PVC Pipe Deflection - Egging 8

[sewerratt]

I have a newly installed 24" Ribbed PVC pipe that is not yet in service. It has about 15' of cover. In the pipe zone it has clean washed sand to 1 ft above the crown. The backfill above the pipe zone is a sandy clay compacted pretty tight, say 95% proctor, all the way up. The pipe was air tested but when a 5% mandrel was tried, it would not enter the pipe. Video inspection shows a consistent egging along the entire length of the pipeline. Short of digging it up and relaying it, are there other options to 'repair' the egging and get it back to near round?

 

[mrwhiteOH]

This is a never ending debate.

How much deflection can a thermoplastic pipe withstand without distress? You say it is a profile wall pipe, which makes this a much tougher question to answer since the amount of bending induced strain in the pipe wall is dependent on the profile. However, my general sense is that 5% is too restrictive, and the manufacturer claim of 30% is way too much.

From a theoretical standpoint, somewhere around about 20% is the maximum most profile wall thermoplstics can withstand without excessive wall strains. However, in field applications, using 7.5% mandrel tesing seems reasonable.

Next point, most research shows that, in the ground, there is no long-term deflection lag. In other words, the deflection after about 30 days is all the deflection the pipe will see.

All the discussion about pipe design, is difficult, if not impossible, to answer accurately for profile wall thermoplastics. The interaction of the soil, the differing geometries of the profiles, residual stresses and myriad other factors make for a tremendously complex problem. Most plastic pipe design theory is based on solid wall pipe(or worse yet, on elastic pipe theory like corrugated metal), not profile wall. The design methods that do exist are not terribly effective at estimating in-field deflections.

Please see Chapter 6 of Highway Design Manual by McGraw-Hill where I spend considerable time discussing plastic pipe design. (plugging my contribution if no one minds)

As far as installation, sand can be utilized very effectively as pipe backfill, indeed, it can be better that #57 since the 57's usually do not fill the corrugation valleys very effectively, and are the "forced" into the valley after load is applied. This typically results in some unavoidable pipe deflection. What is critical is to what compactive effort was the sand placed? If there was little to no compactive effort, consider yourself lucky that there wasn't even more deflection. Sand backfill to 95% modified proctor should work.

Now on to the actual question in the original posting. There are ways to re-round thermoplastic pipe. I would not recommend them. Your real options are to either require the pipe to be excavated and relaid, accept a cost reduction, or require a service/maintenance warranty for some time period. Since the specification required 5%, then that is what the contractor should have provided.

 

[rconner]

With all due respect, I am concerned about possible long-term ramifications of some comments made on this thread. If as some say a plastic pipe system is good for good long-term performance desired by an Owner even if short-term defection is far greater than 5%, why then have standards and a great many significant local specifications for many decades limit deflection?? Another reason for my skepticism was I once attended a paper/presentation many years ago where some researchers at least initially sort of lauded a very deep test installation (I think in Pennsylvania) of both corrugated and smooth-lined, profile-walled plastic pipes, and I believe then even postulated a theory that primarily inward compressive creeping behavior of the plastic (the pipe reportedly ended up being a smaller diameter than it started out!) helped to reduce the heavy earth load. It appeared the intent of this paper (and I later found out similar papers by the same folks in multiple other engineering forums) was to promote some sort of rather high strain plastic/new design behavior phenomena, and I guess perhaps even thereby the use of similar plastic pipes for deep cover installations. If I remember correctly, this particular test piping was VERY carefully bedded with very select materials and bedding/compaction was also VERY carefully inspected/tested, and I’ll note this pipeline at least short-term generally deflected LESS than 5%, I guess as a result of same.
I subsequently also attended a paper/presentation of some competitive (not the same) researchers a few years later, who said that while the original researchers promised some longer-term follow-up of this installation the original reports, it was they instead who along with some other folks walked through this same piping installation just a few years later, inspected this installation, and they were thus reporting on the actual longer-term results. They reported that while the line was not generally severely deflected, the line exhibited “several interesting conditions”, including: “1. Buckling of the inner liner of the smooth lined pipe.” And “2. Circumferential cracking at the inside crest of the corrugated pipe.” They noted that the various pipes in this installation apparently buckled in a couple different directions and/or manners, and they even described buckling as “severe”. There was apparently cracking near every joint of pipe, at least one location not near a joint nor even under the deepest cover, and water was observed infiltrating the very carefully installed test line. While I will note this test installation was of polyethylene pipe (normally most used for drainage), I am not sure exactly how any other similarly creeping plastics would be dependably insulated long-term from similar behaviors (and I also happened to notice above the reference/warning of “local buckling” also from stanier). The newer authors also stated at the end of the newer paper, “Experience from other installations of polyethylene drainage pipe indicate that this type of buckling occurs at more normal depths.”
With regard to the inference in posts that pipe deflections even up to 20% are from a theoretical standpoint somehow acceptable, and while I can’t remember all specific details of problems, I am further very skeptical (even if a plastic pipe WALL happened to survive without long-term bucking or cracking) that all joints would maintain their integrity in a pipeline subject to such magnitude, unpredictable wall movements.
With all due respect, and while I don’t doubt something close to this may have been accomplished in a controlled test installation somewhere, forgive me that I am also very skeptical of the general statement that in practice an installed, unpressurized gravity flexible plastic (subject to creep) pipe will all of a sudden simply stop deflecting say once it reaches 5% or 7.5% deflection in 30 days or less. I believe this is contrary to some experience (and maybe even common sense!)

While I guess I sort of understand the intent of after-the-fact sort of exculpatory statements that deflections far greater than 5% or 7.5% are acceptable (an attempt to help the parties at least initially sleep better, to prevent at least very early re-installation expense that must be borne by somebody, and/or at least initially prevent uncomfortable, embarrassing, and/or hostile contract disputes etc.??), in the long run I would nevertheless be very cautious based on what little I have seen/published to step outside this box/standard of care, lest one (as I heard a colleague of mine in forensic pipe work once say in perhaps a little of an overstatement, in referring I think to some similar very flexible pipe experiences) be in essence “burying their own Chernobyl”.
I suspect some or all of the actual papers concerning what I am talking about in this post can probably be obtained/accessed by anyone for their own reading/consideration at nominal cost, through e.g. ASCE or other publications service with a keyword search. I noticed also some time ago that are also some multiple interesting pictures of perhaps similar, apparently actual installed plastic pipe wall conditions now on the Caltrans site at

http://www.dot.ca.gov/hq/oppd/dib/dib83-5.htm#5-1-4

(e.g. see Section “5.1.4 Plastic Pipe”).
If anyone cannot independently find what they want in this regard, I would be happy to try to dig out some more detailed reference information to help locate – please let me know on this thread if this is needed.

 

[mrwhiteOH]

I must respectfully disagree with rconnor in stating the long term creep of thermoplastics is common sense. Reference is made to an extensive study by the Battelle Institute for the City of Columbus, Ohio in which the 30 day inspection recommendation is made. Further, work at Ohio University with thermoplastic pipe under 40 feet of cover also stabilized very rapidly. This is the same experience generated by Ohio DOT in-house research. The reason for this rapid stabilization is quite simple. Under constant load viscoelastic materials will creep. Under constant deformation they relieve themselves of stress (stress relaxation). This is the phenomenon exhibited in the PennDOT deep burial. It is real. When the side support of the soil is mobilized, the pipe exhibits circumferential shortening due to the load. This shortening will create a positive soil arch and effectivly result in stress relaxation of the thermoplastic. In the PennDOT study the measured stress at the crown of the pipe was approx. 30% of the free-field stress

I agree with rconnor that the limiting factor for pipe deflection is joint integrity not pipe performance. However, it is necessary to understand that the deflection limit is as much marketing as it is technical. It is also necessary to understand the ASTM process for developing standards. This is a consensus specification authoring body. If a specification can get enough positive votes it can pass. This is not to say that bad information is generated by ASTM, on the contrary it is a great organization. It is just information to better understand how the standards came to be. The history of the deflection limit is tied to unibell pvc pipe and their desire to break into the sabitary market in the 40's. At the time 15% deflection was deemed maximum for the pipe. A factor of safety of 2 was applied, hence the 7.5% limit. Since then, in an effort to introduce HDPE pipe to the market a deflection limit of 5% was proposed in order to be "better" than PVC. Somewhere along the line, specifiers have been convinced that the 5% is a structural limit.

From a design standpoint neither deflection nor load alone can accuratley predict the pipe performance. It is the strain derived from the combination of bending and compression. A pipe with high deflection but low compressive stress may have a long life, while a pipe with low deflection and very high compressive stress may exhibit wall cracking and buckling.

In regards to the PennDOT study it is important to understand that the pipe in question, which I have personally seen, is under 100 feet of cover. It was fully anticipated that the pipe would be highly distressed. This was a goal of the study, to understand the limit state of the pipe. Also, the orignial researchers authored a discussion on the second researchers work discrediting most of the conclusions.

The bottom line is that when installed well thermoplastics can perform well for a very long time. If installed poorly, it will not. Period. As I said in my first post, this very well could be a never ending debate. That said, I have offered my opinion based on 15-years of experience, almost exclusively with pipe mechanics, for the benefit of sewerratt. I did not mean to open this can of worms.

If anyone is interested in any of the research I reference, please e-mail me.

 

[stanier]

The problem with standards where it is by consensus is that plastics manufacturers see standards as a marketing tool. They invest heavily in getting members on committees to preserve and push their perspective. Some more aggressively than others. Government bodies and individuals can rarely provide the time( ie cost) to spend at committee meetings.

I am suspicious of any manufacturers claims as they do not provide guarantees. They can always blame the contractor. You destroy the evidence when you exhume the pipe.

AS 2566 is written to provide a conservative design where the soil/embedment carries a large proportion of the load.

Lars Eric Janson has provided data supporting the deflection of pipes in the longer term. The problem with initial deflection (ovality) is that the soil is also oval and can thus deform more readily and buckle under external loads. If egging has taken place and the top soil is compacted then there is a tendency to square the corners of the pipe. This subjects the pipe to far higher strain rates that calculated for the oval of the egged pipe. This does result in failures at these corners.

 

[rconner]

mrwhiteOH,
I guess we can agree to disagree that high compressive/bending/or whatever strains and deflections can be potentially injurious to the long-term integrity of systems such as discussed; however, I am not sure exactly how a specifying/inspecting engineer, Owner, or tax/rate payer for that matter are protected by such less unconservative opinion if/when? any one or more of these multiple bad things happens (particularly if it happens beyond say a one year contractor warrantee period). We can also agree to disagree that as you apparently indicate there is some sort of magical quick stop to deflection, say within a 30 day time period; however, I believe much more notable folks than I, including a gentleman from my alma mater named Spangler (as early as 1941) and perhaps many others, including one Amster Howard et al have also found/taught (I think including crawl-thru examination of a great many installed actual flexible pipelines all over the place) that at least some movements (manifested also as increasing deflection) and as a result of relative soil/prism movements/natural consolidation etc., continues over a very LONG period of time. I believe they have also found/said this movement is generally more substantial when cohesive soils are involved than non-cohesive even when rather highly compacted, and there appears to be even some comparative data indicating that pipes like plastics move at least a little more than steel etc. I suspect many other folks dealing with sewer pipes (normally deep) would probably also agree earth load can be rather heavy and unrelenting (and the pipe wall effects it influences) over 50 or 100 years or so etc.!
With regard to your comment that the original researcher had "authored a discussion on the second researchers work discrediting most of the conclusions", I was not aware of this and hope at some point I get an opportunity to read this more recent work alongside the original. It is obvious you are quite close to this situation, as you reported you had even seen this piping. In this regard, could you please confirm at least whether or not the specific observations of the second researcher I thought I remembered/and repeated above are correct (in other words, did this short pipeline in fact develop over a period of apparently very few years many various cracks, buckles, and a leak etc. that as far as I know were not reported in the initial paper?)
Frankly, I do not remember from the original presentation I attended that they predicted piping would in any fashion fail or as you say comments to the effect, "It was fully anticipated that the pipe would be highly distressed...". In fact, right or wrong I got the entirely opposite impression from the presentation I saw (I even assumed that some disbelief on the part of the second investigative group when they similarly heard of this pipeline and alleged load forgiving theory etc. may have sparked the competitors interest/decisions to go look at the installation themselves!) As this was admittedly many years ago, I guess I will apologize upfront for any fuzzy memory or mischaracterization if what you say is true and not some attempt for whatever reason to re-write history. I of course also did not state that joint concerns are "the" only ones with this pipe, just one among the others discussed.
Some comments in above multiple posts related to these standards sadden me. I personally think that what appears in standards or specifications for that matter should be reasonably understandable and generally also for performance/technical and practical manufacturing or measuring/testing etc. reasons, and while no standard nor piping material is perfect one would think that above all the intent of any business in it for the very long haul should be that the result will work in practical application (and not be part or centerpiece of some big marketing shell/blame game when there are problems)! It makes one wonder also who is looking out for maybe now huge numbers of pipe specifiers/buyers who might not have the access or be able to afford the benefit/experience of either more critical/specialized engineering analyses (perhaps looking beyond the standards), nor armies of strong-willed jobsite inspectors to make sure even generally well-intentioned contractors spend adequate contract effort/money on the extremely high-quality construction that is apparently required for such pipes (and not on other things)? Perhaps extreme levels of price competition between manufacturers, contractors, and maybe even engineering service providers could also be unhealthy. In any case, maybe there are reasons for/value of some "never ending debates".

"The bitter taste of poor quality remains long after the sweetness of low prices is forgotten."

 

[rossmail]

its poor instalation ,take the top off ,pack the sides and the pipe springs back up.

 

[boo1]

Is it the product or the installation (or both)?

 

[rconner]

Suffice it to say the piping product at least in this case did not have adequate effective stiffness to meet the installed specifications, at least with the initial installation conditions and inspection provided. It would appear also that if all parties are not able to agree or work such matters out, any party with “deep pockets” (including those mentioned) might eventually be drawn in to the fray and the quality of expert witnesses and attorneys might then also be a factor!! I suspect all would agree it is probably much smoother sailing if the pipe consistently meets the installed specifications with the prevalent quality of soil conditions, installation, and inspection etc.