Placing Skids on Gravel Pads 1

I'd start by deciding if the IBC actually governs. Is a skidded piece of equipment really covered by the IBC? I haven't really looked into it, but I can tell you that I wouldn't take the NBCC as governing a skidded piece of equipment at grade level. I'd use some of the loading provisions but a lot of the other considerations I wouldn't consider binding.

Think of it this way. If it were trailer mounted, would it fall under the building code? Does removing the wheels make it a building?

I can tell you that this is a pretty standard setup for a lot of movable (and some non-movable) equipment in mines and similar locations.

When you're looking at static equipment, it's fairly easy to make this situation work. With dynamic equipment you need to have discussion with the owner with regards to risks. The equipment may be stable against overturning, but lack of hold-down can be pretty damaging to the equipment. It can significantly decrease the lifespan of mechanical systems because the motor and equipment will come out of alignment. As soon as your mechanical train loses alignment you start getting signficant vibration and that affects equipment life. In some cases, this may be just fine for the owner.

With regards to frost protection, what's the actual danger? If the heave wouldn't significantly affect your equipment, or if fixing the situation is not difficult, then it may not be a concern. Once again, this is a risk to discuss with the client but isn't necessarily a show-stopper.

Oh, and you can protect from frost with a gravel foundation.

Either extend gravel to the frost line, insulate or provide drainage to avoid the freeze condition.

Look also at the equipment manufacturer's requirements. Many will require a foundation, particularly with reciprocating equipment due to alignments, vibration, and other effects on the equipment and its performance.

You might find this thread helpful.


A few weeks ago a colleague, who does a lot of work in the natural gas industry, was telling me about a settlement problem with a skid compressor. I asked why wasn't it installed on a concrete pad, and he replied, "who puts a skid on concrete?"

Any suggestions on designing gravel foundations? I've done some research, but can't seem to find information anywhere. I sat through an ASCE seminar on foundation design for equipment and they mentioned using gravel foundations, but did not go into any specifics. I'm assuming you want to use non-frost susceptible fill a maximum of 6% passing 200 sieve similar to what is specified for Frost Protected Shallow Foundations. How do you keep the foundation area drained so it doesn't fill up with water? Any good ways of estimating an allowable bearing pressure to make sure there is no settlement?

It really depends on what you're doing.

If it's really temporary and rough you might just throw a leveling pad of gravel down and call it a day.

If you're doing something a little more permanent it's pretty similar to how you'd design any foundation. Check your loads against the bearing capacity of the soil. If it's not adequate, use gravel to carry the load down until it's either at a competent bearing strata or it's spread out enough to work. Skidded structures are generally comparatively light, though, so none of this is generally an issue unless you're on bad soil. If you want written guidance, I bet there's some stuff in design guides for large tanks. Gravel rings and pads are reasonably common for those.

Double check that sliding and overturning aren't a concern. With larger scale equipment it's not generally an issue, but there can be problems on lighter objects that may come on skids. If these are issues, you either have to weigh them down or build an actual foundation.

Generally try to put your foundation a few inches minimum above prevailing grade so water stays below your top of gravel pad.

As far as materials, it's not really all that sensitive unless you have specific needs. Avoid fines if there's a freezing issue. Otherwise, check what they like to use at whatever site you're doing this at. They probably have common suppliers for material or crush their own. Review what they've got. A well graded gravel will lock together nicely which is great, on the other hand if there's lots of water you might want voids to allow better drainage through the gravel.

There may be corrosion concerns, in which case you may want to raise the skid slightly using railway ties, jacks, scrap material, or something else.

This is the sort of thing that they likely already do at the site(s) in question. See if you can take a look at their installations and use that as a starting point. You obviously have to be comfortable with and check whatever you eventually tell them to do, but there's no point fully reinventing the wheel if people are already using something with a history of adequate service.