Density of soil in large planter

[ajk1]

We are designing some heavy temporary shoring for a roof that supports soil including soil in a large 15 foot square planter with 36" depth of soil in it. The planter has been there for about 35 years. The unit weight for soil around basement walls of buildings is usually taken as about 130 pounds per cubic foot. Should that be the density that is also used for soil in planters, or would the planter soil be significantly less dense because it would not originally have been compacted? I know that we can do the conservative thing and use a 130 pcf density, but I just wondered if that is what would be the normal procedure when determining the weight of soil in planters.

 

[Ron]

If the planter has been there for 35 years, the soil is likely reasonably compacted; however, the soil is likely a sand or loamy sand so as to be relatively free draining. Have a testing lab go out and do an in-place density test using either a drive sleeve or nuclear method. Quick, easy and inexpensive compared to guessing incorrectly!

 

[oldestguy]

130 maybe is ok as an average, not particularity conservative, but it can go higher. Depending on saturation from rain, some soils can reach 150. If I had to guess, I'd not go below 140.

 

[JStephen]

It seems like the potting soil they sell in stores is very light, and I would think the density in a big planter like that could be all over the scale. Good chance it's not uniform down through there, either.

 

[dhengr]

Ajk1
In that big a planter, 15'x15'x3' deep, you may want to consider a fairly large tree, its concentrated self weight, and its root system tending to lift on one side and add load on the other side during a high wind event. You probably won’t find a whole chapter on this in ASCE 7, but if they get wind of this thread, watch out for the next edition, another 30 pages. You might want to consider this if the wind moment on the tree could distribute much of the planter’s weight over only one spanning or supporting member. Within reasonable engineering judgement, no body is going to reward you for saving a couple pounds of steel or timber in your shoring design, so be conservative on this kind of design.

 

[aaronPTeng]

Ensure you allow for water build up in the soil, Dont cut it fine, suspended planters = expensive. No need to re invent the wheel with this one don't lean out too much.

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[racookpe1978]

OK. So you have the weight of the planter. Write it down.
Now you need the weight of the soil in the planter. Write that down.

Now, somebody fills the planter and soil with water - which seeps in between the little nodes of the soil and completely fills the planter volume with water some night because he/she forgot to turn the water hose off. Very heavy water, filling the total volume of the planter (minus a little of soil) interior with very heavy water. Write that assumed weight down.
Now, over 35 years of tree growth from CO2 in the air (never measured yet!) the trees in the planter weigh ??? 800 lbs? 1200 lbs? 1800 lbs? 2800 lbs?

OK.
After you add all of those, THEN you can guess the total weight of the planter. 8<)

 

[ajk1]

This is a planter with only quite small shrubs, no trees whatever. I should have said that in my original posting. I'm surprised how many responses this generated. Thanks all for your comments. You have convinced me not to go lower than 130 pcf.

 

[TDIengineer]

I have always used 120 pcf for soil. The soil over the roof must be drained, so I think considering saturating is overkill.

Not to nitpick, but I find it hard to believe soil weighing 150 pcf (the density of concrete!) unless the soil is actually solid bedrock which it wouldn't be in this case.