Calling the soil specialists [Engineers welcome as well]

[Shimmy1]

I have been asked to prepare a site for a large steel grain bin. It will be 72' diameter, and 110' tall. There was a geotechnical survey completed before construction of an identical bin about 6 years ago. At that time, the engineer allowed the base to set on 2.5' of compacted fill, which consisted of pit run sand/gravel. Since the plan is 7 years old, it was suggested to do a new one, but the customer has decided to roll the dice and use the old one. I'm not concerned at all about the sub-grade material, I know based on the location that it will be identical. What the customer is wanting to do is increase the depth of the modification from 2.5' to 4.5' to be safe.

What I'm wondering from you guys who read this is the material I'm planning on using has a small percentage of 3-5" rock in it. It's roughly 70% coarse sand, 25% rock from 0.25" to 1", 4% rock 1" to 3", and 1% rock 3" to 5". I am mining it out of a pretty deep pit, and in the process of pushing it up, piling it up, loading it, re-piling it onsite, I am able to shake out 99% of the bigger rock. Naturally, since it probably only has about 1% of rock larger than 3", I've reduced that to probably 0.1%. Is this going to be a problem in this situation? I'm only asking since the geotech says I should use material that passes 100% through a 3" sieve. I've attached a couple of snaps of the engineer's report. Sorry for the long-winded post.

 

[Shimmy1]

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[redneckracin]

I don't think the larger rock will be an issue. Generally you want a placement thickness 2.5 times the largest nominal size of stone so if they "stack" it wont bridge off. I would be more concerned about the moisture content, the lift thickness and the overall compaction achieved before the next lift. Having an onsite nuke gauge might be a good CYA investment.

The bigger question is why the increase in depth? Was there an issue with the other bin? Adding geo-textile will accomplish a similar outcome at a cheaper cost?

 

[mowingman]

I would want a proctor test on the material that is being used for fill. I noticed the report mentions one. Also, you should then take compaction tests as the fill goes in, to see if it is comparable to the original proctor tests. If I was doing the work, I sure would want to be legally protected from future problems. I was a geotechnical/soils engineer 50 years ago, but have tried to forget everything I knew about that line of work.

 

[Shimmy1]

I don't think the larger rock will be an issue. Generally you want a placement thickness 2.5 times the largest nominal size of stone so if they "stack" it wont bridge off. I would be more concerned about the moisture content, the lift thickness and the overall compaction achieved before the next lift.

Thanks. This is the kind of info I'm looking for. I plan on 6" maximum lifts.

The bigger question is why the increase in depth? Was there an issue with the other bin? Adding geo-textile will accomplish a similar outcome at a cheaper cost?

Since they are having me use a 7 year old report, they just want to be safe. I'm confident that the soil is identical to the last bin, but the wording about being cautious about everything down to 4' reflects a possibility of needing it.

 

[terex herder]

I expect that is around a $750k-$1M project, cheaping out on a $5,000? professional evaluation seems like false economy to me. I don't know about the weight of the steel or concrete, but a 110' column of grain alone will top 5,000 lbs/sq ft.

 

[redneckracin]

I should also mention I have better than a basic understanding of soils, compaction, etc., but I'm not a geo-technical engineer. If this really is a nearly $1m project, I think id want things done the correct way. Everyone is friendly till the suing starts unfortunately....

 

[Welder Dave]

If you have clay soil for the base is a good application for mixing quick lime or hydrated lime to firm up the soil. It's used for airport runways and parking lots. I didn't know much about it but have been reading a lot on the internet from reputable sources. It's used a lot more than most people think. I can't remember if it was a million tons or 10 million tons used annually to stabilize soil in the US. Might be worth looking into. That's a lot of weight. There's a national lime association that should have all the info you need.

 

[Shimmy1]

I don't know about the weight of the steel or concrete, but a 110' column of grain alone will top 5,000 lbs/sq ft.

Actually,iit figures out to about 3500 pounds per square foot, concrete, bin, and 220,000 bushels of grain at 64 pounds per bushel, which is as heavy as would ever be in this bin, and not at all likely since that would be wheat.

Corn is 58 pounds per bushel at the very most, soys here about the same, so the most likely scenario would be 3000 pounds per square foot maximum.

 

[terex herder]

Not sure how you got that number. 60 lbs/ bushel is 48 lbs/ft3. So 48 x 110 =5280 just for the grain.

I get 298000 bushel without the peak

edit, 298,000 is wrong, see below.

 

[Shimmy1]

Bin slab is 5000 square feet. 80' diameter, so 40' times 40' times 3.14.

Weight of 220,000 bushels of 60 lb per bushel grain is 13,200,000 pounds. That weight sitting on 5000 square feet of area calculates to 2640 pounds per square foot. Add 1,600,000 pounds for the weight of the concrete and the steel, and you have close to 3000 pounds per square foot.

Am I doing this math too early in the morning??

 

[terex herder]

I'm not rated for calculators this early. 80' diameter slab is 4' outside of bin diameter. Thats a lot of space, not sure I've seen that wide outside bin. Implies a 8' wide foundation ring. I don't feel you can use entire area of foundation to calculate soil loading, as I doubt the slab is sufficiently reinforced to transmit the loads from the center to the foundation.

I get 358,292 bushel without peak, add 17,000 bu for peak. Not sure how I got the earlier capacity.

Is this going to be a raised aeration floor, or are the aeration ducts going to be formed in the slab?

I'm using the 110' for the sidewall. Or is that the overall height? If 110' is overall, prolly a 90' sidewall. 90' would be 293,148 without peak.

 

[Shimmy1]

Raised aeration floor. I could be misinformed on the height, maybe the 110' is to the top of the leg? I do know the customer has said there will be 8' instead of 6' for the extra diameter of the footing. I also know the actual intended capacity is 220,000 bushels.

 

[AzIron]

I just got done doing a 100 ft diameter by 40 foot tall fuel tank pad now our soil conditions are not even comparable with that said my project required 5 foot over ex and backfill with native 10 ft outside the dimension of structure so we cut a 120ft wide circle we also had about a 30 percent shrinkage rate so we imported spec base and mixed it in from the start

In the process we found 2 clay pockets that were sludge and had to cut that out and fill with suitable material all of this was done under the watchful eye of a geo

The intended design was looking for 4000lbs per sq ft or above and the actual load was a hair under 3000lbs per sq ft

On anther tank we did that was 600 ft away that was 60 ft wide and 60 ft tall we had to geopier the pad then cut 4 foot and fill entirely with spec base native was deemed unusable the geo wanted more than 6000lbs per sq ft bearing capacity

I would not take on a project like a tank or grain silo without a geo report there is no margin of error for settlement and those silos will unzip Over nothing

Your methods and plan seem spot on in my opinion really I don't think the larger rock won't hurt in such.low quantities

 

[terex herder]

Ok, that implies a 67.5' sidewall, maybe 18 ring bin. Lots less scary.

 

[Shimmy1]

I would not take on a project like a tank or grain silo without a geo report there is no margin of error for settlement and those silos will unzip Over nothing

Your methods and plan seem spot on in my opinion really I don't think the larger rock won't hurt in such.low quantities

I do share a bit of your concern on doing it without a geo report. The only reason I am even considering it is because we are adding close to 2' to the depth, the other bin has been standing for 6 years with absolutely no issues, and we have had some extreme weather with precip and temps at odd times in season. Obviously this bin is about 33% larger, and the builder is increasing the footprint as well. If I wouldn't have substantial experience with building pads, and have done several projects that required testing, I'd be concerned. Also, I have laid down thousands and thousands of yards of this material and it sets up like concrete with minimal effort.

 

[CM1995]

The owner needs to hire a geotech firm to oversee the fill and placement. I would also have a proctor run on your fill material by the same firm. Let them tell you the material meets the specs or it doesn't.

A testing firm paid for by the owner is the best insurance policy we contractors have. They can be a PITA sometimes but that's why it's called work.

Did the sitework for a new building at a private school that had 1500 or so CY of structural fill import. Did the normal 2 5 gallon buckets of soil and sent it to the lab. Proctor comes back good (or so we were told) and we set out to build the pad. This was a clayey chert with a lot of rock in it. Built the pad, the cassion drillers came in and did their thing and didn't leave a mark this material was so hard.

Well the slab was poured and they were erecting the steel when the civil engineer sends an email out saying the fill material submitted didn't meet spec, it was over on plasticity. The GC's response was in a nice way - "Well you're a day late and a dollar short - there is a building on it!"

It was the owners testing company so it just went away..

 

[cfherrman]

Seeing your guys calulations I found them very weird way about getting the answer and mostly not figuring the height, maybe can't read good. I'm not a engineer or anything just my thoughts on how I get the answer. Instead of comparing concrete psi to bushel weight I figured I would try to find bushel psi. Values shown let me know if I got something wrong

Bin is 110' tall, I assume at the center so we need to know the psi concrete needed at the center. I looked up a bushel volume which is a cube at 1.25 cubic feet, or a cube with a height of 1 5/64'. Wheat has a weight of 60 lbs a bushel and a area of 1.16, which is 57 psi per 1 5/64 in height. You could just round up to the weight per bushel. There's 102 bushels stacked vertical in 110'. I think you could just use 110 again.

102 x 57 = 5814

Or quickly

110 x 60 = 6600

Geo wanted more than 6000 psi.

Unless you count chickens I'm not a farmer either.

 

[Joe H]

Assuming it doesn't work out, who's ass is on the line ?

How much money is owner saving?

I always found the customer forgot who's money it was that we saved when things turned to ****.

This sounds like an expensive project, couple ot thousand dollars gonna make that much difference ?

Joe H

 

[KSSS]

You could put Helical Piers under the circular footing as insurance to prevent the 3-4" of downward settle ment and the 1.5" of side settlement. Give me a call Shimmy I would love to set those piers and get a paid trip home. LOL