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Self-Compacting 3/4" stone

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  • Self-Compacting 3/4" stone

    I have a question about 3/4" crushed stone. I read several places that 3/4" crushed stone is "self-compacting". That is, you dump it in a trech (as in under a concrete footing or floor) and you don't have to compact it with a plate compactor - it is already as "compacted" as it needs to be. Can anyone comment on this? Thanks.

  • #2
    Re: Self-Compacting 3/4" stone

    I think the proper classification is Non-compressive soil. As long as it is retained, ie: held in a trench, or level filled, it is in fact non compressive. If not contained, it will run out of the space intended. It is used for footings under wood foundations, pre cast foundations, and obviously is a great water diverter. The Alaskan pipeline, and any set of railroad tracks are other good examples of crushed/washed rock as a footing. I believe that anything above 3/8 inch to 3/4 inch, as long as it is consistently the same size, and any crushed product of the same size will meet this classification.


    • #3
      Re: Self-Compacting 3/4" stone

      I've always considered pea gravel to be self consolidating. Anything 1/2" plus we've compacted before building upon. I have known this as a "rule of thumb" and don't know if it has a basis or origin in science.



      • #4
        Re: Self-Compacting 3/4" stone

        I read a post somewhere that said rock had to be compacted, that surprised me. Years ago I ran out of dirt to compact in a garage slab, so I backed up a dump truck and filled the hole with 1½ rock and poured. It work so well that I decided to forget compacting dirt, and have just filled the foundation walls with 1½ rock and have never had a garage slab settle since I started doing that. I can't even imagine that rock would compact. 1½ rock seems better than crushed rock to me, since I see no reason to have fines in the mix, it seems that large rock would provide better drainage, and allow the concrete to settle down in between the rock giving a stronger slab. Am I wrong?


        • #5
          Re: Self-Compacting 3/4" stone

          The crushed rock I was referring to, is, in my area, like a red quartzite, and it is still sorted and washed according to size and to remove fines.

          The larger the rock, the more reservoir capacity as far as drainage, because of more opens in the aggregate. 3/8 pea rock is the product of choice around here for AWWF footings, and 50% backfill. Seemed funny, that 30 years ago, they couldnt give pearock away as it was a by product of larger screenings. With the advent of wood foundations, and mandatory backfilling of wood foundations with crushed rock, it now commands a higher price.


          • #6
            Re: Self-Compacting 3/4" stone

            The more uniform a product is, the less it will need to be compacted. If you have pure coarse sand even, it is at 100% compaction if you don't touch it. You still have to worry about above ground and subsurface "erosion" or slope stability if you are on a slope and there are shallow ground water acquifers. We have condition here on the east coast where a foundation will be clean sand which will not consolidate and does not need compacting but there will be settlement due to high velocity subsurface water eroding away the sand. It won't compact but it will displace.

            With soils, you also have to take into account the moisture content (but not with relatively pure sand).

            A well graded round stone should not need to be compacted. Think of a can full of marbles all the same size. They will lay together as close as they can and all the compacting you do will not change that. If for some unusual reason your stones are flatter or irregular in shape such as some limestone after being crushed, you must consider the possibility of bridging. Vibrating is a good way of settling down stone. Water and tamps don't do much good.

            Don't forget, if you disturb the existing material under the stone while excavating, this could cause a problem and need recompacting.

            Since there are no organic material in rock, you don't have to worry about settlement due to consolidation.

            Overall, well graded stone makes a great foundation.



            • #7
              Re: Self-Compacting 3/4" stone

              Many times a set of plans will call for rock to be compacted. This can not happen. Even with sand generally all you do if a vibratory compactor is run on it is to cause it to flow out from under the compactor. Things that have alot of fines like 304, 411 or soil can be compacted.


              • #8
                Re: Self-Compacting 3/4" stone

                I once dug out my crawl space and built new knee walls that were about 3 ft inside the old very shallow footers. I then filled in the back of the walls with the smallest bluestone grave avvilable. When I went looked behind the walls several years later the level of the gravel had dropped abou 1 1/2 feet. Either it compacted or someone was stealing gravel from my old crawl space which is now a basement. Any expainations appreciate.


                • #9
                  Re: Self-Compacting 3/4" stone

                  Two other possibilities -- and I'm not commenting on how likely they are:
                  1. Uneven settling or frost action allowed your new "kneewall" to heave.
                  2. The soil under the peastone was compacted by the considerable weight of the stone, resulting in settling of the soil beneath the stone.


                  • #10
                    Re: Self-Compacting 3/4" stone

                    I would have to agree that the soils settled. Although that amount sounds to be very large.


                    • #11
                      I realize this is an old thread, but it comes up on a google search, so i'm providing some info.

                      I am a PE who has worked in a lab and at a testing and inspection company. We run proctors on uniform sand, and then field test... uniform sand absolutely compacts and is very very difficult to achieve over 95% compaction with. Volume loss with sand will be high. You can't run a nuclear densometer on washed crushed stone, so compaction documentation is visual.

                      Larger, uniform, washed crushed stone will compact under proper effort. There are three reasons for people's view that is doesn't need compaciton: 1) the bearing capacity of contained, large diameter crushed stone is extremely high, so you are unlikely to cause compaction to occur with application of direct pressure; 2) it is not readily compacted by water due to laminar flow around it and no appreciable pore pressure, as such, getting a rain storm won't cause sinkholes; 3) the actual volume change from free dropped fill to compacted fill is much lower with uniform washed crushed stone. Due to these things, you don't see as many problems crop up the next couple of years from having had uncompacted stone. The rate of compaciton will be between 4% and 10% for 3/4" stone (largely based on partical angularity). If you get less than 4% volume loss, you are probably not achieving adequate compaction. It is actually much harder to get compaciton with something like uniform crushed stone, mostly due to the stone size (larger diameter is more difficult/requires more effort). You need heavy equipment such as a jumping jack or heavy drum roller (preferably sheeps foot) or, best of all, a rubber wheeled heavy equpment such as a loaded 10-wheeler (tri-axel etc). Lift size also needs to be reduced. I was on a job where we allowed 24" lifts of 18" minus blast rock, but they had to have witnessed proof rolling everywhere ... it lost volume. After proper effort, you still will have voids, and it takes training to feel the difference with your feet. ... just watch your volumes change.

                      What WILL happen with uncompacted crushed stone, is that some day it will vibrate (nearbly equipment or an earthquake) and that will cause compaction. A lot filled with uniform boulders will have similar problems.


                      • #12
                        Thanks for taking the time to post this info.
                        Welcome to the forums.
                        Mark Parlee
                        BESI(building envelope science institute) Envelope Inspector
                        EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
                        EDI Seminar Instructor
                        Level one thermographer (Snell)
                        You build to code, code is the minimum to pass this test. Congratulations your grade is a D-


                        • #13
                          Welcome to the forum and thanks. Good info.


                          • #14
                            Thanks for the info!