Grading Tolerances

[wesley_pipes]

I've designed some irrigation ditches for a large farm project. The specified slopes range from 0.03% to 0.07% - very flat. The contractor has told me that even with GPS and lasers, that he won't be able to meet these grades.

From other designs and guidelines I've seen, slopes this flat are not uncommon in large conveyance canals. The California Aqueduct has slopes in the 0.01%-0.02% - with large portions being built in the 1970's and 80's. So, I don't believe I'm designing anything unreasonable.

What should I make of this? I am not an expert on grading or leveling equipment, so any insight on the limitations of the equipment is helpful.

 

[CarlB]

You mentioned 'tolerances', what are they, what is required by the specifications? For a finished surface of earth (vs pavement), a typical tolerance (allowed deviation from design grade) is something on the order of +/- 0.05 feet, or about 1/2 inch. The tolerance would be independent of slope. So the flat slope ditch finish grade tolerance should not be more difficult to achieve. Sure there will be more/larger birdbaths due to any unevenness, but that's unavoidable.

 

[eyeforgeotech]

I'm not sure myself of the GPS tolerances but you could reach out to Trimble or their competitors to ask what type of slopes their equipment is able to monitor. That would give you some independent info to use.

 

[FacEngrPE]

The slopes you describe are flat enough that given that the curve of the earth surface is about 1 foot per mile, if your ditches have significant length you will need to set grade stakes using geodetic leveling.
For work like this it is beneficial to the project if a licensed surveyor sets the grade stakes. If the distance between grade stakes is say 200 ft, and the tolerance allowed between the design grade and the ditch invert grade is say +/- 1/2 inch, a civil works contractor can most likely work to better than that.

GPS system accuracy is not suitable for this sort of work, unless your contractor has a surveyor skilled in using and interpreting differential GPS/GNSS data.

Accuracy of the GPS system w/o data correction service
https://www.gps.gov/systems/gps/performance/accuracy/

Trimble's site indicating accuracy of GPS with various types of signal correction.
https://oemgnss.trimble.com/en/correction-sources

This is a dated manual on geodetic leveling, the principles still apply.
https://www.ngs.noaa.gov/PUBS_LIB/GeodeticLeveling_Manual_NOS_NGS_3.pdf

 

[FacEngrPE]

If you care to become a survey nerd, the National Geodetic Survey has a webanar series that often addresses high precision uses of the Global Positioning System. For example this months offering GPS Campaign Surveys for Estimating Vertical Land Motion to Inform Coastal Management (the video will likely be posted to the webanar library next month) discusses the efforts needed to achieve resolution needed to track vertical ground motion and sea level rise. The short story is that resolving vertical position to 1/2 inch is possible using differential GPS.​

This does not change my opinion that GPS based elevations are only part of the survey solution needed for your irrigation channel. It needs to be combined with conventional survey, and either lots of time to get all of the parts of the solution working together, or find a surveyor that already has the skillset needed.

 

[LittleInch]

Sounds like you might need to go back to using water inside a pipe....

Water Level Basics: How to make and use one

Water levels are a great way to find a level spot on items like posts or stakes, so you can build structures that are even and correct. The simple

www.mechdaily.com

 

[TerryScan]

To me the issue is the ability to construct more than measurement.
I imagine this would highly depend on the required construction specifications and materials.

Is this an earthen channel? (@0.03% a 1" high clump of dirt will back water 267 ft. up the channel)

Is this a paved channel? (one still needs to grade the base and set forms to spec).

I do realize there are many larger channels that have a very shallow slope, but I would hope one tolerates some undulation in the bottom (practicallity of construction). This gets back to how does one specify checking the slope and tolerances. I would base it on how much standing water is tolerable in the channel and ensuring there is fall in the channel over the greater distances.