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.