Dunno...like Bill indicated above, I would be a little concerned about horizontal movement over time, but I suspect that the vertical movement will be minimal as Kent indicated (or not...who knows without checking).
Even points on "bedrock" will show some differencial movement over time, even in a VERY local sense (at least in these parts), so it's always a bit of a crap-shoot.
All in all, I think that Kent has a pretty good idea there, subject of course to local variation in soil and weather conditions.
Loyal
> Dunno...like Bill indicated above, I would be a little concerned about horizontal movement over time, but I suspect that the vertical movement will be minimal as Kent indicated (or not...who knows without checking).
Well, we do have a fair amount of experience with shorter (36" to 48") rods in the same soil condition. It's not as if the top 6 ft. of the soil moves around for no known reason.
What is the diameter of the PVC sleeve you're using? You original posts says 1/2" PVC, but I assume that's incorrect since you are talking about driving a 5/8" rebar into the sleeve.
I've seen 4" and 6" PVC used as sleeves for control points around here, similar to the photos shown above. I'm not sure how deep the sleeves run, but I assume posthole diggers are needed to install the sleeve.
> What is the diameter of the PVC sleeve you're using? You original posts says 1/2" PVC, but I assume that's incorrect since you are talking about driving a 5/8" rebar into the sleeve.
Actually, it is 1/2 in. PVC irrigation pipe, a thin-wall pipe that is apparently extruded with the same o.d. as normal Schedule 40 PVC plumbing pipe, but getting the thinner walls by increasing the i.d. I know, it sounds wrong, but what I buy is labelled by the manufacturer:
1/2" IPS PVC 1120 315PSI@73F 2/1SF ASTM D2241 NSF-pw
BTW this is the same PVC pipe that I use for laths that last for years. I carry 48" lengths of this stuff around for that purpose, so it's also in the truck for sleeving long driven rod monuments.
"Well, we do have a fair amount of experience with shorter (36" to 48") rods in the same soil condition. It's not as if the top 6 ft. of the soil moves around for no known reason."
Unknown to WHO, you, the mouse in your pocket, or both?
🙂
Loyal
> Unknown to WHO, you, the mouse in your pocket, or both?
Well, the mechanism of horizontal soil movement in the expansive clay soils of Central Texas is mainly the result of the action of gravity on the pillars of soil that the deep cracks in the top layers of the soil forming in the dry season subdivide the soil into. On level ground, there is negligible downslope movement. So, if a mark is claimed to move under those conditions, it supposes some mechanism beyond those known.
Kent,
I know that Wayne Terry has been using your technique for awhile now. However, he informed me that he had talked to a local and well known Soils Engineer with a Ph.D. (who I know), who suggested that you would typically see what David describes above. Wayne is re-thinking the technique. My comment to him was that I couldn't think of anything else that might be more stable. Even if you drill into bedrock, the top would stil be susceptible to lateral forces. Of course, depending upon you soil conditions, your mileage may vary.
> I know that Wayne Terry has been using your technique for awhile now. However, he informed me that he had talked to a local and well known Soils Engineer with a Ph.D. (who I know), who suggested that you would typically see what David describes above.
Would the soils engineer be Marshall? He'd be right if the mark is installed on a slope, but all my experience with shorter rods indicates that he's probably mistaken if he thinks that the top layers of clay soils migrate in any consistent direction on essentially level ground. At any rate, I haven't seen significant movement over ten years.
Your monument is probably quite stable in the vertical but not as much in the horizontal. With the NGS steel rods the PVC at the top is filled with (food grade) grease, to keep the PVC from filling with water, freezing, and blowing out the PVC.
I suggest an improvement might be to use a 1" PVC (or larger, but that would be tough to drive) and fill it with grease, then drive the 5/8". That would allow for the PVC to move around a little with changes in water content of the soil and freeze/thaw without pushing the iron rod within.
BTW - you say "remove the 48" iron pipe" like it's nothing. I'm sure that is sometimes the greater part of the work. Still, it's a pretty cool low-tech, low-cost idea.
I imagine the depth of permafrost is pretty shallow in Austin.
Yep, that's him. To be fair, I don't recall if he qualified his statement with whether or not the ground was sloping. I thought that he may have said that water penetration around the PVC from overland and subsurface drainage would move the pipe and subsequently the rod. I'll have to ask Wayne about that conversation again.
I'm sure it freezes at least occasionally. Once would do it. Besides, there is a world out there beyond the Texas border.
> Yep, that's him. To be fair, I don't recall if he qualified his statement with whether or not the ground was sloping. I thought that he may have said that water penetration around the PVC from overland and subsurface drainage would move the pipe and subsequently the rod.
I'm personally very skeptical that the small amount of water that might possibly pass between the 48" sleeve and the rod (it's a close slip-fit) would be enough to make a difference. If the sleeve were longer and larger, and if the drainage pattern directed quite a bit of surface runoff toward it, sure, but otherwise color me dubious enough that I'd probably wager lunch on it.
In fact, that rod and cap (10 ft. rod) that was set about a year ago would provide a good test case for stability since I have it connected horizontally and vertically to a nearby brass disc in a bridge abutment.
> BTW - you say "remove the 48" iron pipe" like it's nothing. I'm sure that is sometimes the greater part of the work.
For the rods I've installed so far, that has been the easy part. You just give the capped end of the pipe a smart sideways rap with the sledge to loosen it, and pull it out of the ground. If it's tight, vise-grip pliers work well to turn it as you withdraw it.
The advantage of the 1/2 in. PVC irrigation pipe is that the slip fit is so tight that there really is very little room for water. One thing we don't have to worry about in Austin is the ground freezing. :>
I recently had to remove a couple of reinforcing bars that were much too long for the topsoil depth that I encountered. (It was an open field near a road and I always try to drive flush to prevent tripping hazard or tire damage). It is surprisingly easy to remove a reinforcing bar by twisting it. The deformation pattern on the bar quickly opens up the hole when spun and even though the bottom 3 inches was driven into caliche, it quickly let go with a few spins. Haven't had to use vice grips yet, as the channellocks seem to do the trick. The shorter rod was inserted into the resulting hole and driven flush, capped and then checked again for position. I haven't pulled many reinforcing bars, but it seems to be easy enough.
Around here they sometimes pull easy but sometimes it's an hours work. About 50-50. And that's for partly driven 30" bars. The best reason I can think of for driving a 10' rebar is not the lateral stability, but the high unlikelihood that it could ever be withdrawn.
> Around here they sometimes pull easy but sometimes it's an hours work. About 50-50. And that's for partly driven 30" bars.
Wow! What sort of soil is that? I can't imagine that.
Well, you start driving and about 18" down you start to get resistance. So you pound a little harder thinking that it's probably just a rock and you will get through it. Soon you are 24" in, it won't go no matter how hard you pound, and you realize it's more than just a rock and you've driven it 6" into a crack in the bedrock. Good luck twisting that out.
> Well, you start driving and about 18" down you start to get resistance. So you pound a little harder thinking that it's probably just a rock and you will get through it. Soon you are 24" in, it won't go no matter how hard you pound, and you realize it's more than just a rock and you've driven it 6" into a crack in the bedrock. Good luck twisting that out.
Oh, we have plenty of shallow soils in Central Texas. If you want to set an 18 in. rebar on many sites, you'll need a rock drill for the lower 12+ inches. If you pound a #5 rod into a 5/8 in. hole, it will take some very serious effort to extract the rod.
This whole discussion, though, is about deep, clay soils where driving a rod 10 ft. down requires nothing more than a t-post pounder and a sledge. If the rod is 36 inches long and in soil all the way, it's hard to imagine more than a pair of vise-grips being needed to pull it out.
For those sort of conditions, I agree.