Earthquake - ground motion amplification in fill

[Ihatelawyers]

1)Is there a fundamental difference between how seismic waves amplify through an engineered fill vs. an in-situ soil profile? i.e. 30 feet of engineered fill on bedrock vs. 30 feet of alluvium (assume similar soils).

2)If you have an engineered fill 30' thick over bedrock, does that impact design of the structure when compared to a natural deposit (again ... assuming similar soils) - from the soils end? Seems like a Site Class D is a Site Class D regardless of how you get there (but perhaps my ignorance for question 1 is showing).

I have a site that is being scrutinized because we have 30 feet of fill, whereas, if we had 30 feet of alluvium no one would care - we'd just proceed with Code ... and I'm confused (or the other guy is ). I understand that a fill would potentially respond differently, i.e. maybe the associated strains are greater, but I'm having a tough time envisioning that in 30 feet we could get a substantial difference in wave motion when comparing a fill to alluvium (again ... assuming the soil properties are similar).

 

[Qshake]

1. Yes. Wave propagation is largely a function of the bulk modulus which for a soil matrix is a function of the compactive effort and/or vertical pressure acting on the soil near the surface or at depth. The higher the modulus the less attenuation and greater the risk for unabided high frequency waves.

with that said, what type of soil is important as various soil types will amplify the base rock motion. Hence you will always see a soil factor equal to or greater than 1.0.

2. With engineered fill you can improve the site classification and or densify the soil to provide the same effect of the structure on bedrock as on soil.

I hope this helps. Engineered fill is good pending on what type of soil and compaction. It will generally perform better than insitu soil.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[dik]

When a seismic wave goes from a dense medium to a less dense one, the amplitute increased proportionally. If using compacted granular fill, then this is not normally an issue.

Dik

 

[moe333]

If you look at it in terms of attenuation relationships, the only variable would be the shear wave velocity. If the shear wave velocity of the compacted fill is at least equal to the that of the insitu soil, then there would be no net increase in spectral response of the profile.

 

[Mccoy]

What I'd do, I' carry out numerical simulations of seismic response. Then there'll be no more doubts.

In concept, the principle should be the same as in natural soils, but, as Qshake points out, compacted engineered fill may have different elastic properties. The consequent seismic soil calss could even be more favourable than an alluvium soil.

If you have a technically sound counterpart, you may reason that the input parameters for amplification are Vs, density, G (shear modulus) degradation curves and D (damping) degradation curves.
If it can be reasonably shown that such fill parameters are not more unfavourable than your average 'alluvium soil', than they should be all right.

 

[TDAA]

So you have engineered fill underlain by bedrock at 30 feet, and you have a Site Class D? Sounds like your fill may not be very well engineered.

 

[Ihatelawyers]

First ... thanks everyone for taking the time to provide your input.

I plotted up the spectral response curve for each Site Class C, D, and E(ultimately a lesson in futility, but it made my nerdiness feel complete). From there I compared the equivalent base shear and simplified base shear (ASCE 7-05) for the varying site classes (that was easy since the only thing that changes is Sds when the structure is constant). Essentially what I got out of this process was that for a short period structure, there's very little difference in seismic loads when varying between site class C, D, or E. Which is bolstered by the fact that when you run through Code ... all three lead you to a seismic design category D2. So it seems to me, that until your structure gets taller, longer, more complicated... worrying about seismic amplification (except for Site Class F) and its effects are kind of overrated??? Please feel free to refer me to Earthquakes for Idiots ...

 

[moe333]

Depending on your location, going from one site class to another can also force you into a new seismic design category. So site class determination can be important.

 

[TDAA]

Your last post sounds a bit lackadaisical to me. If you are not providing relevant information based off of the actual conditions, then why should they hire you.

I have gone through the same exercise as you , and often it will not matter, but that is no reason to provide incorrect information.

 

[Ihatelawyers]

Understood TDAA ... I would not provide incorrect information. And I'm not lackadaisical, I evaluated the site based on blow counts in the native soils(Site Class D), based on the soft bedrock present(Site Class C), and based on the soft soils described for a site class E (because that profile could also be created based on cut to fill scenario). And going through all of those iterations, I still come up with a design category D2.

My question is ... if everything circles back to a D2, is the structural design impacted by whether or not the Site Class is C, D, or E. It appears to me it is not. If it is ... where/how is that impact manifested?

Hey Moe. Thanks; yes, I've seen how variation in location can impact design catergory. The scenario I've presented seems kind of hinky in that I can have a Site Class c thru E and it doesnt impact the design??? I'm trying to make sure I haven't missed something.