Slab design with no geotech input 4

[atrizzy]

Hi all,

I have a very small little equipment slab that I want to design without requiring a geotechnical engineer's signoff.
Wondering what you folks do in my situation... do we:

1) Assume a ridiculously low bearing pressure (50 kPa)?
1.1) Inspect the native soils' strength using the rebar-prod method, foot imprint, proof roll, or some such method to test for rebound/compression?
2) Require native soil excavation/replacement with engineered fill for a certain (300mm) depth?
3) All of the above?

Eager to hear everyone's thoughts, and thanks in advance.

 

[r13]

What is the weight of the equipment, size of foot print, static or dynamic?

 

[JStephen]

In the US, the common building code, IBC, has a section 1806 for presumptive soil bearing values, which can be used when geotechnical information is not available. Bearing pressures are low, but not "ridiculously" low.

 

[atrizzy]

Static load, think a bit larger than your average neighbourhood transformer pad. Maybe 3m x 4m.
Most utilities just use standard un-engineered designs for this sort of thing, so I'm trying not to break the bank for this private client.

 

[dik]

JStephen: "In the US, the common building code, IBC, has a section 1806 for presumptive soil bearing values, which can be used when geotechnical information is not available. Bearing pressures are low, but not "ridiculously" low."

I don't have/use the IBC. Can you possibly scan the section on allowable bearing values.

Thanks


Dik

 

[r13]

Use your numbering system, I would,

1.1 - Visually inspect the soil, maybe dig a small hole down to frost depth.
1. Determine soil type and bearing strength per suggestion of JStephen.
2. Yes. Down to frost depth.

 

[atrizzy]

retired13, that sounds like a great plan.
I'd then fill to u/s slab with granular engineered fill to avoid the need for insulation.

JStephen, if willing, I'd also love a copy of the table you refer to.

 

[SlideRuleEra]

I have a very small little equipment slab that I want to design...

The key words are "very small".

I would (and have) use prescriptive bearing pressure, per JSteven's recommendation.
Soil replacement/compaction, your #2.

Make the slab, say, 20% thicker than your calculations indicate. For a "very small" slab, preparation, placement & finishing extra concrete are virtually "free" for the following reasons:

Excavation/backfill/compaction are not related to concrete thickness.
Forming is essentially independent of concrete thickness.
Concrete placement... maybe a few minutes extra.
Finishing & curing... independent of concrete thickness.

Even cost of the extra concrete may be "free" if the slab is so small that there is a minimum price charged for concrete delivery.

http://www.SlideRuleEra.net

 

[r13]

IBC Chapter 18 is linked here, see section 1806 (Table 1806.2). Link

 

[atrizzy]

Hi all, FYI found this table JStephen was referring to on this website.

https://up.codes/viewer/utah/ibc-2015/chapter/18/soils-and-foundations#18

Can't vouch for if it's up to date...

 

[dik]

retired & atrizzy: Thanks very much... I always like to have references...

Great


Dik

 

[skeletron]

Bracket the solution:
-What do you usually expect for the size?
-What is a design you have used in the past in the area?
-If you had to design that previous slab again, but in worse soil, how would the design change?


Put a note on your drawing stating the area should be clear of organics, dry undisturbed native soil, etc. to prevent poor construction. Inspect the area yourself. Have the client understand the risk.