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@norman-oklahoma
Interesting. The center of the MH plate vs center of structure makes a difference, that is certain.
I was wondering about that. I thought 0.05 was a bit large to miss rims by, if they had good control.
I was taught to shoot the rim nearest the center of the structure. Makes it easier when another crew follows behind to match my work. But, I have since learned that there are many ways that people consider to be “the right way” to measure a manhole.
Based on your procedure, I am guessing that your method is far more accurate than the previous work.
-All thoughts my own, except my typos and when I am wrong.@dmyhill I was taught, and therefore taught my crews, that we shot the rim elevation over the “out” pipe. It is about repeatability. Normally the lids are within a couple of hundredths of being level if steeply canted we would shoot the lid over both the out invert and at the high point also.
Andy
I carry an inexpensive carpenter’s clinometer I’m the truck for use in dipping manholes. It’s only good for a couple of degrees, but when the invert is way back from the rim it’s better than guesstimating.
I also carry a Pipe Mic, but it’s such a hassle to use that I only bother if I’m doing a lot of manholes.
Interesting. The reason we shot nearest the center was for repeatability, but to also be able to use the shot for horizontal calculations.
If you shot over the out, did you also shoot the center of structure?
As for the slope on the lid, 2% cross slope equals 0.04′ drop over the lid (and it is common for there to be closer to 3.5% cross slope – match curb top at crown, which would make it 0.07′ across the lid). But, none of that is relevant to the accuracy of the dip, if you always measure from your shot.
The other advantage of shooting the center of the lid, is that in a typical 48″ structure, (if you shoot and measure from the center) you know two sides of your triangle, making it easy to reduce the third leg for vertical.
-All thoughts my own, except my typos and when I am wrong.Many years ago I had a job locating sewer manholes (or “maintenance” holes as they are now called in this day of gender neutrality) for a small city. We duct-taped a four foot lath to the Philly rod with two feet of the lath extending below the rod so it wouldn’t get all yucky. While probing I noticed a distinct drop below the inlet pipe that was about 6 inches deep at every MH. Two city workers accompanied us to open the lids. One explained that a former employee dug a “stilling well” at every inlet to catch metal items that got flushed down toilets, whether on purpose or not. He would then go into the very shallow manholes and scoop out coins, rings and other heavy items. He said the guy recovered many diamond rings that probably fell off while washing hands.
@dmyhill Just another procedure. We shoot centerline lid, centerline barrel, and measure point for the dips. Depending on the style of manhole this is 1, 2, or 3 different shots. Size of barrel is also determined so slant can be converted to vertical for the distance below the measure point. I have remeasured my own and others dips and typically only find .03′ or less or a significant blunder. Someone is making a mistake if you cannot match within .2′ on a 10′ deep manhole.
My MH code has an attribute field for center of lid or center of structure. Depending on the clients requirements, the MH will get shot twice.
I’d like to come up with a way to shot them directly. First, I hate sticking the fiberglass rod down it the sh___t. As hard as you try you just can’t sanitize that thing. Secondly, it just in not that accurate.
I’ve been toying with getting one of the Leica hidden point poles with multiple prisms and some long sticks rod.
@jim-frame
I have one the Pipe Mics. You are correct, they are a hassle. Then you have to clean them.
Yes, everyone wants the manhole location…and getting the field notes and drafters to get that on the plans, it is usually easier to simply add in a shot for the manhole lid.
-All thoughts my own, except my typos and when I am wrong.@lurker I’m only talking vertical for shooting over the out pipe. For horizontal we did shoot the center of the lid.
- Posted by: @dmyhill
I was wondering about that. I thought 0.05 was a bit large to miss rims by, if they had good control.
To emphasize my point …….. 0.05′ is a lot if you are shooting the same point on the structure. But considering the topography of the typical lid and its frame, it’s not so much. Then you should be doing your measure downs to the point that you rim tied. Which, if you tie the center of the lid – which I think is very commonly done – isn’t possible to do. Then there is the matter of correcting for slope and getting the rod on the actual invert. All these things are potential sources of error that do quickly add up to well in excess of a tenth, IMHO.
Has anyone tried lowering a scanner in to a structure to obtain data that way? I have one of these tripods, but have tried lowering our scanner in yet.
@lurker entering a manhole for any reason is never a wise thing to do for than a few reasons, the first being that your insurance company would be extremely unhappy about it, another being, at least in my area of practice, most are a minimum of 4′ deep and, by definition, a confined space entry. OSHA would have a field day if they caught us doing that. The reason I mention it is because shortly before I started with one of the companies I have worked for, a crew chief climbed down into a 10′ storm manhole and collapsed at the bottom, the IM went down to help him and they both died as a result of methane gas being present. My crews are not allowed to enter manholes for any reason, they are also not allowed to walk foundation walls. we do most new building and home construction with basements. I refuse to locate a foundation that is not backfilled.
We have one.
Woohoo baby does it ever flex and shift when the scanner is panning.
Boss won’t let us invert the sx10 to get covered in cobwebs roaches and the other well know substance found in SAniSewers.
We elevate it as little as needed.
Otherwise use the pipe Mic and do it with teams for traffic and even developed a cheat sheet that has the added measurements needed to convert when deflecting the arm into deep and or smaller diameter drops in a vault.
Amen brother!
We had a sanisew manhole that required a 35′ rod to measure.
I asked the GIS tech I supervised who trained him how to get the rim elevation with that big of a triangle…..
Tyler. Tyler left before i got there…..
Tyler was a computer programmer turned GIS. Tech.
@john-putnam The fun part is when the bend in the pipe is NOT made at the manhole. There is a large (42 inch) sanitary sewer line near me that has manholes and then a joint downstream there is a precast bend. Don’t ask me why it was done that way, it just was.
Andy
Yes. It worked great. It was rather too time consuming for everyday use. Might possibly be worthwhile if as-building a subdivision’s worth of new construction in one go. In the circumstance it served to confirm the measure down I got using a Bosch “Disto”.
You don’t need to lower the scanner all the way into the manhole. In fact, you want to keep it high enough that it will pick up the rim as well as the bottom in the same scan.
The Disto method (ie/using a handheld EDM in “indirect mode”) works great for as-builts of new construction. “Indirect mode” means applying an internal clinometer reading to correct for slope. It doesn’t work for in service pipes because of the flow….. and the stuff that is in the flow. Neither does scanning for the same reason.
@jitterboogie Manholes can be entered in compliance with OSHA and confined space regulations. Why does everyone assume I’m talking about entering them in an unsafe manner?
This may sound picky but when it comes to dead flat grades (0.5% or flatter) I prefer somebody to get down in the manhole with the grade rod & a 4?? carpenters level for the inverts coming in so long as the flow & fumes allow. Men up top hold the rod level & shoot with the automatic level. The invert out can typically always be rodded with just a vertical rod as the manholes around here are eccentric with the lids & steps over the inverts out.
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