Thrust Block Factor of Safety 1

[CPENG78]

Hi Everyone,
I've searched if there were similar posts with my question but couldn't finding anything specific, other than on a thread that was closed so here it goes.

When it comes to thrust block sizing, NFPA 24 calculation methodology includes a Factor of in their equation and at least NFPA 24 2010 refers to that factor of safety as "...usually 1.5" Can someone point me to literature as to what determines what that factor of safety should be? Is it dependent on the quality of data collected during the soil investigation? Or is it a designer's call based on engineering judgement? Thanks for your help.

 

[fel3]

I'm not sure where the 1.5 safety factor originated, but I've seen it in several documents that deal with thrust restraint design. For example, the DIPRA publication "Thrust Restraint Design for Ductile Iron Pipe" uses it . You can find the DIPRA publication here:

https://www.dipra.org/ductile-iron-pipe-resources/technical-publications/design

(it's the fifth document, unless you want it in French, then it's the sixth document).

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[bimr]

Is "no one knows", a good answer?

"The use of thrust blocks should be considered with great care. They are only as good as the stability of the soils used for reaction backing. In locations where the soils may be disturbed by future excavations (yard piping, treatment plant sites, busy streets, etc.), reliance on thrust blocks (particularly for large diameters and high-pressure pipeline systems) is not a very good idea. Instead, the use of restrained (lugged or harnessed) joints and trench friction is a better approach."

from Pumping Station Design by Garr Jones

Thrust blocks are no longer considered to be good piping practice. This is because thrust blocks are normally paired with undetermined soil properties and poor construction techniques. These uncertainties lead to unknown performance parameters for the piping system.

If you read the fine print general criteria from DIPRA, it states thrust blocks should be installed "Undisturbed Soil". Is that realistic?

In addition, DIPRA states "Although the above bearing strength values have been used successfully in the design of thrust blocks and are considered to be conservative, their accuracy is totally dependent on accurate soil identification and evaluation. The ultimate responsibility for selecting the proper bearing strength of a particular soil type must rest with the design engineer."

"These allowable bearing loads are estimates only, for horizontal thrusts, and for pipe buried 2 ft deep or deeper. When doubt exists, safe bearing loads should be established by soil bearing tests."

From Buried Pipe by AP Moser

If time you ask geotech for a horizontal bearing pressure to use in thrust block design, they will put so many factors of safety on it you end up designing your block for a soil with the consistency of custard.

Finally, what many Contractors do, instead of constructing a thrust block, they will throw a bag or two of cement behind the pipe and say that it is good enough.

The use of mechanical restrained joints instead of thrust blocks provides an easier, more predictable and safer installation

 

[jgailla]

What bimr said.

 

[CPENG78]

Thank you all. Your comments make complete sense. I was curious about the origin of that initial "1.5". And yes acquainted with placement of the block in undisturbed soil which is ok in some but not necessarily in others. While the use of blocks may be a bit out of date, it is still part of many water and/or city agencies and ends up coming up as part of the review process. Specially in projects in the public right of way. So maybe something to change in our industry as we move towards restrained joints. Greatly appreciate your input.