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Hello all.

I would like to “poll the class” regarding the above. We have a current application that includes a sanitary trunk at 9.0m depth and greater. We spoke about this topic at the May 2016 DEC meeting, which was very helpful. But not having experience with stacked sewers (or sewers at this depth in general), could you please provide your municipality’s standpoint on the following:

1. Do you have a “magic depth” for which you tell designers that they cannot connect the individual service laterals to the trunk? (i.e. must do stacked sewers)
2. Do you have a “magic depth” for which you tell designers that it is simply too deep a trunk (even if stacked) and they simply must pump via lift station and forcemain?
3. What is actually more costly for long-term maintenance: operating and maintaining the lift station, or the very deep sewer trunks?

Our immediate thoughts on these deep sewers is to make them pump under the water crossing that is causing the gravity sewer to have to be so deep; but then my question leads to #3 above.

Your thoughts on this and comments regarding experiences with this would be appreciated. Thanks in advance, and everyone have a wonderful holiday season!
Mike, based on experience in Toronto, Hamilton, and London (UK); given a choice between a deep trunk (tunnel) and shallow sewers with pumping stations; with the initial construction cost being paid by the developer, and maintenance / life cycle by the Municipality; I’d be amazed if the deep trunk / tunnel wasn’t a better option for the Municipality. Connection to the trunk may require a system of local sewers with internal backdrops in trunk manholes, and energy dissipation on the drops. About the only reason I could see for not going with a deep sewer versus shallow sewers and Pumping Stations would be if the ground conditions likely to cause tunnel failure, (as tunnel repair is unlikely to be cheap).
Straight from our Region of Durham Design guidelines:

No sanitary service connections shall be permitted to sanitary sewers exceeding 7.6m in depth. Depth is measured from the final center line finished road elevation to the top of the sanitary sewer.

We try to avoid pumping stations at all costs. For your information, we are currently building trunk sewers at a depth of approximately 15m in Pickering right now.
Thank you for your comments Mike.

Curious about two things:

1. What size is the trunk in your Pickering example? And what material?
2. What is the minimum right-of-way width that you have installed stacked sewers?
1350mm Concrete and the ROW width is 23m.
Mike
Peel doesn’t have limits on the sewers depth. We have trunk sewers at 25+ metres deep and even a few deeper than 40 metres; these were tunnelled of course and they don’t have any laterals connected to them. We have deep sewers (around 10 metre deep) constructed previously by open cut method as part of greenfield development with long vertical risers for lateral connections, in the past Peel Ops were comfortable with laterals up to 10m deep however recently this was revised and max 7m deep laterals are permitted due to Ops concerns. In cases where sewers laterals are deeper than 7m an additional shallower local sewer is installed. So far, Peel has not taken the stacked sewers route but pros-vs-cons are being closely examined as this might be the future. Peel installs gravity sewers unless the only option is a lift station. Gravity sewers are more cost effective option long term and are not dependent on pumps, hydro, etc., unlike lift stations. Deep sewers in Peel, where possible, are relined instead of being replaced. Deep sewers are oversized to allow for relining as part of long term maintenance without impacting design capacity.
Hi Mike
Just to let you know that the auto subscribe system is working. ie if someone subscribes to this post then they will get a notice anytime a new post on this topic is made

John S
Thanks again to those that have replied so far.

One thing I realize is that the responses I am receiving are from larger municipalities that are referring to examples where the deep sewer pipes are very large and therefore, the cost for such a sized pump station would be quite high and therefore you would be better off with a deep pipe than a huge pump station.

With my example I am referring to, the size of the sanitary sewer trunk pipe crossing the creek is only a 250mm dia. pipe, so the lift station and forcemain needed only to cross the creek to a higher sanitary gravity trunk on the other side would not be very large; whereas with the deep sewer trunk option we would have about 2.0km of 9.0m+ deep 250-350mm dia. sani. pipe. Would this change any of the perspectives / thoughts received so far?

I am curious about examples of deep sewers with smaller pipes, as well as how smaller municipalities may have dealt with similar situations.
8 posts Page 1 of 1