Support Members in Stockpiles Loading 1

[eit09]

I would like to know how others approach calculating/ applying soil loads to structural support members located within granular stockpiles. I am also looking for reference material for these types of designs.

I apply the Rankine active force the full height of the support member located in the pile. This Load case applies to the configuration when excavated material is removed by bulldozers/ front-end loaders operating on one side of the support and not the other. When the support becomes excessively large & the stockpile material is free of silts and clay I normally reduce the active force by 50-75%. I do this with the understanding that the material for the most part freely flows around the support providing resistance against the other face of the support. I do not have any reference material backing my approach it is mostly intuition and back engineering of similar existing projects.

 

[aggman]

First answer...Good luck finding references other than your typical soil mechanics books. I've been doing this type of work for 15 years and haven't found much.

On the balance, I would tell you to be very conservative with your designs on these. We have seen a lot of piers moved, cracked, reinforced, etc. due to designers accounting for passive restraint on the low end. While we have been told our designs are too conservative by plenty of non-engineers, we have not had any failures to date. We've probably designed 40 - 50 stockpile conveyors that have piers in the reclaim piles since I've been here alone.

Here are some things to consider:
1) How much rotation is required to engage "active" pressures?
2) How much rotation is required to engage "passive" pressures?
3) How consolidated is the backfill? (Level of compaction)
4) Is the backfill granular? Does it have cohesion properties or sticky materials in it?
5) What affect does the pile slope have? (Both uphill and downhill)
6) Can the operator dig out behind the piers without the material spilling around? If the material can stand up vertical, is it in an active state?

Remember that the material does not come in behind the pier very well on the low side of the slope. Think of wind (or water) going around a cylinder and the turbulence on the leeward side. The material flowing around a stockpile pier will do the same thing. So the typical worse case design is going to be building a vertical conical pile that flows around the pier at a 30 - 40 degree slope. The diligent engineer looks for some restraint from the downhill slope material. It's possible to get some, but how much is the true amount? You have to account for the loss of material on the leeward side due to turbulence. Then, what state is the material in? For it to be active the pier has to rotate away from the material. These are actually rotating towards it. For passive we have to rotate into the material quite a bit, and it has to rotate into soil consolidated enough to actually generate resistance (so we have to compact some of that soil). It could be at-rest, but the pile is sloping downhill. Even if the pile is full on passive, it's downhill sloped so it's more or less at the active pressure level from a numbers level. We have looked at this time and time again and ultimately, never were able to convince ourselves to account for anything to hold the pier back.

That's where we approach the design from. Maybe we can have some further discussion and see what you think. A lot of people use the conveyor to help hold the piers. It's a good option if you've got the steel to do it and we account for it when we can. One thing is for sure I would be cautious on what you allow yourself to get talked into because when it doesn't work you will be hearing crickets chirping behind you...

 

[dhengr]

Eit09:
There was a pretty good thread on this topic within the last few months. It pertained to material loads on horiz. structural members or a bracing members, I believe it was on a support tower in a stock pile of material. As I recall active and passive material pressures were discussed, but the bigger issue was the material flow, and settlement/compaction, and the drag/frictional pull on the member, as Aggman suggests above, over and above just the material weight above the surfaces of the member. I don’t recall which forum this thread was in, but I’d bet it was the structural, civil or mech. forum. Try searching this general topic, I don’t have very much luck on searches.