Do I need a 4" gravel base in my instance


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Old 05-15-12, 03:20 PM
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Do I need a 4" gravel base in my instance

I live in Long Island NY, and I am installing a clay brick patio/walkway.
I have very sandy soil here. It wouldn't even call it soil - it's basically sand. I've had a hard time getting grass to grow because of it.

I know that the correct way is to install about 4" of gravel and then an inch or two of sand as a base. I'm wondering if I need to do this. In the front of the house I built a clay brick walkway to the street with bricks that I found on the property. I just dug down, tamped and peeled out any rocks so that it was very flat, and installed the walkway. I've never seen any heaving or issues with it.

That said, this area will be more showcased, and I bought some really nice clay bricks, and edging, so want a pro job. I'm feeling torn on whether I need to remove 6" of sand only to then install 4" of gravel and then sand again. I read that the purpose of gravel is not really for draining but rather that is what a small pitch is for.
I will have a reasonable pitch for run-off. I'm really tempted to just install on my existing sand.

Thoughts please..........

Thanks, Tom
 
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Old 05-15-12, 09:49 PM
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If your current project will ever receive any amount of abnormally heavy loads (like concrete being wheeled across in repetitive heavy wheelbarrows, or dealing with milling machines and grand pianos on a regular basis, etc.), I'd suggest you go with at least 2" of well-compacted gravel. Use the stuff with interlocking ability (at least 50% fractured faces, not the smooth rock), which will set up and resist heavier loads quite a bit better than pure sand. Compacted sand is great when kept at optimum moisture and density, but if it gets too wet, say good-by to its ability to withstand loads. There's a good chance for differential settlement of your brick pavers should the loads or moisture content become too great.

A preferable alternative (with a lot less work) would involve working in a few sacks of Portland cement every 10', going down 3" or 4", and adding enough water for hydration. The finished product is called soil cement, and is quite strong. If pitched properly, drainage shouldn't be an issue.

If you don't anticipate severe loading conditions (and don't expect any 3-day rain storms), then the pure sand (compacted, of course) should be OK.
 
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Old 05-16-12, 03:15 PM
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Thanks for your reply Bridgeman. No , there won't be anything heavy going across this area. I think I've pretty much decided on a compromise. I'm figuring on 2" of the crushed concrete as a base and then a thin layer of sand for leveling.

The problem I'm having is understanding the whole theory of the base. If the recommended 4" base is for weight, then what happens if the ground under the base is weak? Can't that sag and take the base with it and then cause problems with the bricks?
If the base is for drainage, I also have issues - The frost line is 36-40" here. So if you have a 4" base, couldn't the ground below the base heave and take the whole base with it again causing problems with the bricks?

So, I'm wondering how important the 4" of "whatever" is. I'm thinking that the most important thing may be to solidly tamp whatever you may have as a soil, and be sure to then provide a pitch for water run-off.

Or, am I just missing something?
 
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Old 05-16-12, 04:43 PM
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I think the idea with the gravel is that when water does freeze, it has all those nooks an crannies to make crystals in before it starts pushing things around.
 
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Old 05-16-12, 05:00 PM
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I see - well that does make sense - I've never read nor thought of that! I hope the crushed concrete has enough nooks and crannies - it's pretty fine. Selling point was that it compacts really well.....
Thanks.....
 

Last edited by tomfmal; 05-16-12 at 06:28 PM.
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Old 05-16-12, 06:55 PM
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If the subgrade is lousy and weak enough, no amount of compacted base will prevent applied loads from causing problems. Your subgrade doesn't fall into that category, so it's a mute point. The theory of a well-compacted granular base is that it both bridges the weaker material under it with a stronger platform (such that passing point loads don't reach the subgrade in their entirety) and it also provides a ready path for surface drainage. Literally millions of miles of streets and highways around the world utilize the system, so it must work.

Your question of heaving caused by freezing of saturated subgrade is valid, but usually such heaving is uniform, and occurs in a manner that it is not normally a problem (and is mitigated to a point by the granular base, as stated earlier and by drooplug). However, paved surfaces (streets, highways, sidewalks, etc.) with proper bases will still heave if the frost depth is deep enough. As a Wisconsin DOT bridge engineer many years ago, my common (and the State-wide) practice for measuring vertical clearances under all bridge overpasses was to always take measurements in the coldest part of the year (January), as that's when the clearances would be at their smallest because of pavement (and support structure) heaving when frozen. Doing so provided a safety factor, meaning all bridges' signed clearances during the rest of the year were always greater. Clearance differences between cold and hot times of the year at any given structure could be as much as several inches, depending on the amount of moisture in the ground and the pavement support structure.
 
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Old 05-17-12, 07:29 AM
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Thanks for that detailed explanation - understood now.....

I hope the RCA (recycled concrete) is a better base than crushed gravel as the yard recommended, because that is what I bought.

Thanks again!
 
 

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