Sandbox Foundations

[Althalus]

Does anyone have experience designing sandbox foundations?

I've inherited a job when I took a new position replacing a now retired engineer. Some vibrating equipment (1200 rpm/150 hp) is supposed to be "temporary". So the client tends to put these on sandboxes. Literally, a box made of 4x12 (6" in the ground, 5.25" above ground) and filled with sand.

I understand how this would work with low or non-vibrating equipment. But at 1200 rpm I'd think that it would eventually dig itself down.

I've never heard of this method before. So, I'm just going off of my general engineering knowledge.

Help?

 

[BAretired]

If it has worked in the past, why not continue to do it? If it "digs itself down" just throw in a little more sand to level it.

BA

 

[dhengr]

I agree with BA, if it has worked in the past why change it. I suppose this has always been true to some extent, but I’ll bet that when BA and I started out, we had older engineers that we could go to for advice. We had old plans and specs. that we could look at for guidance; and the codes were somewhat less complex, so that we worried less about missing some obscure sentence 300 pages removed, which also applied to the problem. Why can’t you guys go to the older fellows and ask why and how? Can’t you contact that retired engineer to gain some of his thinking and insight or talk to your boss? Is the sand just a cushion, being retained by the timbers? How big is this machine in plan and how does its base distribute its load to the sand, in a good uniform fashion? Does what you suspect actually happen and literally pump the sand out from under its base? Look at some of those earlier installations. Is the sand primarily a cushion to limit vibration transmission to surrounding structure? 1200rpm and 150hp, is no big deal, how much does it weigh and what is the amplitude of any vibration, what is the stiffness of the machine base? There are all kinds of 150hp machines, that are well balanced and would sit there as if on your pillow. The sand is certainly an easily leveled base, and it will vib. a bit and adjust to fill any voids under the base. Obviously, a dry powder sand which could be blown out by any vibration would not be a good base. Maybe that sand should have a little binder in it, so once it settles in, it firms up a bit. I almost always have more questions than answers, and that is to make you think about what you are doing.

 

[Althalus]

I understand the "if it worked in the past, then it should work in the future" idea. And that is why I'm not pushing to change it just yet.

I feel that when I'm supposed to put my stamp on it, then I've got to feel comfortable with it. It isn't enough to simply be told. I've got to see it, understand it, feel it, etc. This is something I've never even heard of before. I've only seen a couple pictures of recent designs. I have no idea how it works over a longer term.

The thoughts about it being self-leveling is a good point. That helps. It is supposed to absorb some of the vibration because it is loose. That is specifically why we can NOT have a binder/stabilizer.

No one else in the office knows much about this. I'm supposed to be the expert on this matter and I've never heard of this particular design.

I've got some questions into the retired engineer who designed most of the previous ones. One of them is about the soil specification for the sand. I had a similar question about the "dry powder sand". But I fear the message will just be "nothing special, just sand."

I'm just trying to get ideas on what physical movement/action will be governing the sitting loosely on the sand and the movement of the sand. If I get enough ideas, I can let it float around in my head until I'm comfortable with it enough to put my stamp on it.

Any other thoughts?

 

[MikeHalloran]

The railroad bridge underpinning the Huey P. Long Bridge in Jefferson Parish, Louisiana (as opposed to the HPL bridge in Baton Rouge) is built on sand, over a thousand feet of mud/clay.




Mike Halloran
Pembroke Pines, FL, USA

 

[BAretired]

Yes, one. Don't put your stamp on it. It is not your design. It should be done only at the express wish of your client and he should accept any risk associated with it. Fortunately, there does not appear to any safety issue involved.


BA

 

[dhengr]

You may want to answer/address some of the questions in my earlier post. If the client wants to do it, and has done it many times before, why do they need your stamp on it? Are you actually in the business of acting as their insurer of last resort, or do they really want your best engineering advice and guidance? What does the machine manuf’er. say about the found. under their equip? Is it on a skid, where, other than sufficient bearing area, the skid is pretty much self sufficient? What happens if it does settle a few inches? What are the underling soil conditions, isn’t the sand sorta intended to just distribute the load? I didn’t mean talcum powder sand which could be pumped up and out, or a dry, round grained sugar sand. I did mean a well graded, angular grained, well compacting sand. When I said some binder, I didn’t mean 50-50 sand and cement, but it should compact well and not turn to powder then. Although, a lean concrete mix within the timbers might not be a bad solution. That’s fly ash, sand and some cement, 2000-2500psi, easy to break up after the fact. What happens when you put any vib. equip. on a found. on sand? It can settle, and a GeoTech guy should help you with that. Maybe dig the sand hole deeper than 12," otherwise the timbers act as lateral restraint just as adjacent virgin soil would around any found, as relates to the lateral shearing of the soil. But, it raises the bearing surface a few inches above grade so things drain better. Maybe you should install a few tie rods btwn. the long side timbers at mid depth, then install/compact the sand. What about a couple 16" deep x 20" wide x 12' long conc. beams, cast in a trench (maybe formed and precast), with some rebar, which can be reused for the same purpose on the next machine? What about a properly sized 8x8 timber crane mat, directly on the leveled existing soil?

 

[a2mfk]

Nothing wrong with sand if it is properly compacted. Most of Florida is sand at the near sub-surface, and often we can easily get 2500+ psf of bearing capacity. Since the machinery will be vibrating, I think you would want to be very specific in your compaction criteria and grain size (larger). What did the previous engineer specify in soil specs?

 

[Althalus]

I got a hold of the previous engineer.

Apparently, this is only valid for non-vibrating or very low amplitude vibration machinery. And for the size of the machine (over 20k) 150 hp is not going to vibrate much. We obviously can't anchor it much because... HELLO! Sand!

I asked about the sand. There is no spec -- just sand. He said it is no different than simply placing something on native soil or structural fill (w/o concrete pad). The purpose of the box is simply to raise it above a certain flood level. We use sand because it consolidates & compacts immediately and drains well.

Then I got another bomb dropped on me. The client never asked for it. It was all being pushed by the previous engineer. But what's worse is that the client just wants to set this on the gravel fill. Still no concrete. I guess it will be fine as long as it doesn't flood.

 

[BAretired]

If you are taking professional responsibility for the behavior of the sandbox foundation, then you must be comfortable with its expected performance. There is certainly a possibility of repercussions from the client if your firm is recommending it and it behaves poorly.

BA

 

[TLHS]

I've done prepared earth as foundations for skidded movable equipment before. Really the thing you need to do is talk to the manufacturer or mechanical engineer on the project and make sure the possible settlement and vibration isn't going to break something and that the piping system doesn't require tie down of the pump for some reason. Then communicate any risk to your client and put it on your drawings.

You may have a different scenario if there's reasonable risk of the thing walking on you (and that's unacceptable to the client), or if you're in a significant seismic zone and there's a risk of it tipping or something else.

Rule out any safety things that bother you, and then just make sure any serviceability issues are communicated to the relevant parties. This sort of thing happens a lot in the construction and resource industries where you aren't going to bother building concrete pads for all your temporary facilities that you plan to move every couple of months.

 

[TLHS]

If it's not a pump, you can obviously ignore the piping system bit.