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Dump Truck Retaining Wall on Edge of Highwall

Dump Truck Retaining Wall on Edge of Highwall

Dump Truck Retaining Wall on Edge of Highwall

(OP)
I'm working on the design of a dump point and retaining wall bunker at the edge of a highwall in a quarry. The sketch I attached should explain the situation. The bunker is three sided with a slab on top. Dump trucks back onto the slab against the bumper and dump over the wall into a bin structure. Gross weight of the loaded truck is 247 kips. I'm using 50% of that weight for a bumping force and distributing it over the 14.5' outside width of the tires giving a load of 8.5 kip/ft.

I'm thinking I'll design the front wall as free at the top and use at rest pressure full height with a surcharge for the truck and slab. I believe the approximately 20' long slab on top will distribute the bumping force out to the side walls and not induce a lateral load at the top of the front wall.

Ignoring global sliding and overturning for a moment, does this seem like a reasonable approach for reinforcement design of the front wall?

RE: Dump Truck Retaining Wall on Edge of Highwall

(OP)
Posted too soon.

Anyone see any issues with not connecting the front wall to the bottom of the slab?

RE: Dump Truck Retaining Wall on Edge of Highwall

Two concerns:

1. If the high wall of the quarry is rock, no worries, but if soil, big worry.

2. I would suggest deadmen extending back from the high wall to counter the continual horizontal bumping force from the trucks. Otherwise, it will creep toward the edge with time. Not good.

Mike McCann, PE, SE (WA)


RE: Dump Truck Retaining Wall on Edge of Highwall

(OP)
Mike,

Thanks for the comments.

1. High wall is rock and will be cleaned of all loose material so I think that should be reliable.

2. Deadmen anchoring is one way of preventing sliding due to bumping. I am using the weight of the concrete and the bunker material. That alone, not counting self weight of truck, is over three times the weight of the truck so that provides a pretty large factor of safety against sliding even using a low (.35) coefficient of friction. Agree or disagree?

I've done a few structures similar to this and while they are all different the same type of questions come up with each one. For example, if the slab is supported on the fill material, it's easy to design as a slab on grade. But if there is any sort of consolidation/settling of the fill material after construction there is the possibility of regions of no support under the slab. Depending on the size of that region it can lead to a thick slab.

Another example is the front wall. If you design it as pinned at the bottom of the slab and fixed at the base you can get a more efficient wall design, but then how much vertical load does the wall pick up from the truck?

I realize these are all judgement calls. I haven't seen any situations like this mentioned on here before. I'm just curious of other's opinions/thoughts/ideas.

RE: Dump Truck Retaining Wall on Edge of Highwall

I would design the front wall as laterally supported at the underside of the slab. And I would assume that the front wall takes the entire weight of the rear axle. I might also design a thickened apron slab for the last few 4-5' leading up to the front wall. That way, the slab on grade would have some spanning capacity should the soil settle behind the front wall.

At the proportions specified, I feel that the front wall would actually tend to span horizontally if you chose to isolate the slab on grade from it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Dump Truck Retaining Wall on Edge of Highwall

Bootlegend:

Ok. I think I would pin the sidewalls to the rock to be sure. I do not like to rely on just friction.

Mike McCann, PE, SE (WA)


RE: Dump Truck Retaining Wall on Edge of Highwall

I look at that curb as the bumper. A pre-cast curb maybe your choice, but such curbs have a habit of moving, even if pinned down. That detail has better be fool proof. Is there any maintenance of this facility? Can there be spillage on the slab and thus less curb height? A higher curb than the sketch seems mandatory. Is there any winter use with accompanying ice and snow? Suppose a driver goes over the edge; insurance coverage then for the designer?

RE: Dump Truck Retaining Wall on Edge of Highwall

Good points OG.

Mike McCann, PE, SE (WA)


RE: Dump Truck Retaining Wall on Edge of Highwall

MSHA would never approve that design

RE: Dump Truck Retaining Wall on Edge of Highwall

MSHA? Did you mean OSHA? Or is that another agency?

Mike McCann, PE, SE (WA)


RE: Dump Truck Retaining Wall on Edge of Highwall

quarries and gravel pits are considered surface mines (non-metal), and are governed by MSHA, not OSHA. Highwalls around here require a berm to prevent equipment from driving over the edge. Usually required to be at least as high as the axles of the largest vehicle.

http://www.msha.gov/READROOM/HANDBOOK/PH01-I-6.pdf

RE: Dump Truck Retaining Wall on Edge of Highwall

(OP)
@KootK,
I'm leaning toward the first option you mentioned. I agree with your second opinion as well, but I think I still end up with shear controlling at the base in the middle of the wall usually. Going off memory though. So if I can reasonably design it as a propped cantilever I should get my wall thickness down.

@msquared48,
Pinning it to the rock can probably be done without too much trouble, so I'll consider that too.

@oldestguy,
The curb shown is the bumper and is heavily reinforced. It isn't necessarily to scale, but the bumpers are not ever high enough to prevent the wheel from travelling over it at a considerable speed. The truck bed only provides 26" to 30" clearance from the ground with the bed up, and the clients want to prevent clipping the bed if the driver pulls forward after dumping before the bed is down. The detail we've used has performed well and despite cvg's claim we've used it many times without MSHA issues. Spillage cleanup is part of normal maintenance.

@cvg,
What exactly would MSHA not approve? The sides and approach are graded at an acceptable slope or protected using berms (as detailed in your attached document) but those don't really influence the structural design so I didn't show them in the sketch. That leaves the bumper, and it meets the criteria in the handbook you attached. Someone could get the wheel over it possibly if that was their intention, but ultimately it's just an hazard of the job that people are aware of and we've never had an issue from MSHA. Thanks for the handbook. I hadn't seen that.



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