Percent of "crush" in a rokfill embankment

[BigH]

There is very interesting issue on a recent project. Somewhere, although it is not in the project requirements, the client has it in his mind that a rockfill (alluvium) where the 400-700 mm boulders are crushed in a jaw crusher and then reintegrated into the minus 400 mm material, can have no more than 17% of the material being "crushed". (not 15%; not 20% but 17%!). This material is in the outer shells of the embankment dam upstream and downstream.

Has anyone ever run into a similar "requirement" or "notion"?? I haven't. Just wondering as we have many engineers from a great many countries following this forum.

 

[GeoPaveTraffic]

Never heard of such a thing and I can't imagine why there would be such a requirement.

Mike Lambert

 

[Ron]

False precision.

Kind of like measuring with a micrometer, marking with a crayon and cutting with an ax.

 

[GeoEnvGuy]

I have seen river run rip rap specified for aesthetics to have all rounded cobbles on the shell of a berm. Crushed rockfill has a higher friction angle than rounded boulders and cobbles so it is not likely a strength requirement. This could be related to a specification on the rock acid leaching or geological formation requirements?

 

[BigH]

GeoEnvGuy - Don't think so - I think it might be a "Russian" thing. The alluvium fill will be subjected to, at the base, fill heights of up to 300 m or so. They are worried, I think, in that crushing of angular particles or microfractures within the crushed 400-700 mm breaking off, and causing, perhaps, more settlement that if the crushed material was not present. The natural alluvium contains a number of different rock types and an observable portion of the boulders are angular to subangular. We have found that the boulders at the surface of the fill layer, as a result of placing (dozer) and compaction (smooth steel drum vibratory roller) do in some cases show "platelike" and "block-like" fractures.

One could entertain the concern - as I have with other aspects of the Dam's "to be" components but have not wrapped my head around such a "specific" number like 17%. If I am doing pile design and the "allowable bearing capacity" of the pile would be calculated as, say 68.2 tons, I would never provide this - I'd use 70 tons. In Indonesia years ago, I saw someone take an "N" value, go to a chart and determine the phi value, go to the chart of Nq to get that value (which do you use? Berentzev, Vesic, Meyerhof, Terzaghi?????) and then calculate the ultimate pile capacity. Then use 3.00000000 to determine the allowable bearing capacity. Yep!

 

[Ron]

Then use 3.00000000 to determine the allowable bearing capacity. Yep!

....my previous point, exactly