steep sewers

[swolfman]

thread161-90643

I am going to have sewer constructed with directional drilling at grade increasing all the way to 70%! It will be a 6 inch with commercial flow from a 911 center..i.e not much flow. I do not see much of an issue although some engineers say there is a max flow for solids seperation...We flush line once a year although this line will be flush in reverse due to grade.

Commnets?

 

[cht13er]

Here's what Ontario's guidelines say:

"In sizing sanitary sewers and selecting sewer slopes, consideration should be given to possible sulphide generation problems. Sulphide problems can be minimized by designing for sewers to flow less than full under peak flow conditions and to flow at velocities of 0.6 m/s (2.0 ft/s) or higher.
The velocities in sanitary sewer systems should not be more than 3 m/s (10 ft/s), especially where high grit loads are expected. Higher velocities should be avoided unless special precautions are taken. Where velocities greater than 4.6 m/s (15 ft/s) are attained, special provision should be made to protect against pipe displacement by impact and erosion"...

"5.7.6.1 Steep Slope Protection
Sewers on 20 percent slopes or greater should be anchored securely with concrete anchors spaced as follows:
• Not over 11 m (36 ft) centre to centre on grades 20 percent and up to 35 percent;
• Not over 7.3 m (24 ft) centre to centre on grades 35 percent and up to 50 percent; and
• Not over 4.9 m (16 ft) centre to centre on grades 50 percent and over."


So there you go - I suggest you check your anticipated velocities and then provide anchoring as required.

 

[bimr]

The usual practice is to design the slopes for sanitary sewers to ensure a minimum velocity of 2 ft/sec with flow at one-half full or full depth. However, a mean velocity of 1 ft/sec is usually sufficient to prevent solids deposition. The minimum and maximum velocities are typically specified in state and local standards.

In general, maximum mean velocities of 8 to 10 ft/sec at the design depth of flow will not damage the sewer. Above that velocity, you should evaluate the effect of the higher velocity on the structure of the sewer.

High velocities in small pipe sewers and corresponding low depths of flow may allow larger objects, which at times enter all sanitary sewer systems to remain on the inverts, where they may become lodged so firmly that the next rush of wastewater will not detach the objects.

You also say 8-Inch on one section and 6-Inch in another. Gravity sewers are usually 8-Inch minimum size.

If you use an 8" gravity sewer, you will have about 30 ft/sec velocity which will require some time of energy dissipation device on the downstream end.

You should consider designing this as a force main with a smaller pipe. The greater head loss associated with the use of a smaller pipe will help to keep the velocity down. You will still need to install entrance and exit structures.

 

[swolfman]

I am using a 6 inch pipe...No need for a larger pipe...other than ease of TV. Will depend on upstream MH and downstream to hold pipe in place and and hope to channel flow in manhole into downstream pipe that has less grade ( will reduce slope to 20% prior to entering downstream manhole. No cost effective way to reduce slope or velocities.

 

[bimr]

Interesting problem. What you are describing would function more like a plumbing stack than a sewer.

The capacity of a sewer stack is set by keeping the cross-sectional area of the water flow to approximately 25% of the pipe area. To do this, you will need a vent the drain pipe at the top to atmosphere so that the discharge pipe is not allowed to completely fill with water (and create surging conditions). A standard sewer inlet will probably lead to surging of the inlet pipe.

The terminal velocity in a sewer stack has been determined empirically to be approximately 10 - 15 ft/sec.

http://xnet.rrc.mb.ca/rcharney/interior.htm

http://digicoll.manoa.hawaii.edu/techreports/PDF/NBS31.pdf

http://www.asahi-america.com/documents/documents/Asahi-English/Air Pro engineer_theory.pdf

I would recommend that you oversize the system because once the capacity is reached, you will start to see violent water surging and water cannon effects.

There will be a hydraulic jump at the base. To avoid the creation of a water cannon at the downstream end, you should consider enlarging the pipe at the base of the slope. If the pipe is enlarged, the water cannon effect (caused by the full pipe at the hydraulic jump) can be minimized.

See Garr M. Jones, Robert L. Sanks, Bayard E. Bosserman, George Tchobanoglous discussion of approach pipes.

http://books.google.com/books?id=bi...=onepage&q=terminal velocity in sewer&f=false

Short clip showing the effects of the water cannon at the base:

http://www.wisc-online.com/objects/ViewObject.aspx?ID=PLU104

 

[rconner]

I guess there is perhaps a lot to think about, some maybe non-obvious, in proposed “steep slope” applications. Stabilizing anchorage of the piping and for that matter the slope (fighting a tendency of both to try to creep downhill), strength of the piping to take concentrated loading at proposed anchorages, erosion (both from conveyance inside the line, as well as water movement around the outside of the piping and/or trench backfill or borepath etc.), and I have even heard allegations in more recent discussions that with steep slopes and modern low water fixtures etc. what fluid is there can in effect “outrun” the solids (normally "floaters" etc.?) in very steep slope cases, and that doesn’t necessarily sound like a good thing!
I think there is mention to most or all of these issues now in various literature including ASCE MOP No. 37 (WPCF MOP No. 37), “Design of Sanitary and Storm Sewers”, ASCE MOP No. 60, “Gravity Sanitary Sewer Design and Construction”, and also the “Ten States Standards” (the latter that I have noticed can now be viewed online at

http://10statesstandards.com/wastewaterstandards.html).