Selecting subgrade modulus value for slab design

[arh13p]

I have a site with variable soil types. Layers of sand, silts and clays. When designing a slab on grade, how many feet below the bottom of the proposed slab should I be analyzing? There will be loaded front end loaders driving on the slab. I alway use the soil type with the lowest "k" value to be conservative but was wondering up to how many feet below should I go. For instance, if I have 2' of sand overlaying clay should I use the value for clay? Can I do a composite value? Up to what depth? Thanks.

 

[dik]

Selecting a subgrade modulus based on the poorest soil will not likely be of much help. It is more important that the subgrade be 'uniform' in bearing resistance. You can use a low value or a 'modified' one based on replacing the upper soil with something more suitable.

The loading is temporary. You might consider excavating the lower 2' or so beneath the slab. The area should be proof rolled with a sufficiently loaded earth mover or something of that ilk. Soft spots excavated and filled using a proper compacted engineered fill. The 2' beneath the slab should be filled and compacted with suitable material.

You might consider using a geotextile layer and if the area is subject to frost heave (silts are a big problem). Talk to a geotekkie in the area...

Dik

 

[arh13p]

The subgrade will be uniform. We will proof-roll it and replace any unsuitable material so that part won't be an issue. I work in the agricultural field so our slabs have to be liquid tight and a specific distance from any bedrock and/groundwater so we do soil borings all over the site. sometimes the sites will have different layers of soil type uniformily dispersed so I was just wondering even if everything is uniformly compacted, how many feet down should I be observering for subgrade type to determine what k-value to use. hope this makes sense.

 

[msucog]

you could always look at a distribution curve as a means to choose a value