Looking at a project where an existing bridge has to be monitored for movement during blasting for a new bridge. The bridge towers it refers to are sitting on 6 piers (3x2 pattern). There are 7 towers and 2 abutments. Spec says this:
Set or define 2 control points to which all measurements will be referenced to on each abutment and tower base. Set another offset control point that ties the base control points in an area that will not be disturbed during the construction process. Per bridge tower all 6 bases should be tied together to monitor movement of the entire tower. Lateral and vertical movements must be taken on the control points. Maximum acceptable lateral or vertical movement is 1/8".
The bridge crosses a hollow and is 500' long and 75' high in the center. It also is on about a 2:1 slope both along centerline and perpendicular to centerline on each side of the hollow.
Logistics of actually being able to do what they want aside, how many shots are they asking for?
Each tower sits on six piers? The 6 piers x 7 towers = 42 monitoring points.
A couple of points on each abutment = 4 more.
You need two control points plus an "offset control point" (whatever that is) = 3 control points.
Does that sound right?
This type of monitoring project should have a design drawing to refer to. Have you talked to the engineer? They probably don't know exactly what they want, but I think I would start with a call to the engineering company.
That would be a perfect project for continuously operating GPS. Unfortunately, the cost is probably still around $10,000 per point.
Don't think GPS will work, in a hollow surrounded by trees, and under the bridge.
Do they still make/sell Mekometers?
I think that would be:
2 per tower (7 towers) = 14
2 per support base, with 6 per tower = 12 x 7 = 84
abutments= 2 each, 2 x 4 = 8
onsite control total = 106
Off-site control should be a minimum of three points. I'd go each side so I could monitor from the opposite bank of the blasting, and if possible, up and down stream of the bridge....maybe four of-site control.
I've used the reflective targets mounted on buildings for this sort of work. Reflector-less would be awesome if you have the range.
Ditto on the reflective targets.
They come self-stick but I've found if you rub a dab of contact cement into the concrete first, then let dry, they are there for good. Set gun to 0 offset.
Why monitor the towers and the piers?.....one isn't moving without the other.
Presumably the tower shift will be more pronounced than any base shift, no?
"2 control points to which all measurements will be referenced to on each abutment and tower base."
So it could be as few as 50 (6x7= 42)+ 8 for the abutments, assuming 4 abutments.
I would use the towers, too, though, since movement will be more visible.
Here's a couple of pics looking each direction
CL new bridge will be 30 ft. to the camera side of the old bridge.
Here's a couple of pics looking each direction
I guarantee you that even without any blasting goin' on you won't be able to replicate your info to a 1/8 inch tolerance...plus if they ask for it in inches...run, or triple your rates!!!
Here's a couple of pics looking each direction
I agree it's going to be hard to do. I'm thinking using a Trimble VX or S8 to measure rounds to either mounted prisms or the stick on ones if they can be proved accurate enough. Maybe a pair of control points between each tower and targets mounted so at least a couple are visible from 2 setup points.
Another requirement is to shoot the top of each rail over each tower. Not going to be possible to mount anything permanent there. Hoping they will allow screwing 5/8" threaded studs into the ties at those locations and mounting peanut prisms to those. Then have control off each end to set up on and be able to get half from each side.
Here's a couple of pics looking each direction
You should also plan on monitoring the structure for several weeks PRIOR to the start of construction to establish if there is movement now.
Here's a couple of pics looking each direction
How much effect to you expect temperature expansion/contraction to have on the positions? Or sunlight heating one side more than the other?
Here's a couple of pics looking each direction
Good point, though I'm not sure the schedule will allow much time ahead.