Effect of ice shear on sanitary manholes

[minnjoe]

Hi,

I'm designing a sanitary sewer that will include a 1,200' section extending through a slough. The slough is subjected to annual ice flows (shallow shields, typically 4" to 6" thick) from an adjacent river and I noticed many of the area trees are severely scarred due to abrasive action of the ice. Two manholes/juntion chambers have to be constructed in the slough; both less than 8' deep (invert to ground surface).

Questions:
Can anyone offer any advice or recommend any precautionary measures that should be taken to avoid potential ice shear on the proposed manholes? The manholes could be constructed with the top castings below grade...but this isn't highly desirable for maintenance reasons. Or, maybe a ring of riprap could be placed around the structures as a sacrificial interface to break up ice before it reaches the manholes? Maybe this is a non-issue?

Thanks!

 

[dhengr]

Minnjoe:
That’s probably flow (or flowing ice) and the nearer the river the worse; and for the most part the scaring on the trees should be on the river side and the up-stream side, correct? Save the trees, they’re added protection. That ice tends to act like a bull dozer with hundreds of tons of potential force behind it. A bulk protection system like rip-rap might just be pushed into the manhole structure, and not be as effective as a number of piles or some such on the two most exposed sides, which just tend to split and break the ice up. The manhole structure might best be cast in place and reinforced against some of this type of loading, rather than be some precast structure if it can’t be anchored down against these moments. While it might seem a bit strange, could it have some sharp angled corners pointing upstream which would split the ice flow, rather than take the force broadside? Take a look at what the cold weather highway depts. do with bridge piers in rivers and ice flows.

 

[cvg]

you must keep the manhole above the 100-year flood level. you will also need a significant mass to resist floatation. This mass concrete could also be used as protection against the ice

 

[minnjoe]

Thanks for the input! The sewer will roughly parallel the river, which is approx. 200 feet to the west, and will cross beneath a creek in the slough area - most of the trees are clustered along the creek banks. Although the ice in the slough covers a broad area, the movement is consdierably more "sedate" compared to what occurs along the river channel - water backs up, freezes and slowly subsides depending on the river elevation and temperature. Doesn't seem like it would do much harm but many of the trees are pretty damaged - hence the concern.

Most of the project, including a portion extending through a public street in a residential sub., lies beneath the 100 flood elevation - there's no way to avoid it (with flooding nearly every year, the folks on wells and septics literally wallow in their juices...that may explain why they vote the way they do!) The manholes will have bolt-down watertight lids. For shallower sections in the slough, we plan to use concrete encased DIP to ease boyancy concerns.

Thank you again for the responses.