Sag in buried drain line

ajk1 · Dec 13, 2016

I am looking into the issue of a sag in a 6" diameter drain line that is buried about 7 feet below the basement floor slab which is a structural suspended slab due to poor soil. The pipe is suspended from the floor slab. The pipe clogged up about 3 months ago, but was snaked out and has been running freely to date. However when the plumber videoed the pipe at the time, he discovered that there was a sag in it, such that the pipe was always full of water for about a 10 foot length. There is a trap in the line just before it exits the building. Due to the structural slab, it is costly do get down to the pipe.

Question: Is there any problem with having a sag in the pipe? Does it have to be fixed? Or is it ok to monitor it and see if it clogs again?

LittleInch · Dec 14, 2016

Main problem is, as you've discovered, that dirt and gunk can settle out in periods of low or no flow and slowly start to block the pipe.

If you don't fix it, it will block again.

If you can, arrange for a large flush on a regular basis ( once a week?) to keep it clear and monitor.

So long as the settlement has finished, there shouldn't be any line breakage caused by the depression, but this needs to be monitored. As the pipe is suspended, there will be higher loads and stresses at the points where it is suspended which may cause premature or sudden failure at those locations, but depends on the material and how far it has moved.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

PEDARRIN2 · Dec 14, 2016

I understand the pipe cannot be supported by the soils under the slab, but supporting it from a slab 7' above the pipe seems a questionable piping design. That is a lot of all thread rod to use. It is almost another level. Is there some type of access

Was the piping plastic and if so, was the support spacing designed for this. Even with metal pipe, if there is water for a 10 foot length, it doesn't sound like it was supported correctly.

Was the piping installation inspected - this sag should have been seen when the pipe installation was inspected. For "buried installations", the pipe is supposed to be inspected prior to being backfilled and prior to obtaining final AHJ acceptance. A similar requirement should have applied in this case as well to make sure the hangers properly supported the pipe. Why wasn't this done?

Like stated, this is a natural "falling out" point for all debris, waste, etc. and unless corrected, tell the owner they will have to deal with this for a long time.

Is the area below the slab backfilled to the slab or is it like a crawl space? If not backfilled, can some type of access be installed in the outer wall to access the pipe? It might be costly but so would having to snake out the pipe every few months.

racookpe1978 · Dec 14, 2016

So, is the basement floor cracked enough to equal the sag in the pipe?

If the drain is now sagging - you can measure the sag exactly by knowing (estimating) the height of water in the borescope compared to the top of pipe (pipe ID). Plumbing drains should be about 1/4 inch per foot of length, so the if the midway point of a 6 inch pipe is half-full of water, you know how much the floor "should have" subsided. As asked above, what is length of pipe?

ajk1 · Dec 14, 2016

To Littleinch - thank you for the information. Much appreciated. How would the large flush once a week be achieved? The recently installed low flow toilets probably don't help the situation of the 50 year old system.

To PEDARRIN2 (Mechanical) - Thank you for your comments. There is no access. The pipe is cast iron with bell and spigot joints, oakum and lead in the joints. Installed in 1966. Hung by hangers, at 5 foot centres (if they conformed to the municipal code, which I expect they did). Has worked well for 50 years. I don't think there is any significant problem with the original design and construction as the building was constructed under the direction of a large respected architectural firm and seems in general to be of above average quality. I think the mostly likely thing is that the hangers have corroded and failed. There is no crawl space.

To racookpe1978 (Nuclear)- I of course cannot see the underside of the floor, but the top side does not appear to have any discernible sag, although of course it must deflect a bit under its own weight. Deflection of the floor is not the cause of the problem, as the pipe is sagging 6" or more. The length of the pipe is about 150 feet but the sag is over only a portion of this - about (roughly 10 feet?) over which the 6" pipe is full of water. Where did you get the 1/4" per foot minimum slope? A mechanical engineer told me a few weeks ago when I asked this question that it should be 1/8" per foot. The original 1965 drawings specify the slope as 1/8" per foot.
Thank you for your comments.

rconner · Dec 14, 2016

This is a quite interesting "design" - as best I can understand not exactly like any I have ever seen. How, where (with respect to the bell and spigot joints, kind of begs for at least a sketch!) and with what was the piping "hung"? The OP proviso "that is buried" and maybe over very poor soils (and if same earth was say originally up to the bottom of the "suspended" slab?) could mean that quite high tensile loads were exerted on whatever hangers are involved, if that 7 feet column of earth along with the pipe encapsulated tries to settle. If the pipe were surrounded with more competent or compacted soils than what is below and that mass settling over time, who knows maybe the loading could furthermore be even higher with those more competent soils really dragging the encapsulated piping down? Are you sure the "municipal code" you are referring to e.g. for support spacing anticipated such loading?

racookpe1978 · Dec 14, 2016

Hmmn. I'm reading 1/4 per foot for smaller diameter sewage pipes, (1/8 per foot only for larger pipes with near-constant flow assumed) in the on-line sources I've found to check that claim.

1/8 inch per foot might be adequate for average residential flow in good conditions in a clean pipe. It is certainly correct for draining a clean pipe of clean water or fast-flowing chemicals with no residues. But it would be (at best!) an absolute minimum slope for a sewage pipe filled with solid waste and sludge and kitchen waste in a dirty pipe that is often stagnant for hours.

Note that NYC requires no more than a 1 in 4 slope (3 inch in one foot) to prevent excess speed of the waste when it hits the main line.

ajk1 · Dec 14, 2016

To rconner (Civil/Environmental)- The design I describe is very common and I cannot explain why you have never encountered it. I have encountered it numerous times across Canada in my 50+ years of experience as a structural engineer. Mechanical engineers tat I have spoken to say it is common then and now. I checked the load on hangers by the ASCE method and it is not terribly large, although I do not know the hanger diameter so I cannot check its capacity. I doubt that there was a blunder made in the hanger design or it would have shown up somewhere along the hundreds of feet of piping long before 50 years. It is now 50 years old. The most probably cause of the problem is corrosion of the hangers. We had a buried water main break 5 years ago and flooded the basement and saturated the subsoil. I expect this may be a factor in the hanger corrosion.

To racookpe1978 (Nuclear)- interesting information. This pipe serves the washrooms and the kitchen in the building, but the kitchen sink does have a grease trap. Thanks. Does NYC have any required minimum slope? Themechanical engineer that I spoke to says 1/8" per foot is standard.

LittleInch · Dec 14, 2016

ajk1 - You've been around here long enough to know you really should provide a link to your previous posts which seem to be the same pipe, but for rconnner and others, this was the main post ( there were three different ones at one time)

http://www.eng-tips.com/viewthread.cfm?qid=415524

Large flush? - you might need a specific cistern or tank which fills up then empties rapidly by solenoid valve maybe once a day?, every 8 hours? probably doesn't need to be that big, but outlet sized for velocity. Hence put it as high as you can...

Especially if the fall is low and the volume of flush has decreased over time, your risk is high of another blockage, IMHO. 1/82/ ft is 1: 100 which is ok for gutters and general sewers, but 1: 75 or 1: 50 is better - but you can't change it now.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

EdStainless · Dec 14, 2016

When they scoped this was there any sign of damage?
In the long term a fix will be needed.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

PEDARRIN2 · Dec 14, 2016

It sounds like there are multiple issues with this piping.

Installing low flow fixtures (depending on how low is low) in an existing system is a recipe for this type of occurrence. You have likely less than half the design flow rate so you will have a lot of settling out of solids.

If the hangers have likely failed and corroded due to age or to being deluged in a water main break, it is only time before the piping starts to leak and crack. Also, with stagnant sewer water, hydrogen sulfide can start to form and corrode the piping from the inside out. It will likely start at the water line and work its way up the pipe circumference. If that happens, having a backed up pipe will be the least of your worries.

I would recommend this piping be accessed and fixed or the owner is going to be chasing this problem for a long time.

racookpe1978 · Dec 14, 2016

As my pllumber - NOT a mechanical egineer! - answered when HE scoped out and routed my drain pipes ... "Every dishwasher, garbage disposal and kitchen drain in every house are my friends. They pay most of my monthly profits."

The grease and waste - especially if not properly ground up by the garbage disposal - from that kitchen will be always blocking the line. Route it out regularly, flush more often, and keep scoping the pipe.

The water buildup may not a sloping pipe or sagged pipe but the "lake" behind the sludge built up over time.

rconner · Dec 15, 2016

Thanks LI for the link to more information (that the OP and some others were aware of concerning this "design" and issue - I had no idea this was a double, triple or more post, with dialogue that has gone on now for more than two months, and I had alas previously read only what I see here on this current thread!
ajk1, I'm hoping you might thus forgive my naivete a little under these circumstances, and I have now scanned one earlier though lengthy thread at the link LI provided. It appears the now "hung" line was indeed installed and backfilled originally/likely up to near the bottom of the slab more than 50 years ago. While the soils appeared described as "swamp" in one post and "10 blows" in another, I am guessing the 10 blows is likely referring to the soil in the last few feet, and the swampy stuff (I'm guessing with this description could be of lesser stability) may be down below? There was also an inference that you have had some access to 1965 project drawings, but maybe not the actual hanger nor material/corrosion protection scheme design details (the latter maybe handled by some Canadian or NYC "municipal code"?) Forgive me also but I really haven't seen any "standard" hanger details or spacing for buried pipes hung below slabs, that in the case of drainage I guess could be at variable distances below said slabs (and with thus variable loads on the hangers and pipe, might sort of defy a standafrd spacing in this case). I noticed some posts alluded to potentially higher than normal buried earth loads, along with the weight of pipe and water in this situation. I believe what they attempted to describe was basically "inverse arch action" of the soil on the top surface of the backfill soil pressing down even more on the top surface of the pipe as the looser soils down below settle naturally or otherwise below. Indeed access to the soil borings to deeper depths, and likewise the hanger details might be helpful in the condition assessment and remediation efforts.
While it appears you have some confidence in the original/basic design, I trust you will also examine the as-built hardware, spacing and pipe etc in the case of planned excavation, lest any more involved remediation or replacement is necessary. While even old gray iron in good condition is quite strong and well-made caulk joints have some flexibility, they do have limits. Let us know what you find.

ajk1 · Dec 15, 2016

Thanks all for your comments. Much appreciated.
I did not put a link to earlier posts because I do not know how. We older guys are sometimes not as prolific with the computer as younger people. If you can tell me how to do that, that would be a positive contribution to my skills set.
I appreciate everyone's time responding to this. I particularly appreciate PEDARRIN2 (Mechanical) and Edstainless's responses, as their response directly address the question that I asked in the beginning, namely "does it have to be fixed", and the reasons why are clearly and cocisely stated. Their answer is yes it does. I will proceed on that basis. The pipe and hangers will be examined for corrosion when we dig down and expose them, and we will determine the cause of the sag. To date the videos of the pipe show no damage to the pipe in the areas where the camera could see, but the camera could not see underwater. We are currently looking at an innovative scheme for pumping the water out so that the camera can see the pipe in that area, but have not yet decided if we will try to do so.

LittleInch · Dec 15, 2016

When you find the post you want to link, just go the web address in the top corner begins

http://eng-tips.....

Just click on it once, right click - choose copy then paste it directly into your post, [right click when in the post and choose paste].

It's certainly better to fix it properly - take a few photos and let us know what it looks like. If its that soft then it sounds like you could undermine the entire slab.... I don't envy you.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

EdStainless · Dec 15, 2016

There are two ways to get the water out.
The safest is to use a small line (plastic) with a weight on the end and go in and suck the water out.
You would want to flush quite a bit of clean water first so that you aren't trying to remove muck through a small tube.
The other option, if you are feeling lucky is to use air to blow the line out. The chances of this working depend on what side branches tie into this line (and where the water and air might go). This is the 'cowboy' method.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

rconner · Dec 15, 2016

[just to answer a question that may have been sort of left hanging so to speak from the prior thread I saw - you will of course need appropriate protocol and trench protection for anyone entering this excavation - can see requirements e.g. with keyword search of OSHA excavation safety etc or other AHJ]

ajk1 · Dec 16, 2016

Thanks everyone for your help.

bimr · Dec 19, 2016

Not an uncommon problem. The only fix is to replace the pipe. Consider adding a clean out above grade to make the period cleaning easier.

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