.
It may be that the best we can hope for is setting permanent monuments outside the tunnels and just leaving it at that, but there's that part of me that thinks about how great it would be to get back in there in 5-10 years and find even one or two of the control points still existing.
Any suggestions or experience in this sort of thing?
Drill bolts into the concrete able to put either 360 or circular prisms mounts. works a treat if there is enough room. This is also a plus if they fear that it is collapsing as you will not have to enter the tunnel for observations and turns it into a 1 person job for further down the track.
Are you thinking about going into the Hetch-Hetchy system above Moccasin?
Be sure to bring Brian with you.
🙂
Don
I seem to remember reading that control points in mines are set in the roof of the tunnel. Never done it, underground is NOT my deal.
Item ten from the chart on page 3 of this PDF:
http://www.tunneltrade.com/pdf/4982.pdf
I do not know where to buy them, but they work well.
> Are you thinking about going into the Hetch-Hetchy system above Moccasin?
> Be sure to bring Brian with you.
> 🙂
>
> Don
I plan to. It's going to be into the La Grange Dam inlet tunnel though. We'll get to see that 1916 construction.
I'll take pictures!
For a different but similar application, we use control "spads" shot into the roof by a Hilti gun and hang strings with plumb bobs for traversing.
I was going to mention spads as well.
" handy tool in mine surveying for marking permanent underground stations or suspending survey equipment. Each spad is 2-1/8" long. Shipping weight of a package of 50 spads is 1 lb. Please note the price shown is per 50 spads.
We monitor a couple of outflow tunnels for settlement. One has bolts in the floor, the other bolts in the ceiling. We run levels through them. The ones in the ceiling are easier to deal with, even though the rod is inverted, because they are much easier to find. The ones in the floor are always under a bit of water and slime, even if they shut down the flow entirely (it pools due to settlement in the tunnel profile).
We also monitor another tunnel that is an air gallery, has no water in it. We measure alignment and settlement on punched bolts in the floor. Also, we monitor cracks in this one by measuring between three bolts that form a triangle across the crack, nominally at 250 mm (10") spacings:
For horizontal I would put large brass bolts with flat heads flush in the wall, in groups of three. Put a punch mark in them, and then shoot them reflectorless. When you go back, do a reflectorless resection. You would have to reflectorless resection the BS as well, but just an extra setup. Since they are flush, they will not be damaged by high flow or debris in the water. For temporary purposes we do the same but with a paint stick, set three points, etc.
When I did some tunnel work in Peru, the tunnels were much larger. They had drilled in some threaded receptacles in the wall for each traverse station, and then they had a device that would screw in there and hold the instrument:
Here it has the gyro on it:
No tripods at all...here I am on a pedestal outside getting a reference azimuth
Use concrete screws with predrilled holes. They make them up to 3/4" in diameter.
We had to as-built a circular 10' diameter tunnel that they would only shut down for 2 hours at a time. So, we set control outside, and took shots on every monolith joint. When the 2 hours was coming up, we had to set temporary points as I described above at the gun and at the backsight, then start up again where we left off after the water flowed for a while. We were not able to set permanent points inside, though:
we used a dual prism rod to take 4 shots at each joint: top, bottom, and at each side. We also shot places where we could see seeps.
The tunnel had two curves in it, we went up to the intake tower, which is where the gate valves were located.
My suggestion?
I'd hire Mr. Hamilton to do the job. 😀
> My suggestion?
>
> I'd hire Mr. Hamilton to do the job. 😀
Wouldn't be a bad idea if the budget was there!
Thanks John, this is really helpful... at least gives me an idea of the plan going ahead. We're a bunch of surface dwellers usually, aside from a few jobs taking reflectorless shots through much larger tunnels that didn't have a full reservoir on the other side of a gate.
The floor is probably not going to be an option, as there is usually about 3-6" of flow even when the gates are closed. Setting bolts in the ceiling is probably the way to go for vertical, and punched brass bolts or even a disc set flush would work for my horizontal component. Debris in the water might make spads less desirable, but it's worth looking into as well.
:good:
