I'm looking to build a few semi-permanent concrete pillar monuments for primary control for a long term construction project. I'm looking for some advice from anyone that has done this regularly and knows the ins and outs of it.
I was thinking...
- Dig a hole below frostline big enough for a concrete footing form
- Attach a round 10-12" concrete form tall enough to get top of the concrete about 4-4.5' above ground. Back fill with dirt that was removed previously.
- Build a quick 2x4 bracing structure to keep it plumb while I add concrete
- Pound three 5/8" rebar into down through the center of the form into the ground spaced evenly inside of the form with about 2" clearance from the sides and 4-5" clearance from the top surface of the concrete
- Fill with concrete, level the top and set a cast in column plate
- Strip the bracing & cardboard concrete form.
- Enjoy!
I have seen good ones and bad ones. I think the bad ones have more projecting above ground than below.
I think that there is some settling that occurs after you set it. Maybe give them as long as possible before you run your traverse and leveling through them.
Just a suggestion if you're in a place that gets any significant frost. Wrap the concrete below grade with several layers of plastic visqueen down below any depth frost might reach. When the ground contracts from freezing and applies pressure to the pedestal the plastic will prevent the surrounding soil from being able to get a grip on the concrete and jack it up.
Just because I'm paranoid, doesn't mean they aren't out to get me.
We utilize them for real time monitoring. While you need to worry about the frost line, unless you can anchor them into bedrock, they really need have at least the same mass below the ground. The simplest way to achieve this is to bury the base in the round about the same depth as the height. In some cases I have decreased the depth by pouring a bigger monolithic slab below grade.
I kind of did the opposite. We used 12'x1' sonotube, 7' of it in the ground. Put 36" 5/8-11 all-thread in the concrete and plumbed it. Screwed on a Trimble 360 passive prism. Run a traverse around. Cut squares on top of the concrete for BM's and dini leveled through. Set 6 of them around the million sq ft building. My carpenters and subcontracting trades love it. It's worked awesome. On a busy site with lots of total stations running, you can't have people parked on the control. We can't have our subs flinging out control either and screwing things up. I established the network and everyone uses it. If we are wrong, we are all wrong together!
I have used many of these types of monuments on deformation surveys. And I have built quite a few. I installed two on my property, intervisible. One I use for GNSS, the other to create an EDM check baseline.
Here you can see the main pedestal and in the background by the barn the other one, about 135 meters apart.
Here is one under construction. Note the holes we cut in the lower section of the pipe. We make the hole somewhat bigger than the pipe so that there is a large mass of concrete, and the holes provide a way for the concrete to "connect" between the inside of the pipe and the larger mass outside.
Finished product:
Probably the best one I have seen is one I used in Canada, it is one of their high order stations (equivalent to what we used to call "HARN"):
note the vertical point (i.e. benchmark) on the left of the pedestal and the tie down eye on the right for securing equipment. Also found nearby was a painted target:
Also, there is an EDM Calibration Base Line (CBL) in Ohio (Culver-Davies) that consists of four pedestals rather than ground marks. Very nicely constructed, although I question the wisdom of putting it at the base of a large earth dam which is subject to settlement due to the weight of the dam.
In the next image you can see the view from the zero pedestal that shows the other three in line, and that they are at different elevations so that none are blocked by a closer pedestal. And you can see the gravel parking area at the next pedestal. There is a two lane highway running parallel to the line with parking near each pedestal
A different take on this is an NGS video of a webinar. It's about driving 4 metal rods; one is vertical and the other three brace the vertical one. They are welded together. It would involve a different tool set than a concrete pillar, which could be good or bad depending on what's available to you.
You've gotten good ideas on depth and such. Two suggestions. First, use an auger and widen the hole at the base. Don't overdig and fill as recompacted soil will continue to move for a long time. Second, do not use sonotube or pvc as a form all the way down. It is helpful in the upper part of the frost layer, but you need the friction of unformed concrete against undisturbed soil for stability.
We have about ten years on a set done this way. Error ellipses on monitoring are running 0.002 with an S7 and monitor prism setup.