Fill under heavy-duty pavement

[sjp8]

I am a site engineer working on a large retail project in a previously developed urban area. The client wants to eliminate pile supported concrete slab/paving in the area of a 30'wide drive-thru (between two four-story buildings), as well as, a loading area for self-storage (there are 3 pile supported stories overhanging the loading area). The client wants to replace the pile supported concrete pavement with heavy-duty asphalt pavement (220,000 EAL, 10-year design). Borings in the immediate area reveal 7 to 13ft. of loose to dense urban fill (e.g. soils, brick, rock, cinder, glass, etc.). Bedrock ranges from 17' to 45', Groundwater 8' to 14'.

This area was originally under the structural engineer's scope, but, the cost pushed it into my court. What should my concerns be other than compaction? Should I be looking into replacing a certain depth of the urban fill with controlled fill? If so how much?

I have contacted the geotechnical engineer on the project and plan to work with him to find a solution. In the meantime, I am posting this to get some additional ideas from the experts as this is not a strong suit of mine.

Thanks in advance.

 

[msucog]

in my opinion, it depends on how loose, how deep and how much movement the owner can stand. complete undercut/replacement, partial undercut/replacement, do nothing (and the client will pay much more later), dynamic compaction, smaller partial undercut with replacement with heavy concrete section, etc...
there's many ways to look at that problem. unfortunately, the actual conditions and owner's cooperation/pocket book will dictate which remediation is the most appropriate/acceptable. the owner can fix it as good or bad as they're willing to pay.
you're question is difficult to answer via the keyboard. your geotech, in combination with the owner's input, should be the most helpful in looking at the problem.

 

[sjp8]

Thanks for the feedback msucog. I realize it's a bit much to expect an answer to such a complicated, dynamic question via this forum. I just wanted to throw it out there and get some ideas that I can start kicking around.

 

[BigH]

msucog covered most methods - but I wouldn't consider dynamic compaction due to proximity of the buildings. You might try stone columns, cut down 1 m of the fill and replace with well graded crushed stone base course (to act as base and subbase).

 

[LHA]

sjp8: "This area was originally under the structural engineer's scope, but, the cost pushed it into my court."

Red flags are going up on this one.

So, the structural was contractually obligated to address this? As I am also a Site/Civil Eng, I wouldn't seal whatever you come up with. This is an issue for a Structural and/or a Geotech Eng to design and seal.

What "costs" affected this change; his cost to complete or the cost to impliment the solutions he designed? If it's cost to complete, he's passing the buck and dragging your budget down. If it is implementation, then his solution was fine, but Client can't afford it. In which case Client is passing the buck, and risking your reputation.

Engineering is the practice of the art of science - Steve

 

[msucog]

LHA, that's sort of my thought...the owner doesn't have to completely fix it...but then again, it doesn't have to last forever if they take the cheap way out. i'd only stamp it as long as i had several options in my recommendations. one of those options would say that if they don't completely remediate the area, there's a real possibility that the area may not perform well. i'd even elaborate on some of things the owner might expect just so that can't say you didn't warn them. i think i'd use the phrase, "we cannot effectively estimate the performance if complete remediation is not performed."
put the responsibility on the owner to decide what they want and what they want to pay for. if they expect you to make that decision AND take the liability, i'd force complete remediation or let them put someone else's neck in the noose. good luck.

 

[sjp8]

Red flags did go up and I am not comfortable with this at all. I plan on addressing all of the liability issues with the client as this process moves forward.

This is a design change for the whole project team that has come about after CD's were issued (the project has had several addendums). The client determined it was an "excessive" cost for implementation after reviewing the structural engineer's design.

I appreciate the feedback.

 

[Prepakt1]

You can increase the load carrying abilities by compaction grouting.This is a fairly expensive process.
If its just heavy duty ashphalt required. You could go the simple way.Compact the base well, 15 ton roller. lay 4" of ashphalt, use the bay and wait for settlement if any. Come back in 1 yr and lay a final 2" course to bring it to level.

Intrusion Prepakt /marineconcrete.com