Pipe to be installed on a very flat slope...

[SMIAH]

Does anyone have any experience with a similar project and would like to share some advice?

Here are the basics:
72'' diameter pipe to be installed deep in high plasticity clay soils on a slope of 0.0013 ft/ft (i.e. almost no slope).

Checked adequacy between design flow and capacity with (normal depth with Manning formula)
Checked self cleaning velocity to be higher than 2.5 fps for a 1/2-year flow rate
Addes small chute at every manholes and spaced them @ every 450 ft or so.

Theoretically this design is feasible (seems to be).
But... Am i missing something? We know that this will be quite a job to do but have no other choice (economically speaking).

Thanks for your comments.

 

[Drumchaser]

I would like to know what is meant by deep....10ft....30ft?

The backfill should be changed to a granular fill at a minimum...or stabilized sand (up to spring line) would be even better if you have the funds. The problem with the heavy material is the compaction under the arches of the pipe.

The % slope is unrealistic. Assuming you are placing RCP....the pipe will not be cast true enough to establish this minute of fall/rise. You cannot place pipe this precise...so I would guess your fall will be non-existent (flat) or greater than specified after placement at best.

***small chute***** might be difficult to attach to the 72".

Would precast boxes be better suited for this placement?

 

[SMIAH]

5 ft of depth for 3750 ft of length...(!)

With between 30 and 45 ft of depth... (!)

But the excavation trench is already very large in this area (over 500 ft)... (!)

This project seems to be unrealistic to me now.

 

[Ryb01]

Something to consider when designing pipes with minimal grade....minimal grade equals minmimal capacity under free flowing conditions. Genrally, these pipes become pressurized(during higher flows)and need to consider hydraulic gradeline and outlet velocoties under pressureized flow conditions.

 

[chicopee]

So, what is to flow in the pipe? storm water, waste water, potable water, drainage from some swamp?

 

[SMIAH]

Stormwater.

And it has been checked for pressurized flow (which shouldn't occur based on the diameter of the pipe).

 

[cvg]

what is a "small chute at every manhole"? Is that an inlet structure? Inlet velocity will be very high and the design of this "chute" to protect against erosive velocities, surge etc. is critical.

I think Drumchaser meant granular bedding not granular backfill. CLSM (flowable fill) would be a better choice for bedding and should be at least to the springline or to 1 foot over the top of the pipe.

Pipe leakage should be kept to an absolute minimum since you have fine grained, expansive soil. Welded HDPE might be better than concrete pipe. HDPE could also be joined up top and then lowered into the trench avoiding the need place any workers in a 40 foot deep trench.

Pre-cast boxes may have issues with joint leakage, cast in place would be better. However, the deep trench and need for shoring will make box culverts very expensive.

Shoring will be a nightmare for this project, maybe tunneling is better?

 

[SMIAH]

Small chute to account for the head losses in the manhole (1/2 inches - i shouldn't mention it as it's not relevant).

Indeed tunneling could be a way to get through this.

 

[beej67]

Do something to make the pipe run water tight, make it very clear in your notes and on the profiles, and document that the contractor did what you told him to do.

This sort of pipe run is not uncommon in areas of the country like Florida that don't have any choice, but Florida also has the benefit of having generally courser soils and less relative suspended solids transport than other areas with more clay. Where's the project?

Echo checking to keep the HGL inside the pipe. What's your design storm?

Get a real Geotech to tell you what your backfill material should be in writing, don't trust an internet forum. (or possibly email one of these guys on the forum and hire them to be your geotech)

 

[SMIAH]

Thanks for your time.

I do have a geotechnical engineer (trusty) here.
Design storm is 50-years and checked for the total range of flow (1/1 to 1/100). Pipe is a little bit over designed (for maintenance issue).

I guess it's now much more of a geotechnical problem (backfill). Thanks again.