Engineered Fill Foundations

[2jdevine]

A client in the natural gas industry has asked me to develop some standard foundation details.
Apparently it is common in many areas of the country to place large equipment skids directly on an engineered fill as a foundation.
What I am looking for is some good references or a design guide to get the background on how to properly design and specify this type of foundation.
Any help would be greatly appreciated.

 

[BigH]

When I have used engineered fill, I would ask for well graded sand and gravel (which invariably led to using well graded road subbase). Placed on suitable soil and checking zone of influence, it was not unreasonable to give 350 kPa allowable bearing. You will need to careful you ensure that you have adequate bearing capacity (2 soil condition -see Button) and that the depth of i fluence is predominantly within theengineered fill if softer soils encountered below.

 

[oldestguy]

Maybe I am of base, but would the client be looking for something like this old nomograph?

 

[civeng80]

Oldest

Thats a beauty. Wish I had a few more of those.

 

[BigH]

Oldestguy - hadn't seen that before - but still, wouldn't he still need to establish the allowable bearing pressure for a particular engineered fill with a specified underlying strata configuration? A good nomograph, though, when you have the load and the allowable bearing values . . .

 

[2jdevine]

Thanks for the responses. That is a good nomograph, especially with the steel included.
But, they are not looking for concrete foundations.
They want a detail or series of details for when they go to a site with X soil it tells them they have to dig down Y number of feet replace and compact with Z soil, top off with N inches of gravel. and they'll know its good enough to plop down their equipment skid that weighs W directly on top the gravel.
I'd like to find a design guide or references. I have some reservations about issues that may arise with a over-simplistic detail. For example what size gravel should be specified to negate any effects of frost heaving? what are the best ways to mitigate water infiltration issues? Are there additional issues with these foundations that I am not even aware of? etc etc.
Thanks again for your help.

 

[CELinOttawa]

Sounds like you need a good Geotechnical Engineer's help. I have a great guideline developped specifically for a client's large alluvial soil site in New Zealand... Nomographs are not hard to develop, you just need the right skill set. I think you need an experienced pro's help here.

 

[BigH]

What I would suggest that you might refer to the following site for guidance. While these might be "road" type pavement structures, skid-mounted equipment is somewhat similar to a vehicle tyre system. They are talking of building "pads" on natural ground rather than digging out and replacing - but the principal is the same - the pad thickness would tell you how much to dig out. The use of geo-grids might be beneficial to you - but if you aren't building a road system - perhaps not that much in savings. I had a paper but will need to look into my files that would also be of help. Will try to get to it.

http://www.tensar.co.uk/jubilee-symposium/pdf/foundationsforroadsandloadedareas.pdf

 

[dhengr]

2jdevine:
I think BigH got it right on his last post. Their raised pad is something akin to the base and sub-base under a flexible pavement, and/or maybe akin to the ballast under railroad ties. It is intended to be free draining and set above grade so the area drains away from the pad. This may require some minimal grading and sloping of the native soils. I think geo-grids may have some constructive application here. And, the more beneficial system might be the one which is actually made up of light steel bar stock, welded together in 6-8" hexagonal shapes which are then filled with smaller gravel, to form the sub-base mat. Also, look at

http://www.prestogeo.com

for a plastic variation. Then the raised pad must be big enough so that the shoulders can slough a bit without undermining the skid. Seems to me the biggest problem with this scheme and this skid support system is developing some foolproof and yet very quick and simple means a evaluating the native soil bearing cap’y., or limiting your liability. Will they allow that a GeoTech Engr. make some determination at each site? Be careful though, once you give them the simple details and a few charts and graphs/nomographs, they will be quick to forget about any soil testing and just plop the skid down where ever it fits best, so be conservative. This is apparently a very common way to support their skids in the field, with or without any engineering.

 

[BigH]

In looking through my files, this might be right up the OP's alley:

http://www.conteches.com/knowledge-...ries/subgrade-improvement-using-geogrids.aspx