Footing on rock or soil

[ztengguy]

Im designing a LRFD footing for a bridge. Its a spread footing, and to help with beairng the geotech is talking about compaction grouting or a stone column foundation. At what point would you consider these improvements to allow the footing to be designed as on rock instead of soil, or would you never consider that?
thanks

 

[a2mfk]

These are common soil improvements with certain types of loose sandy soils and organic soils, but this is with building foundations, I can't speak about bridges.

 

[SlideRuleEra]

ztengguy - A judgment call, so it could go either way. IMHO, since both compaction grouting and stone columns are manmade features they may be subject to human error or unintentional variations in quality. Of course, rock can vary too but that is a "natural" occurrence. I would an consider them soil, not rock.

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[ztengguy]

@ A2mfk, I am aware of the different improvements. I was looking for weather or not you would consider it to be improved to a rock quality for design.

@ slideruleEra. Thanks, the geotech said the same thing you did.

 

[BigH]

ztennguy has the answer - but stone columns and compaction grouting are soil improvements - doesn't make it rock.