DIP Sewer with no gravel bedding?

[EG]

Hi All,

We have been asked to design about 1/2 mile of sanitary sewer (6-inch diameter pipe w/ manholes) up a steep ski run as part of a ski area improvement project (replaces a septic system per Forest Service). The area has very challenging topography and the site is very remote. I have two specific design challenges:

1. Per DIPRA (

http://www.dipra.org/pdf/wastewater.pdf),

supposedly the pipe can be installed without granular bedding/shading around the pipe (Type 1 in the document). If this were indeed acceptable, this could save tremendous amount of money not having to import all the pipe zone material to the project site (1-1/2 hour haul route EW). I would expect the pipe to be OK based on it's rigidity and strength (and shallow cover) if the excavated material is screened and backfilled carefully. However, there are no examples locally where water or sewer mains have been installed without granular bedding/haunch/shading material and I will need to sell this to the review agency and I have a little time for research. Does anyone have any opinions they could share regarding omitting the granular pipe zone material?

AND...

2. The steep grades (>20%) will result in excessive velocities. I cannot reduce the diameter (increase friction) and the pipe slopes match the ski run grades. Although the DIP is tough and probably won't suffer from scour, I am trying to come up with a good plan for where the new steep pipe will connect (T-bone) to the existing line running flat at the bottom of the hill? The connection will involve a new manhole at the junction, but I can only imagine how sloppy things will be inside it from the sewage flying down and would expect nasty buildup over time.

Thanks for your time and consideration.

EG

 

[rconner]

fyi see discussion in thread at

http://eng-tips.com/viewthread.cfm?qid=298070,

and other threads archived. Also, while I believe DIP is indeed tough and is successfully bedded in native soils in normal pipeline projects at normal grades all the time, I fear extreme slopes with any pipes are a special case and could in some case result in movement and erosion of some fine grained soils attempted to be used as backfill. Maybe anchorages and embedment in fine soil trapped in some sort of geotextile bags would however not have at least this problem? [I have also heard some areas require minimum 8" pipe size for at least normal gravity sewers.]

 

[EG]

Thanks RConner. I actually had previously searched and even reviewed the particular thread you referenced. Your point of 'normal' was not lost on me and hence my concern. Our Utility Districts typically will not allow anything less than 8-in diameter and the steepest slope is 18%. Of course, I am faced with an environment that throws 'standard' applications out the window. I share the concerns regarding the possible issues and concerns you and others have raised and hence my post. I am hoping there may be some other references that I may review and potentially incorporate to 'defend' my ultimate design (similar to the Ontario spec included in the thread you referenced. Thanks again!