Settlement calcs for a wide load 1

[muthu856]

I am trying to do a quick calculation for settlements using elastic settlememt formulas. Most formulas require an input for width and/or area of the footing. The load for my problem, extends more than 250 feet and similar on the length. Is there a elastic settlement formula for such loading?

Aprpeciate ur response.

 

[MiketheEngineer]

Just use a smaller piece and analyze that

 

[muthu856]

Based on the length and width of the loaded section, the settlement varies. How can this be normalized?

 

[MiketheEngineer]

Look for worst and best case scenarios.. Go from there

 

[BigH]

Theoretically, you should be able to use it but very cautiously. Need to determine whether you are a flexible mat or rigid mat. The most important aspect will be determining the correct "E" value to use (very difficult) which makes this a crap-shoot at best. Find the point where 50% of load is above and 50% is below. Use the "E" value from this point (assuming that the E value increases with depth). You can, and should, use the E value equal to half the applied loading at this point . . . the applied loading will increase the E values. Also with a mat this size, I would be surprised if you didn't have a layered system - running into rock much shallower than the zone of influence of the loading - see Poulos and Davis' Elastic Solutions for Soil and Rock Mechanics. As I intimated, it will be a crap-shoot at best.

 

[fattdad]

I seem to type this about once every couple of weeks, so here goes. . .

Calculate the stress increase with depth (you can use any number of elastic solutions to do this.

Generate a subsurface profile of soil modulus with depth.

Integrate the stress profile and divide by the soil modulus.

Here's an example:

Let's say you have a soil layer with a modulus value of 100 tsf. Let's say the layer is 10 ft thick. Let's say the top of the layer the stress increase is 2400 psf and the stress increase at the base of the layer is 1,600 psf (i.e., the average stress increase is 1 tsf).

Let's integrate! 10 ft x 1 tsf = 10 t/ft
Let's divide! (10 t/ft)/100 tsf = 0.1 ft
That's 1.2 inches!

If you really have a "seat of settlement" that goes down 250 ft, then you have to make some assumptions and use a hyperbolic modulus model. Elastic modulus is a function of confining stress (for strain softening behavoir), so modulus values increase with depth.

Correlation from SPT to modulus is often misleading. However something like 7 to 11 N is in the range.

A dilatometer is a great tool for in-situ measuring of modulus values.

Have fun!

f-d

¡papá gordo ain’t no madre flaca!