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Grow bed weight
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Author:  Massey [ Jul 8th, '14, 06:40 ]
Post subject:  Grow bed weight
Hi all not expectingany exact figures here as i know gravel weight is affected by gravel size due to fitting in more but does the following figures look like they could be right,

Grow bed 16 inches deep, 33inches wide and 90 inches long so 27.5 ft3 should weigh 1.35 ton dry and 2.13 ton full of water,
Author:  Bufordht [ Jul 8th, '14, 10:18 ]
Post subject:  Re: Grow bed weight
What kind of gravel? Some gravel weighs more than twice as much as others. I think you won't see that much of an increase from dry to wet, more like 600 lbs by my math. I could be wrong though.
Author:  BenBrewcat [ Jul 8th, '14, 10:24 ]
Post subject:  Re: Grow bed weight
It depends also on what kind of gravel, maybe more so than size. Post what you're using and maybe someone has some data points (I use hydroton which has published values).

I'm with you up to the volumetric estimate: I get about 27.43 cubic feet (20.625 ft square, times 1.33 feet deep) for 205.7 gallons, times 8.34 pounds per gallon = 1,715.75 pounds of water. You'd need to subtract the volume of the gravel though from the water weight and add gravel weight...

It'll be easier to ask your gravel supplier how much one yard of gravel weighs (they'll know) and interpolate. Then go over there one day and do a displacement test Archimedes-style if you really want to dial your weight in.

Or overbuild and don't worry about the decimal points :D .
Author:  Stuart Chignell [ Jul 8th, '14, 10:58 ]
Post subject:  Re: Grow bed weight
Some gravels are very light and have a very high void fraction (percentage of air gaps between and within particles) so it depends.

I've used gravels that weighed 1.5t/m3 at 10% moisture content (which is a really weird measure for gravel based on the amount of water contained within gravel that no longer drips after a certain time period from being drained) to gravels that weigh 0.75t/m3 at 20% moisture content.

BenBrewcat wrote:
Or overbuild and don't worry about the decimal points :D .


That is a scary rounding when dealing in m3/s.
Author:  Slowboat [ Jul 8th, '14, 13:16 ]
Post subject:  Re: Grow bed weight
gravel at 0.75t/m³ would float in water is that hydroton?

I reckon your GB volume is 0.778m³ so at about 1.2T dry plus about 0.25T of water

near enough to 1.5T

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Author:  Stuart Chignell [ Jul 8th, '14, 13:58 ]
Post subject:  Re: Grow bed weight
When a density of gravel is given in t/m3 remember that includes the air spaces between the gravel particles.

The gravel that I have used that has a gravel density of 1.5t/m3 at 10% moisture content (dry) has a particle density of 3t/m3. The particle density is the density of the rock itself.

The gravel that is 0.75kg/m3 has a particle density of 1.6t/m3. So if you floated an IBC in the water and then put 1m3 of this gravel in the IBC it would still float but if you dropped an individual particle (piece of gravel) in the water it would sink.
Author:  Stuart Chignell [ Jul 8th, '14, 14:02 ]
Post subject:  Re: Grow bed weight
Slowboat wrote:
gravel at 0.75t/m³ would float in water is that hydroton?

I reckon your GB volume is 0.778m³ so at about 1.2T dry plus about 0.25T of water

near enough to 1.5T


The amount of water depends on the void fraction. The bluestone @ 1.5t/m3 has a void fraction of about 50% so the amount of water to fill it would be about 500L/m3.

The second gravel I mentioned has a void fraction of about 45% so the amount of water to fill it would be a about 450L/m3 to fill it with water.

You really need to either design your beds to take the heaviest gravel possible or get a sample of a lighter gravel and design accordingly.

Most engineers would take the former approach because of their inherent conservatism.
Author:  Charlie [ Jul 8th, '14, 15:26 ]
Post subject:  Re: Grow bed weight
Too heavy to lift? :)
Author:  4x4Eric [ Jul 12th, '14, 17:55 ]
Post subject:  Re: Grow bed weight
Charlie wrote:
Too heavy to lift? :)

Need a bigger forklift :laughing3:
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