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waterline kickblock design 1

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craigburger

Geotechnical
Sep 16, 2002
1
I've been asked by a civil engineer and municipality to provide criteria for design of kickblocks for water distribution pipes.
The question came up because the poor soils on the site likely don't have a bearing capacity of 3,000 psf and the public works department standard detail states "based on 3,000 psf soil bearing capacity".
The lateral forces caused by pipes and water hammers seems to have little to do with the concept of bearing capacity, and I would use a passive earth pressure and friction between concrete and soil approach.
Anybody encounter this before? I checked another municipality, and their standard detail states the same assumption about bearing capacity.
My preliminary calculations indicate that the friction component is relatively small compared to the passive earth pressure component. To have a capacity of 3,000 psf, for a burial of 5 feet, you would need a passive earth pressure equivalent fluid pressure of close to 600 pcf, or a passive earth pressure coefficient of close to 5, which not too many soils would have.
Any comments are appreciated.
 
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I'm not real clear on what numbers are what here. Is the 3000 psf passive pressure cited in the last paragraph based on the passive force you need (at the base? the average?) to prevent sliding of the block, or is that coming from the 3000 psf bearing capacity required by the city, as cited in the second sentence? Can you (economically) solve the problem with a bigger block (same passive pressure times a bigger area)?

It does take a small amount of displacement before the full passive pressure is mobilized, so you should probably design for a decent FS, so the pipes aren't damaged by those small movements. I don't know what kind of movement the pipes can actually tolerate - may not be a serious issue if there is much of a FS.

I don't know why they would relate the block requirement to the bearing capacity either, unless that is a local empirical rule of thumb. Bearing capacity is, of course, not a nice simple number like 3000 psf for a given soil - it is a function of embedment depth and footing dimensions as well.

Bon Chance!
 
There is a thrust restraint design manual, including a section on the design of concrete reaction blocking for various conditions/soil bearing strengths etc. available online from the Ductile Iron Pipe Research Association (DIPRA) from the link
that may be helpful in your efforts. I believe this particular manual/section has been around about a quarter century, but draws on decades of generally successful experience with similar methods before that time. Similar information, though instead for some different metric-sized pipes/readers is also available online from the link I would caution that different types of piping material may involve different sizes of pipes and joining configurations that can affect magnitudes of thrust and other factors, and guidance from other sources is thus appropriate in those cases.
 
Bearing capacity is for vertical forces, use if the centerline of the angle of the turn is up. Kp assumes the block moves into the soil some distance,(hope less than joint leak distance). Use Ko as a conservative value. Restrained joints and mechanical joints for a few lengths of pipe on either side of a turn work where the block is interfering with other utilities or the soil may be removed.
 
Sorry (I understand the sensitivity there and while I did not use the word "capacity" it was not my intent to unintentionally/apparently trample on any sacrosanct geotechnical association/phraseology with my maybe some loose/shorthand use of the term “bearing” etc.) What I of course meant in my perhaps overly simplified generic description is some average (and by some industry experience I suspect reasonably stable/safe in “undisturbed” condition for the generic soil types described) level of effective soil pressure that is assumed (available) in whatever direction to directly counteract the resultant of all unbalanced effect of internal pipeline pressure/thrust forces etc. over the area of the back face of the block as it presses on the soil. I would think in most cases of horizontal bends this might arguably be something like a level of passive pressure assumed at some depth say above the center of the block.
In actuality DIPRA (and for that matter AWWA in their manual M41), in talking about blocking for horizontal pipeline bends, it would appear maybe at least a little more appropriately actually clarifies this some for their readership (I suspect involving many disciplines) with an additional modifier as “horizontal” bearing strength. Unfortunately my old unabridged “Webster’s New International Dictionary” (I just measured at exactly 5.232” thick) didn’t include the modifier “vertical” in any of its several hundreds of words definitions of the word “bearing”! [wink]
 
The term "bearing capacity" was used in original post by craigburger and by dgillette and was stated by the city spec as quoted. Not used in contribution by rconner but he needs to be succinct.
 
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