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base for block wall - use sand or not?

Posted by tomatotomata 10 (My Page) on
Fri, Apr 8, 11 at 16:16

I'm planning a 2 foot high interlocking block wall, and have been watching videos about preparing the trench. All say to use gravel or crushed stone in the bottom of the trench. Only a few advise a 2 inch layer of sand on top of the gravel, for easier leveling of the first layer of blocks. It sounds like a good idea to me, so I wonder why they don't all recommend sand.

What would you advise?

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RE: base for block wall - use sand or not?

  • Posted by laag z6CapeCod (My Page) on
    Fri, Apr 8, 11 at 17:47

Sand is generally a non-compactable material and tends to move around. Two inches is too thick of a layer in any case. A half inch is on the thick side - just enough to smooth the top surface ofthe crushed gravel. The reason that many do not mention sand at all is usually when they are telling you to use larger gravel without any fines in it because it will eventually slip into the spaces between the stones and cause settling.

Tbest way, in my opinion, is to use a grushed stone that has fines - there are all kinds of regional terms, but it would be a mix of 5/8" or bigger gravel with smaller gravel and the dust from the crushing all mixed in like a road base. That has strength and compacts well (with a little moisture). It does leave a coarse surface so a skim coat of sand helps the blocks keep from tipping as you set them. Because the material has fines and compacts well, the sand can not infiltrate and the wall does not settle.

Thanks, laag - very helpful. Another question?

Thanks for the info - in reviewing my previous research, I realize I have mentally combined techniques for wall building with those of building a paver patio (future dream). So speaking of - why do you use sand in patio construction? Wouldn't that sift down between the gravel?

RE: base for block wall - use sand or not?

Exact same purpose as laag described for the wall. For a wall, a patio, a step system etc... when one is constructing over a compacted base of 0-3/4" (synonymous with- road base, flex base, Granular A...) a layer of sand or screenings as fine as one can manage is often required to achieve the perfect level necessary to start building upon. Used sparingly, it will not affect the structural integrity of your patio base and will allow you to lay a perfectly graded patio, wall, or step system.

The sand will not sift between the gravel because the gravel you will want to use will be the same for a wall, i.e what was mentioned above by laag. It has fines in it, hence the name 0-3/4", as in 0" fines to 3/4" rock, and everything in between. These fines at the micro level are shaped like tiny triangles that bind together in a structural fashion while filling in all the voids that are found between the larger pieces of rock (or gravel). Furthermore, 0-3/4" is normally crushed lime stone. Lime is one of the key additives in concrete mix. Wet the 0-3/4" as you compact it and you will see it turn to a near solid mass.

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