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Help needed to calculate soil weight in boxes

Posted by lyonsy 10 CA (My Page) on
Wed, Jul 16, 08 at 23:40

Hello

I am trying to calculate what the weight of soil would be (wet) with box dimensions of 8ft x 3ft x 2ft with the depth of soil being 2 1/2 ft. The box will be a new fibreglass substance and I can calcualte the weight of that myself.

I need this info for a structural engineer so we can see of a garage roof can handle 4 of these sitting on a double span on the roof.

Soil will be regular garden soil with home made compost..not a lighter weight bagged soil.

Any help would be much appreciated

Thanks

Lyonsy


Follow-Up Postings:

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RE: Help needed to calculate soil weight in boxes

I'm confused about the dimensions. You've got 8x3x2 to a depth of 2.5. That looks like a total of 4 dimensions, unless you mean that the box is 8x3x2 and the 2.5 means that it's .5 ft from the top of the 3 ft side?

I would think that different soils would have different weights, so I would suggest getting a sample amount of the soil (maybe 1 or 2 cu ft). Weigh the sample, then multiply it out to get the total weight.


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RE: Help needed to calculate soil weight in boxes

Most often dimensions are listed in the order of length, width, and depth but I would suspect these are Length, Depth, Width which does not really present much of a problem. Soil, depending on moisture content, weighs in the neighborhodd of 75 to 100 pounds per cubic foot, so you need to calculate the Volume (total content of your box) by multiplying the L (in feet) x W (in feet) x D (in inches converted to feet fractions) to get your total cubic feet and then multiply that by 100 (use the greatest weight) to get the total weight that will be on that roof.
Any structural engineer worth his salt would be able to do that quicker than you.


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RE: Help needed to calculate soil weight in boxes

Typically we (people in the engineering community) use a figure of 120 lbs per cubic foot for the weight of an average soil at a "typical" moisture content...

But Bpgreen's suggestion is right. Weigh the soil in a gallon bucket (or a measuring cup), 7.48 gallons is equal to 1 cubic feet . So the divide the weight of the gallon by 7.48 to get the weight per cubic foot. And then multiply that by the volume of the dirt. Which is your case is 8x3x2.5 = 60 cubic feet( if the dirt is 6" higher than the top of th box?)

And typically at 120 pounds per cubic foot, an estimate of that soil weight is 7200 lbs. But I would weigh a sample to be more accurate.


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RE: Help needed to calculate soil weight in boxes

thanks for all those comments

Box is 8ft long by 2 ft wide and 3 ft deep (but only filling it to a depth of 2 1/2 ft)

The soil I make is on the heavier side and I would imagine then I need to use the 100lbs per cubic ft

Thanks


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RE: Help needed to calculate soil weight in boxes

well, even if you use 100 pounds per cf you are talking about the weight of a car up there.


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4 boxes!

i jsut realized you said FOUR of these! you are looking at over 8 tons sitting on teh roof! i hope your engineer and your pocket book can handle that.

are these boxes straight sided or are they narrower at the base/top and then widen at the top/base? if narrow base or top, then you can cut the weight drastically.

what about when it rains, do these boxes drain really well? if not then you could be holding a few extra tons of water as well. and if they do drain really good you will have to get irrigation up there to water them almost daily, it gets HOT on a roof!

i am not saying this cannot be done, just pointing out it won't be as cheap and easy as stick these up there and fill 'em up.


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RE: Help needed to calculate soil weight in boxes

I wouldn't use regular soil for a roof plantings. As you can see that soil is heavy.

Typically green roof designs require an "engineered soil", (sometimes called a growing medium), that has a lighter density and is low in silt (to prevent clogging of underdrain systems).

Here is a link that might be useful: Roof Soil


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RE: Help needed to calculate soil weight in boxes

The best way would be to determine the bulk density of the soil mix you are going to use. As has been stated, a mix of sphagnum peat moss, compost and perlite would be a good starting point.

Next is the water saturation. I assume that these will be open to rain and you will have to use maximum saturation for calculations. This also assumes drainage.

So mix your potting mix into a container and completley soak it until there is no more water draining out. This should give you a good estimate of what will be in your containers.

Now get a 5 gal. pail or metrically a 20 liter pail and fill it with water to either the 5 gal. level or 20 liter level. This should be a couple of inches from the top and mark where that level is. Now fill the pail with the wetted potting mix to the mark. You need to pick the pail up and drop it 3 or 4 times. Fill it up to the mark again and again compact it by the dropping method. Now fill it up to the mark and leave it.

Now you weigh the pail, subtract out the weight of the empty pail. This will give you a weight per volume which is bulk density. This is simpler metrically since 20 liters is 1/50 of a cubic metre and bulk density is measured in kilograms per cubic metre.

Since you already know the volume of potting soil that will go into the container, it will be simple math to get the total weight. A little over-engineering will not hurt.


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RE: Help needed to calculate soil weight in boxes

  • Posted by dorisl 5 NW Chicago burbs (My Page) on
    Thu, Jul 17, 08 at 17:41

And typically at 120 pounds per cubic foot, an estimate of that soil weight is 7200 lbs. But I would weigh a sample to be more accurate.

120 pounds per cubic foot? I dont think this # can possible be right, than a largish potted plant would be "weigh" to heavy to carry around the yard.


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RE: Help needed to calculate soil weight in boxes

HI Lyonsy,

Really can't add much to all the good suggestion. The structural engineer is worried about liability if he comes up with the number used. Pretty slick of him to have you provide it. There are engineering tables for unit weights of all kinds of materials including all kinds of soil.

"A little over-engineering will not hurt." (Good Comment).

This is called a Factor of Safety and should part of your Engineer's calculations. Good Luck. Aloha


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RE: Help needed to calculate soil weight in boxes

Have you ever lifted a 5 gallon bucket full of soil and carried it around the yard? That is less than 1 cubic foot.

You need seven and half gallons of soil to equal one cubic foot. Weigh a one gallon bucket of soil and let us know what you get, we can compare it to the 120 number...

The 120 number is a "conservative estimate" used by engineers for compacted moist soil. We like to design our loads for the worst case scenario. And certainly potting soil is not compacted, but sometimes you just never know what happens after you build it...


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RE: Help needed to calculate soil weight in boxes

The two engineers I talked with about this said they would not trust the clients numbers and would calculate their own and use them. Using the clients numbers would cetainly set them up for liabilty issues.


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RE: Help needed to calculate soil weight in boxes

I can just barely carry a 5 gallon bucket of moist (not wet) sand. A very large potted plant in "potting mix" in a light weight container that had dried out a bit and the plant it's self isn't too heavy might not be so terrible to move about the yard but most really large plastic plant containers I've met tend to break when one tries to move them about the yard. I often use a dolly when ever I move a plant in a container of more than 3 gallons.

Anyway, good luck on the calculations. Have you thought about other options for the roof garden? Perhaps dirt bags

Here is a link that might be useful: Erosion control bags


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RE: Help needed to calculate soil weight in boxes

Thanks for all the terrific suggestions and information

Much appreciated

Lyonsy


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RE: Help needed to calculate soil weight in boxes

I am looking at installing a 150 sq ft green roof on my 500+ sq ft flat roof area. This is a new construction project. I am going with a 3-5" planting medium for the 150 sq ft but I am also installing planter boxes on two floors... each 10 foot x 2.5 foot x 1 foot deep planter will weigh about 1,500 lbs each. I consulted with an engineer and he said the load is fine. But I'm a bit worried. On one 20 foot long, 20 foot high 2 x 4 wall, I'll have a total of 3,000 lbs + 5,500 lbs on the roof. That's 8,500 lbs on a 150 sq ft area. Is it just me or does this not sound safe?


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RE: Help needed to calculate soil weight in boxes

For all intents and purpose, the amount of soil in the box is 3 feet by 2 feet by 2 1/2 feet....60 cubic feet.
but simple arithmatic can be used Length X width in feet, multiplied by depth in inches.
So, 3 feet by 2 feet = 6 square feet multiplied by 60 inches. = 360.
Multiply that by .003 ...giving you slightly more than
1 cubic foot of soil to fill the box...
Since there is no way to tell how much added weight is given the soil due to factors controlling the amount of moisture such soil can hold, you can only estimate.

Since there might be anywhere from 75 to 100 lbs weight of such cubic foot, it comes down to what weight is being borne when the soil is spread out over an area.
This is suggesting the soil will be constantly wet--why is that. Will the soil not dry out and lose a lot of weight.

If a structural engineer cant come up with an exact weight to the square inch, he's not the engineer I'd want to trust.


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RE: Help needed to calculate soil weight in boxes

If you were to put five gallons of compost into a five gallon box and mail it to me, I will mix it with potting soil, leaf mold, and moisture and weigh it, and then I'll publish my findings here.

Ok?


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RE: Help needed to calculate soil weight in boxes

For all intents and purpose, the amount of soil in the box is 3 feet by 2 feet by 2 1/2 feet....60 cubic feet.

The units are correct, but even without a calculator I can see that the volume is incorrect. Try 3 * 2 * 2.5 = 15 cu. ft. Or 3 * 2 = 6; 6 * 2 = 12; plus 0.5 * 6 = 3; which is 15.

but simple arithmatic can be used Length X width in feet, multiplied by depth in inches.
So, 3 feet by 2 feet = 6 square feet multiplied by 60 inches. = 360.
Multiply that by .003 ...giving you slightly more than
1 cubic foot of soil to fill the box...

Now you are mixing units - feet * inches won't work. As well, 60 inches is 5 ft., not 2.5 ft.!

Where did you pull 0.003 from? If It had units, I might understand. Was it just a number you used to get you to 1 cu. ft.?

jeannie7,

What was the point of all this non-math? I don't wish to imply that you should not post your ideas and opinions, but use complete sentences and proper punctuation and capitalization. Separate ideas in separate paragraphs. Then read over what you have written before posting. And lastly, copy the message into an e-mail or other document and check for spelling.

Even dumb things sound good if well presented.


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RE: Help needed to calculate soil weight in boxes

If a structural engineer cant come up with an exact weight to the square inch, he's not the engineer I'd want to trust.

Which is why you are not a structural engineer.

Soil is a combination of organic matter, sand, clay, silt, water, and air. If anyone claims to be able to give an exact weight of any soil, I wouldn't trust them. The weight will change daily with moisture content and compaction...

Engineers deal with multiple variables and design for a worst case scenario.... a fully compacted, moist soil will weigh up to 120 pounds per cubic foot.

I've weighed lots of soil (and rocks). And I would comfortably use that number...


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RE: Help needed to calculate soil weight in boxes

jeannie7:

""This is suggesting the soil will be constantly wet--why is that. Will the soil not dry out and lose a lot of weight. "" I'm not sure this is a question but I understood it as one. For structural calculation purposes, one takes the worst (in this case the heaviest) scenario and there is also a norm of adding a few % for back-up. Of course the soil will dry out but that is not used to calculate things.

We have about 1 ft of snow on my roof and I can't remember ever having so much snow on it since I live in this house. It is not only snow because in between the snow days there were sunny days and rainy days and then more snow. So, there was for sure ice (which weight much heavier than snow/height)underneath all that snow. I prayed that whoever built this house did the right calculation and didn't make any short cuts.

Thank God its gotten 6C for several days now and not much snow left.

Otis


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RE: Help needed to calculate soil weight in boxes

I can confirm the density figures being given - the standard rule of thumb in my business is 1.5 tons per cubic yard, which works out to about 110 lb/ cu ft. HOWEVER this is heavy Missouri clay, which is not what you'd want to use up there. The comments about using engineered light weight mixes for a green roof are right on. If you don't have details on what to use for that, the US Green Building Council might be of some help.

Engineers do have to estimate conservatively, and the roof has to be designed for the maximum wet weight, not the average. Otherwise when the roof collapses you'll be saying, "But on average, that should have stayed up!"


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RE: Help needed to calculate soil weight in boxes

If we designed for "average"; half of all buildings would fall down. :-)


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RE: Help needed to calculate soil weight in boxes

When my clients ask me to do a rooftop garden I always have them get a qualifie structural engineer to assess all the weight issues. Then, and only then do we proceed.

John


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RE: Help needed to calculate soil weight in boxes

Right you are Otis, must have had too much wine at that time....my thinking cap was askew.
The proper measurements....2 by 3 by 2 1/2 = is indeed 15 cubic feet. But we dont want to work with cubic feet....just square feet. 2 by 3 = 6 square feet and multiplied by 2 1/2' or 30".....= 6 X 30....180.
And the amount of soil would, ergo. approx. 1/2 cubic yard.

It might take a little longer than I wish to explain here....but you can indeed multiply feet times inches...

But, nowhere in my original post did I write a result of feet nor inches when I multiplied my two figures.

The .003...is just a simple decimal instead of a fraction.
No need to convert this to that, or that to this....
It is rather very simple to figure.

Joey, far from it, structural engineers do know soil. Just like architects know building materials, building engineers bring out their sliderules and can give you an exactness of soil weight to square inch.
Since water has a definite weight and its portion can be figured out how much is absorbed by a particular soil, I trust such a building engineer can tell what he's working with.


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RE: Help needed to calculate soil weight in boxes

>If we designed for "average"; half of all buildings would fall down. :-)

Reminds me of the saying, "Think about how dumb the average person is, and then realize that half the people are even dumber than that!"


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RE: Help needed to calculate soil weight in boxes

Hey Jeannie, I am civil engineer. I work with soil all the time. Trust me.


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RE: Help needed to calculate soil weight in boxes

Let's assume you fill the boxes with sand. Keep in mind that sandy soils have a higher bulk density than nearly anything else you would fill these boxes with, including clay. Clay IS NOT HEAVY; clay is cohesive. It sticks to itself so that it only feels "heavy" when plowing or otherwise working the soil. But I digress, build the structure strong enough to fill with sand, and you should be ok filling with anything else.

Let's first make some assumptions. We'll say the bulk density (dry mass of soil/total soil volume) is 1.7 g/cm3. This is typical for single-grained structureless sandy soils. Second assumption: particle density (dry soil mass/volume of the solids) is 2.65 g/cm3. This is a typical estimate for the particle density of sands, which is essentially quartz. Knowing these assumptions and the total volume were dealing with, we can calculate the total mass of the solids, total pore space, and the mass of water that will fill that pore space when the soil is saturated.

OK, calculate the volume of the boxes. 8 ft x 2 ft x 2.5 ft = 40 ft3. If you have four of these boxes then you have 4 x 40 ft3 = 160 ft3. If you fill these boxes to 1.7 g/cm3 with sand, the total mass will equal the bulk density times the total volume. First, convert your units to metric. 160ft3 x (1m/35.31 ft3) = 4.53 m3. Now you can calculate total mass of the solids. 4.53m3 x 1.7g/cm3 x (1kg/1000g) x (1,000,000 cm3/m3) = 7701 kg. So, all that sand in 160ft3 at a bulk density of 1.7 g/cm3 will weigh 7701kg when completely dry. That will never be the case though.

If the soil becomes saturated, then all the pore space will be filled with water. We can calculate the total pore space and, assuming the density of water is 1 g/cm3, calculate the mass of water filling those pores. Total pore space is equal to one minus the quotient of bulk density over particle density. Porosity = 1 (1.7 g/cm3 2.65 g/cm3) = 0.358. This number means that for every 1 m3 of total soil volume, there is 0.358 m3 pore space. Just multiply the porosity and the total volume and you get the total pore space. 0.358 x 4.53m3 = 1.62 m3. The density of water is 1000 kg/m3. 1.62 m3 of water would then weigh 1620 kg.

Add the mass of the solids and water, 7701 kg + 1620 kg = 9321 kg. There are 2.20 lbs per kg. 9321 kg x (2.20 lb/kg) = 20549 lbs.
I would say thats the maximum mass you should expect from your solids at saturation.

Keep in mind that I study soils, not engineering. Speak to a license engineer about how to best support your roof safely.


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RE: Help needed to calculate soil weight in boxes

M taggart: thanks for helping to remind me how much I have forgotten about soils since leaving college. Man, figuring that stuff out in school was tough. You did a splendid job of re-explaining the process to me, thank you.


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RE: Help needed to calculate soil weight in boxes

a cubic foot of water weighs 60 ish pounds
a cubic foot of crushed stones weighs 92 ish pounds

So soil is somewhere in between.


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RE: Help needed to calculate soil weight in boxes

I created a little excel spreadsheet based on m_taggart's conversions that will give you the estimated weight of a container if you input the dimensions. There are two sheets, one for a circular container and one for a rectangular one. The circular sheet also gives you the weight if you have a SIP. Send me a message if you want me to email it to you.
(I'm a new member and am assuming here that it's possible to send PMs)


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RE: Help needed to calculate soil weight in boxes

Great thread, could you send me the Excel spreadsheet dobozban?


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RE: Help needed to calculate soil weight in boxes

Great thread, could you send me the Excel spreadsheet dobozban?


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RE: Help needed to calculate soil weight in boxes

Hey dobozban. If you could e-mail me a spreadsheet as well, I'd greatly appreciate it. I couldn't find out how to pm you.


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