Backyard Aquaponics
A place to discuss aquaponics

I know i should do research before i post, but i didn't this time due to time. What is the weight comparison of pea gravel to expanded shale?

Shale is approx half the weight of stone

The fine river gravel I'm using is 1.5 tons per cubic meter, and probably similar to pea gravel.

Once the little one finally went down for bed I got some research time. I am terrible at math so please check my math to let me know if I am in the ballpark. There is a reason I am a chef and not an engineer

The pea gravel that I am looking at is bagged and would get speed up my set up. I found the bags are .5 cf and weight around 45 lbs. With the measurements I have I estimated I would need around 12 cf to fill the IBC tote GB. The math for the gravel then tells me my GB with pea gravel will weigh 1080 lbs (not counting the water or plants). For comparison I found 40 lb bags of shale. It says the bags of shale are .75 cf. Once again doing the math this tells me my GB would weigh roughly 640 lbs (minus water and plants).

So is it worth it to wait a bit and get the shale which will be lighter and give me a little more surface space or grab the pea gravel and get to growing? The water will add another 200 lb +/-. With the GB sitting on the IBC frame does the frame have the strength to hold that weight?

The worth is completely up to you. Assuming the grow bed can support the extra weight, both media are going to serve your purpose. I prefer expanded shale but the lighter only really noticeable when you are filling your containers.

brian

Find a bulk supplier, where they sell it by the ton. Which means expanded shale cause pea gravel sucks. Anywho, getting bags is way too expensive. Also, I've found that pea gravel in .5 cf bags tends to take up less than .5 cf once it starts to settle. I was able to fit a supposed 2 cf of pea gravel in 1.75 cf bed.

I use cracked gravel in my beds. Lots of surface area and spaces between the rocks.

bcotton, is correct in that the lighter weight is really only a factor when you're filling the beds. Any of the porous media is just going to fill with water (hence - porous) and weigh close to what the gravel does. I'm using pea gravel in one of my beds, it works fine and is easy on the fingers. I've handled the expanded shale and it seems rougher to me, not as rough as the lava pebbles I have in one bed, but still plenty rough.

That is, of course if your grow bed is able to take the weight.



Do you have any evidence to quantify this statement? Water weighs significantly less that pea gravel...

Pea gravel is about 90 lbs/ft3 using 6eight's numbers and water is 63 lbs. The gravel would be 1080 lbs for the IBC bed. If you figure on adding about 30% (3.6 ft3) of the GB volume additional water, that would be 227 lbs and the total would be 1309 lbs.

If the shale is 53 lbs/ft3, that's 636 lbs for the 12 ft3. Guessing that you could add 50% (6 ft3) of the volume as water (porous media), that would add 378 lbs and the total would be 1014 lbs.

In this scenario the gravel bed would weight 29% more. That is significant, but if the strength of the GB structure is that marginal, I feel you're asking for trouble anyways. If you're working with a low factor of safety such as 2:1 for the shale bed, you'll be fine with the gravel as well.

I did some wet and dry weighing with the medias I'm using and I'll try to dig that up later to share the numbers.

6eight, in my opinion you'll be fine using either media. Just look at some of the structural designs folks have used on this site and in the excellent IBC of Aquaponics download.

I would be curious to see the numbers thanks gn83tm

Thanks for all the responses and information. I have seen set-ups all over the internet where the GB sits on top of the tank and looks perfectly fine. Some are reinforced with boards a crossed the top of the cage as well. My guess is the totes are designed to hold much more than 1300 lbs. If you think about a full tote (liquid) it could weigh upwards of 2200 lbs if you are using water weight. I have been in a warehouse where they were stacked so the bottom tote was capable of holding 2200 lbs +/-. Knowing that cutting the cage has compromised some of the original weight bearing abilities of the cage is the only thing that concerns me with a 1300 lb GB.

I have been trying to find bulk pea gravel, river stone, or perma till for a couple weeks now and it seems darn near impossible in this area. Everyone sells it by the bag or by the pallet. I did just find someone that sells grey pea gravel (granite) and will have to do some calculations on that as I am sure it will weigh even more than the other choices. I think granite is pH neutral, but will have to check. If I pick it up it is $50 a ton which is about 20 cubic feet.

Home Depot just dropped there bagged pea gravel (small river rock) to $2.50 per bag (.5 cubic feet @ 45 lbs). That would make it about $60 to fill the bed.