Minimum height of grade ring required for concrete manhole 1

[hughits]

The minimum height of concrete grade ring for manholes/sanitary manholes is specified as 150mm on IFC dwgs for a project I am working on.
I got a phone call from Conctractor saying that this minimum height requirement could not be met because the elevation of some manholes installed was higher, and as a result, the maximum height of 75 mm for grade ring was the only option to choose.
As a structural(foundation) engineer, I am wondering what risks I will have by accepting this height (75mm) in lieu of 150mm. By using this height, the final grade will be very close to the design elevation (maximum allowable deviation for grade elev. is 25mm).
Thanks for your tips and advice in advance.

 

[Ron]

Tell the contractor he screwed up by setting the manhole too high...now he has to fix it. He can tear it out and start over or provide a plausible repair method for review.

 

[cvg]

really? why would you not allow a 3 inch grade ring and only 6 inch ones? Why was it not allowed in the first place? Seems extreme to me to remove and replace and I fail to see why a 3 inch ring would not be completely acceptable.

 

[CarlB]

In our area a 6" (150 mm) grade ring is the normal minimum. This is apparently so if the road grade ever needed to be lowered a few inches, or if the road surface settled realtive to the manhole, it could be accommodated by a simple grade ring adjustment. However this minimum is often relaxed, as future road lowering is very unlikely in most cases. In some cases no grade rings are provided.

 

[Ron]

cvg..you're right. I misunderstood the post...thought he had nowhere to go with the height.

 

[hughits]

I checked with our civil designer regarding this matter. As CarlB said, I guess any potential change in the grade elevation in the future is unlikely. He advised me to accept as is. Sounds like 150mm is typical, but the rule could be bent depending on the situation.
Thanks guys for your opinions!

 

[cvg]

our standard also shows 6 inch minimum, 8 inch maximum - but on the same detail it also allows bricks to be used as an alternative to grade rings with no apparent criteria for type of brick or size. For installation in the street, no grade ring is used at all, an 8 inch thick by 12 inch wide minimum concrete collar is used. If the pavement settles or needs to be raised, get out the jackhammer in order to adjust the manhole.

 

[beton1]

Is it possible that your code means 150 mm min., but which can be composed of two at 75 mm? We commonly use 75 mm rings, and as mentioned, bricks are also acceptable. Just a thought.

 

[hughits]

@ cvg, our standard shows 150mm minimum and 350mm maximum. Again, I am not familiar with grading/earthwork stuff, but I guess it depends on who designs, how much room you have for adjustment in the future and such. I didn't know that bricks can substitute concrete grade ring. Thanks for the extra info that is valuable.

@ beton1, I am not sure, but I would say why not? For clarification, what specifies 150 mm as minimum is not a code but our standard drawing (either coming from a code or a typical practice?).