HDPE Anchor Blocks

[Water12]

thread164-290415

I was reviewing the above referenced thread which talks about HDPE anchor blocks to resist poisson pullout effects when transitioning from HDPE to an unrestrained joint pipe system.

We are designing approximately 6,000 lf of 18" HDPE water main that will connect at each end to unrestrained Cast Iron. I have some of the same questions as the original poster and haven't really been able to find any good answers.

Has anyone been able to determing why the equation for the pullout force doesn't factor in the restraint provided by soil friction?

As thr original poster asked, does a 5 foot section of pipe really produce the same pullout force as a 1000 foot section?

Do thermal effects need to be considered when a pipe is restrained underground between two anchor blocks? Or is that force mitigated by the soil restraint and only a factor for aboveground applications?

Any insight would be appreciated.

Thanks

 

[bimr]

A 1000 foot section of pipe has a significantly larger pullout force than a 5 foot section of pipe.

A length of pipe that is restrained or anchored on both ends and placed on a frictionless surface will exhibit a substantially different reaction to temperature change than an unrestrained pipe. If the pipe is restrained in a straight line between two points and the temperature decreases, the pipe will attempt to decrease in length. Because of the end restraints, a length change is not possible, so a tensile stress is created in the longitudinal direction along the pipe.

Suggest you review:

Technical Note 814-TN
Engineering Considerations for
Temperature Change

http://www.performancepipe.com/en-us/Documents/PP814-TN Thermal Effects.PDF?Redirect=1

 

[Water12]

Hello bimr,

I have reviewed the documents you referenced and I find them a little puzzling, hence me questions.

You say that a 1000ft piece of pipe would have more pullout force than a 5ft piece of pipe, however length is not a component of the pullout equation. How then could that be true?

The paragraph you quoted concerning the pipe being restrained and thermal effects also seems to be inaccurate because a pipe buried underground is not a frictionless surface.

 

[77JQX]

It's my understanding that the force is not a function of pipe length, but the total distance moved (if free to move) is a function of pipe length.

In calculating restrained lengths for conventional applications, we make the restrained (rigid) length long enough to prevent any movement. Soil friction provides the counteracting force, and prevents movement anywhere within the restrained length.

The OP's question is whether soil friction will also prevent movement of HDPE pipe. My guess is that the answer is no, because the HDPE is not rigid pipe - it can contract. At the pipe end, the cumulative soil friction is insufficient to prevent pipe contraction. Without restraint against the full force, the pipe will shorten.

The amount of pipe shortening will be reduced by soil friction. If one end is free, the tension in the pipe would increase from zero at the end to the poisson force some distance from the end, and the amount of shortening would be integrated along that distance. I don't know how to calculate that.

As an approximation, I've calculated the restrained length for rigid pipe, the calculated the poisson shortening for free (not affected by soil friction) HDPE pipe. I divide this value by 2, assuming soil friction affects the shortening by a linear amount. Then I restrained the pipe for the full force.

 

[bimr]

re "You say that a 1000ft piece of pipe would have more pullout force than a 5ft piece of pipe, however length is not a component of the pullout equation."

Most of the time, you will be calculating the force for restraint of only the pipe ends, not the entire length of pipe.

"A buried pipe is generally well restrained by soil friction along its length, and with moderate or low temperature change, soil friction alone is usually sufficient to prevent dimensional change and expansion movement. Therefore, a buried polyethylene pipe will usually experience a change in internal stress rather than dimensional change and movement. A very significant temperature decrease may exceed soil friction restraint, and apply contraction thrust loads to pipeline appurtenances. Thrust blocks for underground pipelines are usually not required unless great temperature change is anticipated."

 

[BigInch]

Stress is the same for a 1000 or 5 ft segment, due to Poisson's, or thermal effects. Force is stress x cross-sectional area of material.

How that force is distributed into the soil is dependent on length. If sufficient length is available such that the sum of soil friction and cohesion between it and the pipe x pipe's surface area >= force the pipe will become "virtually" anchored at that point. Until, or if, you reach that point along the pipe's length, movement of the pipe will occur proportional to the integral of stress x length of pipe being considered.

From "BigInch's Extremely simple theory of everything."

 

[Water12]

77JQX:

Regarding your quote below. When you say you calculated the restrained length for rigid pipe, are you using something like the DIPRA equations for ductile iron pipe? If so, what value are you using for unit frictional resistance? Are you then using the restrained length equation for a dead end to determine the required restrained length?

I'm a little confused on the method your using.

"As an approximation, I've calculated the restrained length for rigid pipe, the calculated the poisson shortening for free (not affected by soil friction) HDPE pipe. I divide this value by 2, assuming soil friction affects the shortening by a linear amount. Then I restrained the pipe for the full force."

 

[77JQX]

Water12 - Yes, you've got it right. I used Dipra calcs to determine the required restrained length for PE encased pipe for my soil conditions and dead end conditions at a pressure sufficient to give the same dead end force as the poisson force. I then and pretended that the required restrained length was free to move. I then calculated the movement that would occur assuming there was no soil friction over that length.

Since the "free" length of the installed pipe would not actually be free over its entire length, but would instead be fixed by soil friction at one end, I simply divided the result by 2. My intent was to get at a feeling of the magnitude of the movement. And the result was more than I was comfortable with, so I restrained the pipe for no movement.

Does this make sense? My conclusion was that the manfacturers were correct to recommend restraint against poisson shortening, so I took their advice. If, on the other hand, the conclusion of my exercise went contrary to the manufacturer's recommendation, I think I would still have restrained trhe pipe, because they have more experience than I do. I was really an exercise is showing to myself that the effort and expense were justified.

 

[Water12]

77JQX- I think I understand what you are saying. You were just trying to determine what the movement would be from poisson effects if there was no restraint of the pipe.

Did you also consider thermal effects from the pipe being restrained between two anchors? This is mentioned on page 2 of the 814-TN bulletin from Performance pipe and also on page 7 of PPI-21 (attached below). Page 12-13 of PPI-21 also talks about the restraint provided by soil but it is not really clear whether you can use this soil restraint to calculate a required restrained length to resist thermal effects.

 

[77JQX]

Water12 - Sort of. What I'm saying, is that soil friction is integrated over a length of pipe. For a rigid, restrained pipe we can calculate the length of pipe required to resist thrust forces. The terminal end can't move because the pipe is rigid and the integrated friction over the restrained length comes into play. Contracting and expanding pipes are different. They are different because at the terminal end, the integrated soil friction is zero, and the end can move due to internal contraction (or expansion). Only at some distance from the termination is the integrated friction sufficient to prevent movement due to internal contraction. Obvoiusly, the further away from the terminal end you are, the greater the integrated friction, and the less movement.

So based on this, I think it is wrong to think of a "restrained length" of HDPE sufficient to resist these forces. The forces will be manifested at the pipe termination, even if there are thousands of feet of buried pipe. The question is, how much will the termination move due to the combined thermal and poisson forces.

And my conclusion has been to provide thrust restraint against the full force developed. Does that help?

 

[Water12]

77JQX - OK, I see what you are saying now.

Concerning the thermal effects, did you determine the stress by multiplying the effective modulus by the coefficient of expansion/contraction by the change in temperature? (Standard thermal expansion/contraction equation for restrained conditions)

Did you then combine your thermal force and poisson force and size your anchor blocks for that total force?

This all makes sense to me but the various literature you read seems to always mention that a buried pipe is restrained by soil friction along its length and the soil friction alone is enough to resist forces from moderate temperature change. Hence my confusion about whether thermal effects need to be consderered and what effects soil restraint has.

 

[77JQX]

Water12 - First, yes, I use modulus, temperature change, and coefficient of expansion/contraction. And yes, designed for the combined forces. And yes, for many cases (say, buried pipe handling water from a well) there is little temperature change and not a big deal from pipe movement. Unless your well is geothermal. To me, its worth checking, and restraining against.

 

[bimr]

Suggest you invest in Moser's book, Buried Pipe Design. It should answer your questions:

http://www.amazon.com/Buried-Pipe-Design-2nd-Moser/dp/0070435030

 

[Water12]

77JQX- This design is for a potable water distribution system. Temperature of the treated water can get as low as 35 degrees during winter months. So I have been basing calculations on a 20 degree temperature difference (considering trench to be around 55 at 5 feet of cover).

It sounds like I am following the same process as you, I was just questioning whether thermal expansion/contraction could be considered to be restrained by soil friction.

 

[rconner]

You mentioned you were some concerned of a non-consideration of thermal effects, and you are coming up with some high anchor loads when you attempt to include same. I couldn’t help but notice that you were apparently in your calculations also using a “delta T” of only 20, I suspect meant in degrees Fahrenheit, and also noticed you would thereafter have fairly cold (apparently 35 degrees F.) conditions in the water and line.

In this regard I noticed on page 2 of the site

http://www.ips-frp.com/B5TechBulletins/TechBulletins/HDPE Versus FRP.pdf,

“It should be noted that surface temperatures in HDPE pipe exposed to direct sunlight exceed 150ºF.” While it might not get that hot depending on where you are and where and I would also suspect the full range of delta T indicated might well not be applicable to certain anchorages, time frames or assumptions of movement, there can in fact be a quite high aggregate/actual delta T to a length of pipe, at least from the time/point a length of pipe is snaked into a trench or hole in the summertime, and then cooled down with variable temperature surface source water, or combination of weather and insulating cover, in the winter.

Everyone have a good weekend.