Could you provide a bit more background information? What are the pipes being used for? Foundation drainage? Infiltration tench? How much conveyance do you need? etc
You may not need a"minimum slope depending on the application.
The underground 8 inch PVC pipe is to transport raw sewage from the hospital building by gravity flow to its eventual destination in the lift station from where it will be pumped and routed to the treatment plant.
The site conditions do not allow more than 0.7 % slope for this pipe, and since this is less than the recommended minimum of 1 %, our concern is whether the .7% will generate enough velocity for the needed scouring action to avoid any soil buildup in the pipe. The pipe is expected to be half full in the usual case but in the event of peak flow would run 100 % full.
Thanks and Regards
an 8" pvc pipe at a 0.7% grade produces a flow of approximately 0.95cfs (0.027cms)with a veloicity of approximately 2.85ft/s (0.87m/s). I would think that you'd be fine as long as you could get the pipe at a 0.7% grade, not sure that I would go much less than that. As long as your velocity is above approx 2.6ft/s you should be okay.
I'd check local design regulations for additional guidance. Hope this helps.
Dear Engineer,
We are about to lay drainage pipe in our project where the trench has been excavated and the pipe bedding is all ready. Above this pipe path there will be a road running above longitudinally above the pipe path and the cover above the pipe will be less than 3 feet. The attached sketch details how to install drainage pipe passing under the road with less that 3 feet of cover. It shows the pipe inside another pipe (sleeve) that is totally encased in concrete. The detail asks for the sleeve to be one diameter larger than the pipe.
1) We are unsure about the empty space that would result between the pipe and sleeve. Furthermore, the pipe material is PVC with bell and spigot joints.
2) The precise benefit of the sleeve is not clear since the sleeve would be totally encased in concrete, and there is no likelihood of the cyclic load (from traffic) effecting the pipe that would be encased in concrete.
3) Since the detail calls for the sleeve, we cannot cancel it out but need to know the best way of doing it along the length of pipe and at the locations of bell & spigot joints.
Your kind feedback in this matter will be greatly appreciated.
Thanks and regards
I would advise asking the engineer of record to provide a better detail. The casing appears to be too small. Normally, centralizers are used to center the carrier pipe inside the casing. The casing is generally steel pipe and about 12 inches larger than the carrier pipe. You may want to grout the annular space or fill with blown in sand.
According to the recommendations of the code the minimum slope for horizontal soil drainage pipes is 1% for pipe diameters less than 8 inches.
The maximum slope that is possible on our site is 0.7% and this is fine for 8 inch diameter pipes . However, 0.7% slope is not permissible for 4 inch diameter pipes, so can we simply substitute the 4 inch diameter pipes with 8 inch diameter pipes to come within the recommendation.
Hate to be Debbie Downer, but you really need to be RFIing your engineer these questions, not asking the internet.
If you start changing stuff around without the engineer's permission, and then can't pull your CO because the engineer won't sign off on the as-builts, you're going to have a hard time explaining what the "Eng-Tips Forums" are to your owner.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -
The comment of Beej67 is appreciated in terms of getting the work approved. However I would like to know if it is technically correct and an effective method of meeting the requirement of the code to increase the pipe diameter when the steeper slope cannot be achieved
increasing the diameter is counter productive, it reduces the velocity and makes it more likely your pipe will clog. Unless you have a very large development, you will not have sufficient flow to justify an 8 inch pipe. You need to stick with 1/8 inch per foot (1%) which is generally required by code.
You stated the line (8" @ 0.7%) would generally run half-full, occasionally full. According to Manning's, velocity at half full is the same as at full. I agree with Ryb01, that slope should be adequate. A typical minimum design slope for an 8" sewer main is 0.4%.
You later mention substituting 8" for 4" to meet minimum slope criteria. I've seen this often done for services (actually 6"/1% versus 4"/2%). I agree this would just slightly slow velocities, but sometimes that's the only way to get a reasonable design solution through agency reviews.
assuming the sewer was originally designed for full flow with a 4 inch pipe at 1%, maximum capacity is about 0.19 cfs.
Putting 0.19 cfs in an 8 inch pipe at .7% slope may not provide the necessary minimum velocity you need. Depth of flow will be just over 2 inches and velocity approximately 2 fps.
Now, assuming the sewer was designed for only half full (in the 4 inch pipe), your depth of flow and velocity in a flattened 8 inch sewer would be even less.
Again, this should be an RFI to the project engineer but don't expect it to be approved.
The feedback is appreciated and the way to proceed is clear as regards to the slope.
The main run of this PVC pipe in question is collecting sewage from various buildings of the hospital and is being laid from manhole to manhole that will be cast in situ along with this pipe. The joints are bell and spigot type. The manholes are square or rectangular. Complete 6 meter long pipe pieces are being used and there are no stub pieces in the inlet and outlet of manholes.
Furthermore, the manholes are without any sleeves as the pipe will be in place prior to the casting. In other words this main pipe is running through steel rebars of manholes and so will become integral with the manholes. This will preclude the need of grouting between pipes and sleeves (had the latter been placed in the manholes).
Feedback regarding the technical soundness of this method will also be much appreciated.
make sure the mahole does not settle more than the pipe or you will crack the pipe near the face of the structure. Or put a short stub with a joint near the face...
Based on the invaluable insights about the matter of slopes received, I am now calculating the flow in GPM (from fixture units data and hunter curves) that exists in each part of the sewage network to asses the minimum slope required to generate the 2 feet per second velocity for scouring action within the pipes.
As a mechanical engineer called in to install drainage pipes (with gravity flow in which the pipes might be half full !), the formula(s) to determine the velocity for a given GPM would be most helpful. A sample calculation for any GPM would be a valuable guideline (the pipe material is uPVC), as well as how to determine the fullness status of pipes (half full, etc.). Thanking you in advance.
This website has some neat tools, which may be of assistance to you.....look under simple mannings calculator for pipe flows. Using a uPVC pipe you should be using a mannings coefficient, n=0.013
Mannings equation is only valid for free flowing conditions, this equation is no longer vaild under pressurized flows.
I think this has been hinted at in the previous posts but most engineering standards require minimum sizes of sewer to be 8-10" especially for commercial and industrial applications.
If you are outside the building, the plumbing code does not apply. The IPC only applies for plumbing in the building. An 8" sanitary sewer pipe for outside sewer service is generally run at 0.4%. This will yield a velocity of over 2 ft per second when running full, which is the criteria in the Ten States Standards. Ten States has been adopted in almost all of the US for exterior sewer lines. You really should have a civil engineer look at this, as the regulations for sewers outside the building is different from inside the building.
I respectfully disagree with cvg, who is very knowledgeable, concerning his posts above on 27/08/10 1:04 and 12:09. Ten States clearly states that the minimum sewer velocity (slope) is based on the pipe flowing full. A 1% slope on an 8" pipe is not required by code, at least according to Ten States, which has been adopted by Florida and Georgia in which I practice and I believe by at leat 40 other states. Obviously there are situations in which the standard minimum regulations should be exceeded, but I don't see anything special in this situation as described.
