Pressure on structure engulfed in stockpile 1

[bootlegend]

I'm looking for thoughts on the situation shown in the attachment. A structure supporting a reversing conveyor. The structure will become engulfed in the crushed stone stockpile. What type of lateral pressures would you use? My initial approach was:

1. Assume only one stockpile in place.
2. Use design material height of 56.5'
3. Calculate pressures based on active state with an inclined backfill
4. Apply loads to all four columns and all the beams without counting any "shielding" from the uphill columns and beams.
5. Ignore any resisting pressure on the downhill side of the members.

I feel like this is too conservative, but I'm don't have any sort of reference for the two things that could help.
1. Resisting pressure on the downhill side would help. Would you use at-rest? Passive? The stockpile is gravity compacted from free-fall, so the material will just flow around the columns with no mechanical compaction.
2. How would you handle the shielding? I think there would be some, but just assuming some percentage seems questionable.

 

[WARose]

If it were me.....I'd have active lateral pressures on one side while ignoring any help from any "passive" on the other. (You aren't suppose to use passive for overturning resistance anyway.)

Especially in something that is not compacted.....it's hard to predict what can happen.

 

[bootlegend]

WARose,
That is the approach I have taken but it just seems too conservative at this height. At 20' it isn't that big of a deal, but at this height it's an issue because as the section width increases it carries more lateral pressure. I'm more concerned with the member design at this point. I'm considering a resisting "psuedo-active" pressure on the low side equal to a height of maybe 1/3 the material height on the low side. I've not punched in any numbers yet and I'm not sure it would be enough to make a noticeable difference.

 

[WARose]

What are your issues? Is it the individual members (of the frame) or is it the foundation at the bottom?

If it's the individual members, you might be able to take a "tunnel" type approach (for vertical load) at certain depths.

 

[Earth314159]

My gut feel would be to design the concrete structure to go higher if at all possible. Concrete just seems to be a better option for taking a beating.

I am not a geotechnical engineer and this is really a geotechnical design problem. However, if 32 degrees is the stable slope with no movement in the soil at that angle and the structure is an open lattice, the structure is not required to support the slope. If you remove the structure, the slopes will not collapse. There might be a bit of rolling debris etc. but nothing major. It is the surrounding soil that provides the lateral resistance.

Maybe a good approach would be to look at the actual stable angel (32 degrees) and a stable angle with a reasonable factor of safety (say 25 degrees) and then take the active pressure as the difference between those two angles.

 

[Ron]

I would use at rest pressure, not active. You have to have structure movement to mobilize active pressure and there's no evidence that the stockpile is moving the structure. A phi angle of somewhere between 30 and 35 would probably be appropriate as well.


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[Illbay]

Ron, you are implying that the coefficient of pressure ought to be about 0.5 rather than 0.33? That is quite a leap.

My assumption is that with the "flow" of the material around the structure in question, this results in the lowered value for the coefficient. Unless you have a structure that has been specifically designed to NOT move under such an effect, active pressure is appropriate.

I'd like to hear your response, as this has become a vital issue in the operation of my employer.

"No one is completely useless. He can always serve as a bad example." --My Dad ca. 1975

 

[bones206]

I've never dealt with this particular issue, but I'll share my thoughts. My initial feeling is that the gravel pile would be self-supporting. Therefore, the members that are completely engulfed on all sides would be in a state of hydrostatic equilibrium, with no unbalanced lateral load. But if there is gravel build up on one side of a member only, then an unbalanced force would develop but only along the height of the build up. Not sure if I'm conceptualizing this correctly, but that's my initial take at least.

I would also consider applying the at-rest coefficient to the lateral "build-up" pressure, since I would assume that massive structure isn't going to deflect appreciably.

I look at this problem analogous to a wave passing through a wharf pile, where the water on one side of the pile is lower than the other side. The lateral pressure is only unbalanced over the height of the wave.

 

[bootlegend]

I've been meaning to circle back to this for a while now. I located an article (attached, Loads for Design of Stacking Tubes for Granular Material) that discusses this condition and deals with it through a trial failure surface approach. I intended to use a few of the different methods mentioned in this thread and compare to the method in the article but I've yet to do it.

 

[IDS]

The procedure used for precast arch tunnels for stockpile conveyors is:

- The structure is backfilled with compacted select fill
- Carry out a staged 2D FEA with plane strain elements, modelling the backfill sequence and maximum stockpile loading
- Check for a stockpile to one side only, with the toe of the stockpile over the tunnel, and maximum stockpile slope.

A similar approach seems reasonable for the structure in this thread, provided that there is controlled and compacted fill around the structure that will not be excavated during operation.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[oldestguy]

What would be interesting is a model test first to get a handle on the range of pressures likely. Alternatively placing pressure measuring cells on the structure might be needed to see if any critical pressures develop while first loading.

 

[Illbay]

bones206: this might be a valid theory if you had a stable pile of material, and you somehow were able to put the structure into it post-situ, without disturbing anything. However, consider the reality: the material stockpile forms around the structure, with the material sliding into place as gravity dictates, and the structural members being "engulfed" represents an impediment to the moving material so that a pressure naturally builds.

bootlegend: thanks so much for this resource, it has a lot of valuable information.

oldestguy: I am constantly running into situations that ALMOST make me wish I were a grad student again so I could undertake the research. I think this would indeed make a great research project and could be a M.S. thesis or even a Ph.D. dissertation topic if expanded. There seems to be a lot of research by the Ag Engineering folks into the interactions of containers, silos, etc., with solid granular materials but as stated by the O.P., there's not a lot of information out there that directly addresses this problem. I began working for a mining operation last year, which is why it's become an issue for me.

"No one is completely useless. He can always serve as a bad example." --My Dad ca. 1975

 

[ldeem]

I do almost all my work on mine sites and have dealt with stockpiles in the past on several projects. First off some general tips.

1. It's mining - the structure will be abused so allow for a conservative design. I usually go with smaller thicker sections, angle legs 5/16 min preferable 3/8 same with beam webs.

2. No mater what the design intent is assume the worst possible loading cases. In your example that would be one stockpile as high as it can go and the other completely depleted. This isn't unreasonable when you consider there could be a problem with the reversing conveyor or the reclaim such that they just run one side.

3. Use At-Rest for your lateral pressure coefficients. Structures are typically rigid so they will not get the small deflections needed for active pressure.

Some comments specific to this design.

1. I would not put steel inside a stockpile like this. Fines will collect around the structure and cause corrosion. Since it is in the stockpile it is unlikely to ever be cleared and inspected. I have done many repair projects on columns where fines have been allowed to buildup and the column rusts through. The steel will be constantly abraded by stockpile material so its unreasonable to expect and paint or HDG to last.

2. Pay close attention to overturning moments on the foundation. You need to consider deflection due to the uneven bearing pressure since a small differential settlement will give a big deflection at the top of your structure. It looks like you have lots of room on the footer so this could be a large mat.

3. Pay close attention to existing soil conditions. A lot of mine sites are constantly being regraded so a soil boring is probably warranted.

4. When looking at the stockpile differential loads consider withdraw or reclaim as well. Even if there is reclaim tunnel its possible that loader will be used and they will take from one side of the pile only.

I hope this helps - feel free to PM me if you want to talk though any part of it.

 

[Illbay]

I've found a very valuable article addressing some of our questions.

"No one is completely useless. He can always serve as a bad example." --My Dad ca. 1975

 

[Illbay]

Ideem: Re: Steel. I have taken to using ASTM A588 steel in these structures, which I think greatly mitigates the concerns about corrosion.

I should also say, that I recently replaced a steel structural support that had been in place for about sixty years. It suffered far more from being "beaten up" as you suggest, than from corrosion.

"No one is completely useless. He can always serve as a bad example." --My Dad ca. 1975

 

[ldeem]

I have considered A588 in the past but have so far not used it. Have you had any pushback from fabricators when specifying A588? I am wondering if availability of standard shapes is an issue.

I should qualify most of my experience is in limestone and some in coal. Perhaps other materials don't have the same problem with fines and water.


I have saved both the papers you posted in this tread - I am always looking to add to my library.

 

[oldestguy]

Per Idem, now consider the dozer shoving material off to use it. He backs into the supports. Better be damn strong.

 

[Illbay]

Ideem: There is absolutely no "pushback" on A588, and the availability and costs are negligible compared to A992.

We are a very conservative bunch, but the fact is that this steel has been around since the 1960s. it's not radical by any means.

"No one is completely useless. He can always serve as a bad example." --My Dad ca. 1975

 

[dauwerda]

A588 and A847 are both often used in the utility industry for transmission poles. While it works great in the correct applications, it is not ideal in certain environments - this includes coastal areas and anywhere high chloride levels are expected (certainly some mine sites). The steel must also undergo wet/dry cycles to form its patina, so it does not perform any better than typical steel if it is in constant wet/damp conditions.

https://www.steelconstruction.info/Weathering_steel#Limitations_on_use