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I'm planning to pour a reinforced concrete pad and am looking for advice for how to compact the material underneath.

A plate compactor would work - but can I get by without one? For instance, if I tamp down the soil and later gravel with a 12' sledgehammer head, 4x4 post, or concrete block (etc.) would the results be acceptable?

Notes:

  • We have rocky clay soil, over which will be a base layer of gravel, and then the reinforced concrete.

  • The soil is quite dry since this has been within the interior of the house for ~45 years.

  • The pad will be about 4x6 feet - not very large.

(I don't mind renting a compactor if that really is the best option but for a small project it seems worth considering alternates.)


EDIT: Clearly I was assuming that a plate compactor was like the gold standard and other choices might be acceptable but inferior. But one answer has questioned that and it may be an erroneous assumption.

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  • 1
    Are you aware of a tool called a tamper?
    – elrobis
    May 11, 2018 at 13:21
  • @elrobis vaguely aware but I haven't used one. I guess I wondering if a plate compactor is going to do a different/better job than manual tamping, or if it is just faster but the other methods will produce as good of a result. May 11, 2018 at 13:38

3 Answers 3

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When I was younger and worked in foundation construction we made our own effective manual soil compactors. They consisted of two parts, a pipe handle and a round base. The handle was a five or six foot long piece of 1" galvanized pipe. The base was a cast iron wheel hub from junk yard similar to that pictured below. The hubs were about six or seven inches in diameter and weighed enough that they easily absorbed the shock when tamped to the ground.

enter image description here

The pipe handle was threaded into a pipe fitting that was welded into the center hole of the hub. (Obviously the grease cover of the hub was discarded). If I recall there was no issue of the bottom side of the hub having a small diameter center hole. Use of the compactor was a good amount of exercise and always seemed to be assigned to us young folks at the time by my father who was the foreman.

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  • I totally agree that a hand tamp is the way to go. On this small of a space and the scrap wheel hub would be a good DIY tamp, we had them made out of 1-1/4 or 1-1/2" pipe with thick steel plate welded to the pipe the wheel hub would be cheaper and would do a good job.
    – Ed Beal
    May 11, 2018 at 17:01
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Tool-free method: Level it, walk on it, saturate it with low pressure water, wait an hour, walk on it again, saturate it again with low pressure water, wait a few hours, walk on it once more. If it still doesn't feel compacted, do more watering, waiting and walking. Once you can walk on the compacted soil without leaving a footprint, you're good to put stuff on top of it. (If you're a ninja, get one of your non-ninja friends to walk on it, then check for footprints).

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A plate compactor is not appropriate. It has far too large a surface area for its weight and force, and is mostly intended to flatten the surface of soil. If you want to use a power tool, a "jumping jack" would do, though it's probably overkill for such a small project. We mostly used them around foundations.

You need a small-ended tool with a high weight-to-area ratio. A simple 4x4 post can do well, or a 2x4 of 8 feet or longer, or really anything long and narrow. By applying force along the length of the tool, a very large amount of pressure is created by the momentum of the impact.

You can also do water compaction. By saturating soil, air migrates out and the soil is left in a very dense state once the water percolates out.

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  • Do you think watering followed by drying and then mechanical compaction would be advisable? May 11, 2018 at 16:30
  • Thank you for the comment about plate compactors being too large but also insufficient - I hadn't considered that. May 11, 2018 at 16:30
  • There's no harm in using both compaction methods. I'd probably do water first if you have time available for thorough drying, otherwise reverse the order.
    – isherwood
    May 11, 2018 at 17:08
  • Be very careful about wetting clay based soil. It wants to hold water, not perc through.
    – Jack
    May 11, 2018 at 19:43

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