Large Site Mechanical Pad - Rural Grassland - 3/4" crushed or Roadway Type Subbase

[structuralsteelhead]

Having an issue with a client who 'has always used 3/4 crushed aggregate below his mechanical pads without a problem'. Current site within flood plain, has high water table and little to no drainage plan on site. I am recommending a mixed aggregate and clayey graded 'roadway' type (Class 2 AB) subbase material that in hopes it will act less like a reservoir below the mechanical pad during wet weather than a pourous 3/4" . Am I being 'too conservative',....again! Thinking about this one too much? Any thoughts? Thanks in advance, Bill

 

[SlideRuleEra]

I don't have any problem with crushed aggregate. Think about your statement concerning aggregate being a reservoir below the pad during wet weather. With a high water table and minimal drainage, "wet weather" does not make any difference... it's always wet under the pad. Not a problem. Problems start when wet and dry conditions alternate.

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[oldestguy]

If you are in cold climate, that hi P=200 material is likely to frost heave and loosen it, voiding the strength you need. Sick with the crushed rock.

 

[dhengr]

Structuralsteelhead:
I tend to agree with you. I would like the immediate area to drain away from the pad. Thus, a bit of attention to pad elevation and finished grading is needed. And, I have never liked the idea that the pad base excavation might act like a sump or bath tub under the pad, particularly so in cold climates. If you don’t have to worry about freezing, and the water table is near grade anyway, the whole area is a sump, so what have you gained? One advantage of the crushed rock, maybe with some geo-textile surrounding it, and with a high ground water table is, that your base material with a fair amount of fines, could be subject to some pumping at the edge of the pad. Make a list of your pros and cons of the two base material system, and let the contractor pick and respond to any issues you might have. Then record his selection in a letter, one for him and one for the file.

 

[cvg]

generally, roadway aggregate base is not "clayey". It is a well graded mixture of crushed aggregate, usually with very few fines and very low plasticity. In other words, no clay or silt is allowed. If you are in a floodplain, than suggest the pad be elevated above the flood level

 

[structuralsteelhead]

Thank you all for your comments. If left to the contractor, he would choose 3/4" crushed as it takes little to no compaction considerations provided placed over native adequate subgrade or prepared subgrade, and frankly cheaper. In Napa, CA. Heavy rains in Winter, moderately high temperatures in the Summer. Typically either dry or wet, not much cycling. Will stick with the 3/4" crushed for now. Thanks again, all. Bill

 

[Ron]

The base material you propose will retain a lot of moisture. Using crushed gravel provides a capillary break for rising groundwater. As SRE notes, crushed stone works fine and getting it wet is not an issue.