ᐅ Trench Construction – Channel Bottom Elevation vs. Cover Level

Hello!

I’ve obtained the connection heights and the finalized street plan for my property. The driveway is going to be prepared so I can drive a car onto the plot.

I’m thinking I can use the manholes as a rough reference for determining the final heights. Now I’ve come across two terms: invert level and cover level.

Can someone explain where the invert level and cover level of a sewer are located? Is the invert level the lowest point?

Thanks in advance, Torsan

You want to use the manhole covers to measure your house? That’s risky! Especially in new residential areas, they often shift or get raised. Usually, there are officially calibrated reference points where you can take measurements, otherwise you should hire a surveyor – that’s their job.

Of course, I don’t want to survey my house. The house is already built.

I just have height measurements from several manhole covers and want to roughly plan my driveway.

Regards, torsan

I see
It was similar for us. We had no slope, so we could measure from the surveyed house. The manhole cover was actually 5cm (2 inches) too low – it was then raised using spacer rings.

Wastl schrieb:
You want to use manhole covers to survey your house? That’s risky! Especially in new development areas, they often settle or shift.
Normally, there are calibrated reference points for measurements, otherwise you should hire a surveyor—that’s their job.

What exactly is risky about that, @Wastl?

Nothing shifts or settles unless the road is completely resurfaced or something similar.

What matters for water flow is the slope between the house connection and the invert level, and water doesn’t care whether it flows downhill from 100m to 99m or from 2m to 1m— the slope is the same. Therefore, the relative height difference between the invert level and the house is what counts. The absolute elevation of the manhole cover or its invert level is completely irrelevant, although it is, of course, documented and checked.

The calibrated point you’re probably referring to is a benchmark. Elevations are usually not transferred from these benchmarks unless the slope is very tight or there are discrepancies in the sewer records that cannot easily be resolved.

Generally, you can determine a manhole cover and its invert level using GPS—with around 5cm (2 inches) accuracy. If that matches the data in the sewer records, that serves as a good verification—exceptions may apply. Yes, even for determining the elevation of a newly planned house!

@torsan:

Cover height = height of the manhole cover

To determine the invert level, you have to open the cover and measure the distance from the top edge down to the lowest point of the invert. If the difference between these two values matches the sewer records, it is a strong indication that the sewer is installed as documented and has not been altered. If you check a second manhole the same way and the relative heights between both shafts match, you can be quite confident.

Ultimately, you don’t need to worry about the absolute elevation. You can set your slope relative to the street level and/or invert level, defining the invert or manhole cover height as “zero.” Depending on the distance and planned slope, your driveway will then need to be higher.

Best regards,
Dirk Grafe

Dirk Grafe schrieb:


To determine the invert level, you need to open the cover and measure the distance from the top edge to the lowest point of the invert. If the difference between the two figures matches the sewer registry data, this is a strong indication that the pipe is positioned as recorded in the registry and has not been altered. If you check a second manhole in the same way and the relative heights between the two shafts all align, you can be fairly confident.

Ultimately, the absolute height may not matter to you; you can set your slope relative to the street and/or invert level, defining the invert level or manhole cover height as your “zero” point. Based on the distance and planned slope, the driveway will then simply need to be higher.

Best regards,
Dirk Grafe

Great! Thank you very much!

I did exactly that and found that I need to make my driveway “steeper.”

Best regards, torsan