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A 4-Step Sand Test Rig

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  • #16
    My apologies to those who may be hanging on further information around the sand test rig but my relatively sudden departure for Vietnam has brought that to a temporary halt. I'll be away for about three weeks...and then I hope to test the samples.


    • #17
      Very interesting topic.

      We started building a test system in a poor rural community in Venezuela and, instead of buying sand, we decided to test the plenty local supply.

      We did a series of tests, similar to those you describe here, but the setup is a lot smaller. We mainly tested for permeability, water retention and silt/clay content.

      Although we used somewhat "subjective" criteria, we decided to go ahead with the local sand. We're setting up a more formal experimental framework so that our experience can be replicated.


      • #18
        Sounds great, Angel. Feel free to set up your own system thread in the iAVs sub-forum (which is now open to you) and tell us all about your set up.


        • angel.ramirez.isea
          angel.ramirez.isea commented
          Editing a comment
          Thanks! I will, as soon as I gather all the pictures and dimensions. I'm not working directly on the system (silly fall from a motorcycle has me bed-ridden), but as soon as communications improve, I'll start the thread.

      • #19
        Over ten weeks ago, I set up the sand test rig - with a view to developing a process that would enable prospective iAVs builders to determine if available sand is suitable for the purpose.

        For a variety of reasons (including six weeks of travel), I hadn't got around to actually testing sand...but today that changed.

        I had obtained four samples...and carried out the preliminary tests on them.
        1. Washed and graded filter sand...0.8mm - 1.6mm...turbidity OK -
        2. Washed and graded filter sand...0.56.. - 0.7mm...turbidity OK -
        3. Double-washed River Sand - from the local big box outlet - high turbidity due to powdered sand.
        4. Sand from the local garden/hardware supply outlet - turbidity high suggesting high clay content.
        A vinegar test failed to produce any evidence of carbonate in the samples...and a pH test confirmed that all of them were inert.

        By way of a reminder...the sand test rig is a process by which we establish that the hydraulic conductivity of available sand is suitable for iAVs purposes. In using the test rig, we are not concerned with sand fractionation - the range of particle sizes. Most sand vendors don't have that information and, without some experience, prospective purchasers aren't able to do much with it even if they have the information.

        Our sole concern, at this point, is whether the sand drains effectively - or not.

        I only tested samples #1, #2 and #3. Sample #4 was rejected due to the high clay content.

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        We put 15 litres of sand into each bucket...and 20 litres of water into each of the water reservoirs.

        Sample #3 took about forty seconds for the water to drain down through the sand (not a huge issue of itself)...but, initially, as much sand drained out the holes as water - suggesting a high proportion of fine material in the sample. The water was turbid...confirming my earlier concern about the presence of powdered sand.

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        The water was also murky...confirming my earlier concern about the presence of powdered sand. Suffice to say, sample #3 is off the list.

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        Samples #1 and #2 were (not surprisingly )very different. Within 20 seconds of the pump starting, water was issuing from the drain holes in the sand buckets. Very little sand came out of the drain holes...and there was almost no settling of the sand.

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        At one stage, I went off to do something else. I switched on the first two pumps and, when I returned about 20 minutes later, the samples were draining away happily.

        It should be said the Samples #1 and #2 are a known quantity. They are both triple washed and graded filtration sands...and priced accordingly. I've included them in the testing process to provide something of a benchmark.

        More tomorrow.


        • #20
          So, after several hours spent monitoring the sand test rig, what can I say about its effectiveness?

          Based on what I've done, so far, I'm satisfied that it is a simple and cost-effective way to measure hydraulic conductivity.

          But being able to measure how quickly water moves through a media is only part of the picture.

          The key question is...compared to what?

          Well, the obvious answer is...the iAVs-suitable sand specification:
          • Very Coarse...1mm - 2mm - 40% +/- 10%
          • Coarse...0-5mm - 1mm...40% +/- 10%
          • Medium - 0.25mm - 0.5mm...20% +/- 10%
          • Fine...less than 0.25mm...0 - 5%
          ...and all of the above should be inert and free of silt and clay.


          Because the iAVs-suitable sand specification was what was demonstrated to work so well....and produced the results that were documented by Mark McMurtry.

          Our next challenge is to replicate the iAVs spec...and to measure the hydraulic conductivity...against which we'll compare all subsequent test results.

          I had the advantage of access to technical data for sand Sample #1 - washed and graded filter sand...0.8mm - 1.6mm...and Sample #2 - washed and graded filter sand...0.56.. - 0.7mm.

          Unfortunately, the harder I look at a data sheet, the more I feel like going to sleep, so I came to the conclusion that, if I mixed equal parts of Samples #1 and #2, I should have something approaching the iAVs-suitable sand specification.

          Feel free to set me straight if you don't agree...but you'll need to be quick about it. I want to put this part of the Sandponics process behind me.


          • #21
            Today, I attempted to establish the hydraulic conductivity of an equal parts blend of Samples #1 and #2...which I'm suggesting approximates the iAVs-suitable sand specification.

            To ensure that I had a 50/50 mix, I combined both samples and stirred them thoroughly...and then divided the mix between two buckets. I figure that, if both buckets behave the same, then their contents have to be very similar.

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            The depth of sand in each bucket was 300mm (very close to one foot). The sand was moist but had been draining for over 24 hours.

            I then started the pumps and timed the interval between when the pumps started and the water began to flow from the drain holes at the bottom of the sand buckets. It was 20 seconds - for both buckets.

            I then doubled up the pumps in one bucket and flooded the sand to saturation. I measured the drain time from when the water was level at the surfact of the sand to when it began to drip at the drain holes. The elapsed time was 45 seconds - for both buckets. I'm satisfied that the both sand samples (#1 and #2) were mixed adequately given that the behaviour of the water is identical in both buckets.

            I repeated this exercise six times - allowing an hour between flood sequences - and each sequence has been within a second or two of each other. The level of the sand in the buckets is within a few millimetres (less than a 1/4"). That suggests to me that the sand is stable - and not subsiding which would indicate the migration of finer particles in the sample.

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            While subsidence was negligible, I want to flood (to saturation) and drain the sand buckets - at least six more times - just to ensure that everything is consistent and stable.

            At this stage, however, the hydraulic conductivity of this sample is one yard - or 900mm - per minute. If our assumption - that this sample is equal to the iAVs-suitable sand specification - is accurate, then this is the benchmark against which we should measure all sand samples.


            • angel.ramirez.isea
              angel.ramirez.isea commented
              Editing a comment
              Thanks, Gary, for this.

              All standards are arbitrary, and we could well take this as a reference. It's well documented and repeatable.

          • #22
            On my most recentl trip to the mainland, I found a couple of new sand samples. They're on tomorrow's To Do list.

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            • #23
              First, the bad news. The Rocky Point Coarse sand...ain't! It slumped quite considerably which pointed to a high proportion of fine material. This was confirmed by the hydraulic conductivity test where it quickly revealed itself to have poor drainage qualities. Persistent turbidity also pointed to the inclusion of clay in the sand. Whatever use it might be in other garden situations, it's not suitable for iAVs.

              The Easy Mix Tiler's Sand fared much better. I confirmed it to be inert (using both the vinegar and pH tests) and, after several flood and drain sequences, the water remained impressively clear. It was better, in fact, than some of the "triple washed" filter sands that I had tested previously. The sand also performed well in the sand test rig....taking 20 seconds from commencing the water flow to seeing it issue from the drainage holes in the bucket. When flooded to saturation (with the water level in the bucket level with the surface of the sand), it took about a minute to reduce to dripping. This is about 15 seconds longer than it took for our "iAVs-suitable" benchmark sand to drain.

              There was little subsidence in the sand and it continued to drain at the same rate after at least 10 flood and drain sequences....and a continuous flood sequence that extended over about three hours.

              I judge this sand to be iAVs-suitable.

              Now for the cost. The sand costs $8.00 for a 20kg bag at Bunnings. A 20kg bag contains about 13 litres of sand - and at $8.00 per bag - that equates to 62 cents per litre.

              My next move will be to determine how I get the price down.

              The specifics of this sand have little meaning for those people who can't access a Bunning store.

              What these tests do tell us is that you can use a simple sand test rig to determine if the sand to which you have access will drain effectively for iAVs purposes. I know nothing about the sand fractionation range of the Easy Mix Tiler's Sand...and I don't care because I know that, for the purposes of iAVs, it works.


              • #24
                Hi Gary,

                62 cents a litre is expensive isn't it? It's about the same cost as the sand I'm looking at. But the convenience of getting the sand must be taken into consideration IMO.

                My sand, if purchased in a 1 tonne bulka bag, works out considerably less, you will need to look at the cost of delivery as well, although I would use a trailer and collect. Bunnings also have free hire trailers available for a maximum of 2 hours.

                Thanks for showing us the results of your test rig, it has been very interesting for me.



                • #25
                  The Sand Seleciton guides are not working at the moment.



                  • #26
                    ForeverRaiden...there some links that need to be addressed and I'm hoping to get them sorted in the coming days. In the meantime, you can find the sand particle size range...HERE. Outside of that, the sand needs to be inert and free of silt and clay. Of course, if the sand is close to that specified in the link, then it will drain effectively.