Problem w/ Steel Casing Bore &Gravity Sewer (NE1 up for a challenge?)

[itsleighton]

We have performed a 320' dry auger bore in soft, sandy clay. The steel casing is 18" that carries a 12" yelomine (PVC) gravity sewer line between two manholes under a DOT highway. Site conditions are as follows:

downhill manhole at Station 0+00 is flowing very good-- fast- no problems.

Uphill manhole at station 3+20 is surcharged and is flowing at about 6-7 inches off the bottom of the invert at less than an inch a minute. The difference in the elevations of the uphill and the downhill manhole inverts is 1.5' for a slope of approximately .5%

So, needless to say, there is a variation in the slope somewhere either in the carrier pipe, the casing or both-

We did a sewer video of the 12" line and what we saw was that the carrier pipe is basically shaped like an "M". Directly uphill from the bottom manhole, it flows very fast, after about 16', it visibly drops and the flow begins to slow. It remains in this condition unitl 232'. then the flow picks up considerably until approximately 290' then it remains slow until the uphill manhole and the cam stops at 328.

Over a run of 320+ feet, it appears the pipe is shaped like an "M". It goes up, down, up again, then down until it reaches the manhole at the end.

This bore is 320 feet long. Horizontally, we came in right on target- Not even an inch off. Our target slope was .4% I have done alot of bores and over 320', I believe the bore is a great success.

There is a problem with the slope of either the casing or the carrier pipe or both.

My guesses are as follows in order of what I think is the problem

1) The carrier pipe is bent from the force of the excavator pushing it from behind. The casing spacers are not eccentric-- meaning they are symmetrical and roughly 1" casing spacers all the way around.

2) There is water in the casing that entered from a heavy rain before the ends of the casing was sealed off (We poured the downhill manhole first) causing the carrier pipe to float.

3) Some of the casing spacers have come off and have created an obstacle possibly jumbling up and causing a variation in elevations.

4) the casing did not go straight during the bore. Most unlikely IMO because 18" steel casing does not bend as fast as the pipe is showing to be bent. The clay is soft thus unlikely to cause the pipe to veer up or down due to more friction on one side than the other. The other reason I dont think this is the case is because we are right on target horizontally.

We are about to spend a considerable amount of money and time in an attempt to remedy the problem. Our client is not buying it as it is. I cannot open cut the casing since it travels under a major highway, but we can pothole it in the medians and cut open the casing to try and get an idea of what is going on inside of it.

I know the video is long, but I would like for many of the sharp minds and many years experience to take a look at this. If there is any input anyone can give to help me fix this problem, I would greatly appreciate it.

A youtube link to the video is below. Feedback is greatly appreciated. TIA

http://www.youtube.com/watch?v=0N5wZVEsW-w

It may take a while for youtube to process this file, so if it is not available now (5:08PM) give a it a few minutes.

 

[semo]

Looks like no one wants to tackle this one. Probably because anything said is strictly speculation and not enough information was given on the installation. Was RACI type spacers used (or wooden blocks banded to the pipe)? What spacing were the spacers placed at?

You will have to pull the carrier pipe or get some kind of camera between it and the casing to determine what has occurred.

My initial guess would be that the spacers were not installed at the proper distances and allowing the pipe to sag; but, I wasn't there to see the installation. Depending on what kind of spacers, it's possible they could have slipped as well.

If it took enough pressure pushing the pipe to bend it, something was wrong and should have been noted during the installation. The pipe should have slid in easily.

I think you need to follow your instinct and do some more investigation.

 

[cvg]

1) your casing is way too small for installing a relatively flat sloped gravity pipe. One inch annular space does not allow any adjustment in the carrier pipe to maintain proper slope. typical specs require "The inside diameter of the steel liner plate or steel casing shall be a minimum of 12 inches larger than the largest outside diameter of the carrier pipe..."

2) with such a small space between the casing and the carrier pipe, it is unlikely that misaligned casing spacers could cause such a variation in slope of the carrier pipe

3)It is likely that the casing pipe was either bent or not installed to the proper line and grade, regardless of how close it came out at the receiving pit. Pushing the casing with an excavator is not a generally accepted method of jacking...

4) the CCTV should have been run through the casing prior to installing the carrier pipe.

 

[itsleighton]

Thanks for your replies-- I appreciate the time and thought-- CVG, just so you know, we were not pushing the casing thru with an excavator-- we jacked the casing with the bore machine in the manner it was intended-- it was the carrier pipe we pushed thru the casing with the excavator-- which is pretty standard in this part of the world.

Just in case anyone was wondering-- we are digging up the casing in the median and cutting into it next week to see waht we can see- I will post my findings if they are significant if anyone is curious to know.

 

[rconner]

While I of course don't know exactly what alignment your casing is now in, I believe (in accordance of course with ol' Archimedes) at least some areas of an empty installed 12" pvc or other plastic gravity sewer pipeline, or even one carrying much less than full flow, may indeed likely float when the annular space in a casing gets filled with ground water. Where areas of the pipe etc. become full(er), due to sags etc. or whatever, they will not tend to flow as much, and may even sink if the pipe becomes near full.
What else do you need to know?

 

[rconner]

(Oops, I meant to say they won't tend to "float" as much in the casing when the pipe becoems full [The submerged weight of the non-full plastic pipe is much less than the weight of water displaced, whereas if a pvc pipe were full it might be very slightly negative buoyant.])

 

[rconner]

(Oops, I meant to say they won't tend to "float" as much in the casing when the pipe becomes full [The submerged weight of the non-full plastic pipe is very much less than the weight of water displaced, whereas if a pvc pipe were full it might be very slightly negative buoyant.])

 

[rconner]

[see also linked site e.g. mentioned at

http://www.wef.org/technicaldiscussions/Topic11376-11-1.aspx

]

 

[bimr]

I agree with CVG, you should have had a 24" diameter casing pipe (3/8" thick) for a 12" sewer.

yelomine pipe is also not used for gravity sewers, yelomine is pressure pipe. I am thinking that the yelomine pipe is too flexible to use in this application. You can see in the picture that the pipe is too flexible.

http://www.intpipe.com/files/yelomine_409045e.pdf

You probably need pipe supports on 10" centers and both sides of each joint. You probably need an expansion joint as well.

The yelomine is also lightweight, so it will float like the other posters have stated.

An 18" casing is also not rigid enough to go that distance. If you put a laser down the casing, it would probably look like spaghetti. Larger diameter casings have more ridgidity than the smaller stuff.

You probably would be better off just filling the holes and starting over with a better design staff and an experienced field crew. If you would have had either one of them, you probably would not be in this position.

 

[rconner]

This is an interesting thread. I certainly agree with the (at least hypothetical) principle offered by others on this thread that a smaller casing might be easier to install in a non-straight profile than a larger diameter casing with much greater axial stiffness.
However, I also know that AWWA Manual (of Water Supply Practices) M23, "PVC Pipe - Design and Installation" on the other hand says 18-20 inch casing pipe is "Recommended" for 12" pvc pipe.
For whatever it is worth, I believe the AWWA Manual M41 or C600 standard guidelines talking about casings for 12" ductile iron pipe (and as approved by other committees etc.) does in effect direct one to use ~24" casing pipe.

 

[cvg]

first, these (AWWA) requirements are for minimum casing sizes - many agencies have discovered that larger casings are better for a variety of reasons.

second, precise alignment and slope is not as important with pressurized waterlines as is presumed to be the case with the AWWA guidelines. Slope is much more critical in gravity lines which either requires greater accuracy on the casing pipe, or some means to adjust the slope on the carrier pipe to meet the designers requirements for slope.

According to our local standards, the "minimum" casing size for the 12" pvc would be closer to 28" which accounts for the pipe wall thickness and the thickness of the bell and spigot joints.

 

[rconner]

I think we are all in agreement that what you are saying is pretty much understood, but while not speaking to any wisdom of the guidance the word "Minimum" does not now appear in the Table 7-1 of "Recommended..." casing sizes for various size pvc pipes in AWWA M23. Incidentally, I suspect larger casing sizes are probably also most helpful (as long as the pipes are not grouted in!) if in the future someone wants to remove and/or upsize the line!

 

[itsleighton]

Just to give those who were curious an update-- we potholed the casing in different spots and took shots. The casing is straight- horizontally and vertically-- We cut into the casing and it had filled up with water causing the pipe to float- when we cut it, the water flowed out and the pipe dropped- the manhole is now flowing at the correct speed and depth.

I appreciate the insight. Even though the points some made were not the case, I will keep them in mind next time we are told to do a bore that long with that size casing and carrier pipe type. Again-- thanks for your replies and interest.

 

[bimr]

I am glad to see that your situation worked itself out.

However, it is still somewhat surprising to hear that the casing is straight horizontally and vertically over that distance. Must be an excellent Contractor.

Also, how do you plan to keep the pipe centered in the casing when it fills up with water again?

 

[semo]

I've always understood that the minimum casing ID was to provide 2" clearance all around the maximum diameter of the carrier pipe. That is minimum and works fine on waterline; but, I think it smart to use larger casing on a sloped pipe in case the casing is not installed correctly.

I too am glad it worked out without considerable expense.

Towards Bimr's last post, we had older designs which called for the void between the carrier and casing to be filled with sand. A sand blaster hose can be pushed into the void to the center, sand blown and the hose pulled out as the void fills. The ends were then grouted closed.

Newer designs use casing spacers spaced appropriately to prevent the pipe from floating or sagging then end seals used at the ends. Sounds like the spacers were inadequate in number or were cut at the top allowing part of the pipe to float up unevenly. I would fill the void with sand to prevent the pipe from floating again. Sand is easier to remove with a vac truck than grout is.

 

[rconner]

While I was not involved in deliberations that I suspect went on and that probably involved much thought concerning many design, installation, and maybe even future aspects (and likely based on some user experience existing even before that time with gray cast iron and ductile cast iron water pipelines), for whatever it is worth I believe however that consensus AWWA standard C600 and subsequent AWWA manual M41 for ductile iron pipe along with some other guidance for such installations have contained the wording, “The casing pipe should be 6–8 in. (150–200 mm) larger than the outside diameter of the ductile-iron pipe bells.” [This would appear at least to allow the user of ductile iron piping to sort of base the sizing on the actual ductile iron pipe and joint etc. they are e.g. asking the Contractor to push or pull etc. through the casing on that specific job.] Special structures that ductile iron pipelines could be run through can of course theoretically be some smaller, and obviously can be even very much larger, witness “pipe galleries” in plants etc., utility corridors, or “utilidors” etc. where that is desired for whatever purposes of installation, maintenance, or future expansion etc., provided the pipes are sufficiently supported/fastened/restrained etc. for such passages.

With regard to putting “sand” around a pipe in a casing, I believe this has been done many times over the years at least with iron piping, and I wonder incidentally if in some cases when this is used if it might even be possible once any end bulkheads are removed to in effect “wash” the sand out one end with a simple water hose (I guess that might be easiest facilitated if there were a slight design vertical slope to the casing, at least if that were feasible for the pushing/guidance operation of the casing etc.) if this were desired for any future maintenance or upsizing etc.?