Determining the Pipe Invert Level

Hello,

Pipe diameter will have no influence in the outcome since the invert is the "inside surface of the bottom of the pipe".

1:100 across 20m gives 0.2m fall from one manhole to the next. Thus relative to the original manhole the next one will be 200mm lower.

However, you will need to adjust for the soil surface "slope" or contour condition unless you can survey back to the same datum.

By the way, check the calculations. It may be different if the 20m is from (centre to centre) or (edge to edge) and where in the manholes the invert levels were taken.

Just an engineer said it exactly. But I felt the need to add a pedantic methodology for you

Open the second manhole.

You have measured from the rim down to the invert on the first manhole and found it to be 1.5m. If the elevation of the second manhole rim is exactly the same as the first, AND the pipe installer managed 1:100 on the pipe, then the second invert will be 1.7m below the rim (if the end to end measure on the pipe is 20m, and not center to center of the manholes).

This is usual practice to 'as-built' a site (but doesn't address pipe sag).

That being said, if you determine the manhole elevations (by survey) and find the invert fall to be different than 200mm, you don't have what you thought.

Never assume what was on paper exactly matches what was built.

I agree with the posters. Is this existing or proposed? Is the line to carry storm water or is it a sanitary line? If sanitary, a 1% slope doesn't seem to be enough to keep the solids flowing.

You're wrong. One-percent is quite steep. A 12-inch concrete pipe sloped at 1 percent will have a velocity in exceess of 4 fps (by Manning's equation).

A velocity of 2 fps is required in wastewater to prevent settling.

Sorry guest. I don't design sanitary sewers. My experience is from designing network undergound ductline. We always shot for 2% slope to be sure there was no buildup of silt in the ducts.