I've mentioned this topic before, but will again since it is fresh on my mind. I've spent the last couple of days surveying a network of control points around a building for monitoring of horizontal and vertical movement. One of the elements of the exercise was establishing reference points that would be expected to hold their horizontal and (particularly) their vertical positions well. Because of the problematic nature of the site itself, I decided to just stab control points in GPSable location off the site and connect the network to them, both vertically and horizontally.
The control points were a design I've been using for over a year that so far has performed well. It's cheap and relatively easy to install: a 7-10 ft. long #5 rebar with an aluminum cap, the top 48 inches of the rebar being in a PVC sleeve to isolate it from the most active zone of the soil.
So, how does one drive a 10 ft. long #5 bar into fairly stiff clay soil? Stilts or a tall ladder? I've mentioned the trick before, but it's worth posting again in case anyone else would like to experiment with the mark. To install it:
1) Get some 1/2 in. thin-wall PVC irrigation pipe. Cut it into 48 in. lengths. Note that the important thin is that the PVC slide over the rebar you'll be using, but without too much extra clearance.
2) Get a 10 ft. length of #5 rebar, taking care to make sure that the bar will be a loose fit in the PVC irrigation pipe.
3) Use a sledge to drive a 48" long 1/2 pipe nipple with an end cap on the hammered end. Drive the pipe flush and pull it out of the ground, leaving a hole.
4) Drop the 48" length of PVC irrigation pipe into the hole, cutting it to length as necessary.
5) Stand the 10 ft. rebar up and insert it into the PVC sleeve in the ground, leaving approximately 6 ft. out of the ground.
6) Use a t-post pounder to drive the rod (driven end of rod protected with a drive cap) down to the point that the sledge hammer can be used to finish it off.
7) Affix an aluminum cap to the end of the rod if you want. I like to leave at least a couple of inches clearance between the top end of the PVC sleeve and the bottom of the cap just in case the soil gets a better grip on the sleeve than I think it will.
It takes about fifteen minutes to install this mark. The cost of materials is about $7.50. If you want to get fancy, I suppose you could set the thing in a plastic sprinkler valve box. I haven't yet.
Kent - it's kinda' like a NGS monument of sort. Have you noticed any movement - does rain effect it?
dla
How about a picture of the site? In my experience I have always used unmovable objects for control such as existing buildings outside the the subject building.
Kent,
I've never tried that.
As a matter of fact, I've never driven a rod much longer than 48"
I'm wondering however, how precise one can drive a 10' length of #5 rebar into the ground with a t-post driver.
What is your procedure to ensure complete vertical placement?
Does the rebar 'get a mind of it's own and begin the vary from vertical?
i'm with you, merlin. i like to use a headwall with wingwalls, c shape. has good horizontal and vertical stability.
We got lotsa dat expansive clay soil. Would need a 10' long rock drill bit to get more than 2-3 feet deep, though.
> Kent - it's kinda' like a NGS monument of sort. Have you noticed any movement - does rain effect it?
A couple of the 10 ft. rods I installed a year ago are very close to NGS tablets in bridge abutments. They were set as GPSable auxiliary points with elevations transferred from the NGS marks with very little uncertainty. Sometime I'm going to recheck the elevation transfers to confirm the vertical stability of the rod and cap monuments.
So far, over three hot, dry summer months, I haven't seen any detectable vertical movement between two marks in soil about a mile apart. The rain wets the clay soil around the mark, causing the soil to expand upwards. However, the soil loses shear strength when wet, so the combination of the smooth PVC sleeve around the top 48" of the rod and the deep anchorage of the bottom of the rod works to counteract that upward movement.
> How about a picture of the site? In my experience I have always used unmovable objects for control such as existing buildings outside the the subject building.
The site is basically a slope on a mixture of different soil conditions that were cut and filled. Into that mixture, there is subsurface water transport via presently poorly characterized means including possibly utility trenches intersecting permeable strata. The nearest buildings are behind security fences about 1000 ft. away, so would not be suitable candidates for control points.
I assume you put these on flat sites.
I've seen the top of long pipes move downhill at the surface. I know the 1" I.D. pipe set in 1964 is 6' long from the field notes.
> I'm wondering however, how precise one can drive a 10' length of #5 rebar into the ground with a t-post driver.
>
> What is your procedure to ensure complete vertical placement?
> Does the rebar 'get a mind of it's own and begin the vary from vertical?
Well, the pilot hole is that made with the pipe into which the PVC sleeve is inserted. Once that's in place, the rod doesn't seem to wander very much as the remaining 6 ft. is driven with the t-post pounder and sledge. With an aluminum cap, there is of course some room to adjust the exact station mark when the cap is punched.
On the rods I've set so far, I've been careful to make sure that the driven end of the rod was a clean, square-cut end instead of a sloppy sheared end that would be expected to wander a bit.
> I assume you put these on flat sites.
Yes, for horizontal stabilility in clay soil the mark can't be on a slope. It has to be nearly level.
The USGS Geologist at Parkfield showed me their network of passive control points along the San Andreas Fault. These are a series of points which cross the fault. They are a rod with cap in roughly a 6" diameter riser sleeve. You can see the sleeves have moved horizontally until they touch the rod/cap so the rod/cap may be fairly horizontally stable until the sleeve contacts it. These points aren't being used anymore in favor of Plate Boundary Observatory CORS and strain meters plus a bunch of other monitoring equipment.
> i'm with you, merlin. i like to use a headwall with wingwalls, c shape. has good horizontal and vertical stability.
Ed, the problem is that in the soil condition at the subject tract, even the concrete structures such as headwalls and inlets are moving. The building is itself on piers and shows movement.
> You can see the sleeves have moved horizontally until they touch the rod/cap so the rod/cap may be fairly horizontally stable until the sleeve contacts it.
What did you think accounted for the movement of the sleeve? Was it just normal wet-and-dry cycle downhill movement of a clayey soil on a slope ?
I can't see how a driven rod of any length in any soil can be depended on to be horizontally stable to better than cm level.
I've seen lots of USGS and NGS driven rods in tile or monument wells where the rod or disk was off center due to movement of the outer pipe. Some of that has to get transferred to the rod. And I've wiped off the caps on rods to read them and seen them move a scary amount just from that gentle touch. Hope they sprung back to near original position.
This one was due to a tree root, but the cause is not always obvious.
No root was visible, and I suppose there might have been some grading near the RM that caused vertical movement of the tile as well as horizontal.
No it's on a pressure ridge next to the fault line which looks like a swale but not a downhill swale and too straight to look like a drainage. The soil doesn't look clayey to me. As the fault moves it pushes the soil next to it.
Bill
At Parkfield, on top of a hill inside of a building there is a triangulation station. The windows of the building can be opened all around to take observations. They have a very heavy duty tripod in there (you wouldn't want to pack that monster around).
They are very proud of the old EDM that can measure to 1mm or something like that.
Like you I had to touch the cap and it moved back and forth at least a centimeter 🙂
It is inside of a big pipe.
DG2502_ROD/PIPE-DEPTH: 13.4 meters
(Not my picture)
We take it for granted - over 90% of our area is some sort of yellow jack. Sands and more sands - there's some clay here and there, but it's an oddity.
All that clay you guys deal with...
dla
> I can't see how a driven rod of any length in any soil can be depended on to be horizontally stable to better than cm level.
Well, it will be interesting to watch the performance of the pair of control points that are in place near the current project. both have horizontal reference marks nearby and are within 12km of the Austin CORS antenna. Considering that GPS vectors between them are consistent with no horizontal movement over a hot, dry three months of Texas summer, I'll be surprised if their longterm performance is substantially worse.