I'm working on a subsidence monitoring project that's a follow-up to a survey made in 1991. The surveyor on the original project installed a bunch of bench marks, typically 8" diameter by 10' deep holes filled with concrete and with a 1/2" rebar in the center. At most of these sites he set a pair of steel guard posts in line with and on each side of the mark, and about 6 feet apart. He also set an access cover over the mark. Here's a photo of an example:
Of the 8 of these I looked for, I found 7 as described. The 8th, however, has me mystified. Here's what I found:
That's a 4-foot probe sticking up out of the hole. At the other sites the guard post tops were about 3-1/2 feet above grade. I got no hits at all with the probe, and no magnetic signature at the hole, though I suppose it's possible that the equally-spaced guard posts were nulling the signal. The area doesn't look like it's been filled, but a lot can happen in 20+ years (like the trees!).
Having this mark would have been nice for the historical data, but it's not critical to the project. But I'm still wondering just what happened at that site.
Could still be down there from the looks of it. Personally, I probably wouldn't quit until I had at least excavated down to the top of the concrete on the guard posts.
From the looks of the guard posts there could still be a lot of overburden to move.
Digging holes is back-aggravating for me. My suggestion would be find something to dig with that that burns diesel. OR....you could invite J. Penry down for some BBQ and a dig-fest! 😉
Time and business-wise, if you've got seven you're good.
Could they have sunk? I have heard that given the right soil conditions the ground can turn into a kind of quicksand during an earthquake. Liquifaction I think. I'm sure you know more about it than me. We don't have many earthquakes in NY!
The guard posts look pretty short in that photo with the missing BM. I think the others may be right about settlement or sinking.
I'd say that subsidence is an issue. 🙂
Wow, could a single location liquify to drop a monument like that? Or were the posts obscured by fill?
I recollect when I was a teenager (slave labor family labor) running levels on control points on and around a drawbridge to check for settling. We lost a control point to a snow plow...
It is probably about a foot and a half away from your dig and half foot deeper up under that root cluster.
B-)
My first inclination was to think it is not there any more. Was removed for some reason.
Can you use the historical report of elevation to determine whether you've dug far enough (ASSuming that subsidence is not a substantial factor)?
Has the area been filled?
Hard to tell from the picture, but I don't see much of a root collar exposed on the tree; therefore first guess would be fill.
Agreed. Looks to me like a bulldozer pushed into that area as the triple tree looks like the roots are about five foot down.
Can you tell from inspecting the guard posts if they have been cut off? Is there any reason, legitimate or otherwise that someone may have wanted to remove that particular BM?
If it was subsidence the bm plus post would be sticking further out of the ground than as originally set.
So, the opposite would be some uplift action.