Boulders and foundations

[Stinja]

Hi. first time on the boards and i have a question.

There was a soils report done on a site and it said the boring logs were terminated because it hit boulders. The boulders went from 4.5ft to 6.5ft below the surface. The client wants to build retaining walls and some of these retaining wall foundations will land/cross the location of these borders but will not be in contact with them.

Would there still be a problem with building the foundations if the bottom of the foundations were about 2ft above the boulders? If it isn't possible, how would i get around to building the foundations of the walls?

Thanks for any future input.

 

[ishvaaag]

Well, you may use some poor concrete layer to get to the boulders, this of course if you think the foundation layer is not competent enough itself. Likely for most situations some light compaction of the intervening layer since since thin enough for a well reinforced wall and footing will reduce to nil any bad effects of its existence, except that the soil is as bad that warrants removal.

Since construction of the footing will expose the boulders where present, you may use there also some additional steel in the wall and footing to forestall any effects of differential settlement; only very long continuous walls -on very stiff supports, out of differential stiffness- should make that steel a significant amount.

 

[fattdad]

This is a question of geologic setting. If the natural geologic layers include boulders and there is some geologic reference as to how this layer performs under new loading, then there may be nothing to worry about. If the geo-firm just gave up 'cause the drilling got difficult and provided no geologic context, they the risk is yours to take. I'd want to drill through the boulder layer and evaluat the density/consistency of the soils that will be under the new loading.

Not much help, eh. . . ?

f-d

¡papá gordo ain’t no madre flaca!

 

[blgeo]

Re-emphasizing fattdads points, the borings need to be viewed in the context of site geology.

Based upon your comments you seem fairly certain that boulders will only be encountered in a certain area of the project over a very specific depth range. I would point out that you cannot know this from the boring logs alone. Hopefully there is some degree of interpretation that went into this assumption based upon the presumed origins of the boulders.

 

[msucog]

how big are the boulders (do you guess)? are the boulders in fill, residuum, other? i'd at least dig test pit and do some coring to see what's around or on the other side.

 

[msquared48]

In what area of Washington is the project? My guts tell me that this may just be glacial till.

Mike McCann
MMC Engineering