Consequences of a Deflected Pipe? 3

[MarshfieldTimC]

A developer recently installed a new section of sanitary sewer in our City. On one segment of 8-inch PVC sewer (8-feet deep), the pipe has failed a deflection test (5% mandrel gets stuck). We televised the pipe and it looks good, other than a few scratches due to repeated mandrel pulling attempts (including the use of a vibrating mandrel, which would only go through while vibrating).

My questions/concerns include:
What are the long term consequences of accepting a deflected pipe such as this?
How much service life could be lost due to the deflection?
Are scratches on the inside pipe walls a concern?

Any comments or opinions are appreciated!

 

[bimr]

Flexible pipe design specifies the allowable deflection, and specifies the soil embedment necessary to ensure the pipe will not deflect more than its allowable limit. This is done by specifying a trench width that is appropriate for the native soil condition, using the proper granular backfill materials and compaction effort, and using the high quality flexible pipe products.

Excessive deflection affects the pipe’s hydraulic performance such as decreased flow velocity and sediment deposition; but it may lead to harmful strain levels in the pipe wall that can trigger structural failure modes such as cracking, inverse curvature, and wall buckling. Excessive deflection may also compromise the pipe joint performance which can lead to exfiltration or infiltration, and the loss of backfill material causing sinkholes.

The deflection indicates that something was done incorrectly in the installation and should be repaired.

 

[cvg]

excessive deflection may also indicate there is a sag in the pipe. sags really cannot be easily detected by either mandrel or video inspection. however, a sag will definitely reduce your capacity.

 

[rconner]

You have gotten very good responses. You are not the Lone Ranger in this happening -- many Engineers specifications that you may now find online specify exactly what is to take place when such pipe/bedding/backfill fails a deflection test (and I've not read a one saying that the Engineer should then try to find a reason/excuse? to accept!) Incidentally, and for whatever reasons, some also prohibit pulling grinding/vibrating gizmos pulled through in attempts to re-round an overly deflected pipe. If there is poor soil outside the pipe, I could see it may not do much at least long term good to shake it up! Also, scratching or gouging a pipe material that has very low fracture toughness to begin with will not do it any good. The question are there also "sags" is also a good one, as sags per many references can also contribute to debris, odor and hydrogen sulphide generation etc.

 

[MarshfieldTimC]

Our spec required clear crushed stone 1/2-inch minus as bedding (6-inches below) and cover (24-inches above) with a 12-inch sidewall clearance. The stone was hand tamped at the pipe's springline and plate compacted at 12-inches and 24-inches above the pipe. Native clay/silt soils above the cover material were compacted with a hoe-pac.
The flowline of this pipe appears to be unaffected, not sagging.
This sewer is presently a dead-end/'start' line with very little potential for extension, so I am not concerned about capacity.
If anyone knows of a good spec that addresses requirements or steps-to-take due to a failed mandrel test I would enjoy reading this.

 

[cvg]

open graded stone placed against silt or clay can be problematic. typically that would not be allowed because when water gets in the trench, it can flow through the rock and then the silt or clay particles can migrate into the voids in the stone bedding and cause settlement. sand bedding would have been better for bedding and pipe zone material. otherwise, a filter fabric should have been used. the recommended steps for the failed mandrel test are simple: remove and replace the pipe.

 

[amlinerrichard]

If they won't remove and replace, you could say we won't pay for it. Then put in a CIPP liner to reinforce the existing pipe.

 

[mknighton]

I know this is an old thread, but since the contractors appear to have followed the bedding/backfill spec, can the engineer still hold them at fault for the deflected pipe?

 

[rconner]

I don't know enough about the specifics of this work to answer this latest question (however, I would think a successful mandrel test was likely required). I have noticed the presentation now posted on the organization's website at

http://www.facers.org/wp-content/uploads/2011/01/Role_Risk_Responsibility.pdf​.

This presentation reveals to the viewers that the standard installation specifications for thermoplastic pipes (i.e. hdpe and pvc et al, with relatively low long term stiffness) may now actually put a great deal of responsibility on the ENGINEER. [It is perhaps not hard to guess what parties may have been most instrumental in that spec development?]
While I guess I should let the presentation speak for itself, it also explains that various different types of pipe that might be "substituted" are not equal in their interactions with the soil, and maintaining their shape etc in practical circumstances (as perhaps some might think, or wont). The Engineer should take a very active role to assure a good installation. See the very last/summary slide 53:
​
"Engineers design, specify, require and ENFORCE
The Engineer has a sworn duty to protect the public
health safety and welfare
Forgetting any of the above results in the Engineers
assuming the role of the Responsible Party with all
the risk, liability and consequences thereto."

 

[MarshfieldTimC]

We recently resolved the issue - decided to excavate the deflected pipe and 'cap' it to prevent further deflection and improve the load bearing. We found the contractor used excessive compaction force which caused the pipe to deflect. With clear stone bedding/cover, groundwater can flow along the outside of the pipe so we require clay trench plugs (placing our native dense clay material in leu of the stone, approx. 6 LF long) to slow the groundwater flow. Apparently the contractor compacted this clay trench plug with a hoepack (vibratory plate on a backhoe). To remedy we cut an HDPE pipe in half longitudinally and placed it over the deflected area. The HDPE fit like a glove and actually snapped onto the PVC, essentially removing the deflection. We did not have to cut out/remove any of the PVC and thus did not disturb the grade/flowline. The mandrel slid through the pipe easily. We were very happy with this remedy.

 

[peter246]

thanks for following up! the post is an interesting read...

 

[stanier]

Australian standard AS 2566 provides the guidance for design and construction of flexible pipelines. Iowa State University and Books by Watkins also cover the subject.

The thing needed to give consideration to are not only deflection but strain, stress, combined loading and buckling. Being a flexible the soil embedment and native soil properties are important as they provides a composite structure.

ôThe beautiful thing about learning is that no one can take it away from you.ö
---B.B. King

http://waterhammer.hopout.com.au/