Estimating the Allowable Bearing Capacity of Clay or Sands 3

[smcc23]

I realise that allowable bearing capacity is a function of strenght and settlement characteristics. However, based on solely yhr estrenght, what is the general rules or correlations for allowable bearing capacity to field tests or lab tests (eg, SPT, Unconfined compressive strength tests).
SMc

 

[fndn]

There is a great book that covers this subject very well and I was fortunate enough to learn from it-"Essentials of Soil Mechanics and Foundations-Basic Geotechnics" by McCarthy. 1990's edition is more favorably priced and is still valid. Once you have a handle on the manual computations you would find it best to have a spreadsheet perform the tedious computations.

 

[BigH]

For sands, it is ever seldom the bearing capacity (or allowable bearing capacity) that governs - it is the settlement. So, for that, as a first approximation, I would use the charts you find in the various texts on the SPT vs Allowable bearing pressure (usually 25mm settlement). For Clays - on strength alone, I usually use 2xSu for the allowable bearing capacity - rule of thumb (e.g., qult = 6xSu/3 or therefore 2Su. Of course, you have to take into account settlements and check this out subsequently. For soft clays, you never know which will govern. I never really saw the McCarthy book - I do like Tomlinson, myself as well as Tschebotariof.

 

[smcc23]

Thankyou to both of you. Will have alook at those text books. Also, for clays, I always thought it was 2 x Su but couldn't actually find it mentioned in any of my books. Perhaps because it is just a rule of thumb. Anyway Much appreciated

 

[jdonville]

smcc23,

I have seen the 'Nc' value for phi=0 in the Terzaghi bearing capacity equation reported between 5.14 and 5.7, depending on the source and the particular application.

Using Nc=6 provides the easy rule of thumb.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[BigH]

jdonville: For square/round footings, I've usually seen from 5.14 - as you indicated to 6.2. For the latter see Skempton - he has a chart for varying Nc values ranging from 6.2 (i.e., 2pi) to 9 depending on the foundation depth ot breadth ratio for square or rectangular footings; from 5.2 to 7.5 for strip footings. (Skempton, A.W., "the bearing capacity of clays", Building Research Congress, Div. 1, 180 (1951) - as in and referenced by Tomlinson's Foundation Design and Construction book. Scott (1960) does a detailed analyses and shows the 2pi value.