Seismic Site Class

[VAD]

Would rockfill be given an automatic seismic site class of C. If rockfill for a building site is contaminated by virtue of construction (to fill voids and create a dense matrix) with overburden soil - silt,sand and minor amounts of plastic fines, would this classification still hold. The assumption here is that the SPT N is 50 plus despite that one would not do such a test on this material if encountered on the surface.

 

[civilperson]

No, not unless the fill was 100 feet thick. The average N for the top 100 feet is the N to use or use shear velocity.

 

[VAD]

Thanks civil person for your feedback:

Lets look at a site that has bedrock that is massive but undulating requiring the rock to be blasted and the blasted rock used to level out the site which results in variable rock fill thicknesses far less than 100ft. generally the SPT test would not be done as one would obtain refusal almost immediately by bouncing on rockfill. Would you do a shearwave velocity on this site. There is a tendency not to do this test because of its cost etc.

Many argue that this site can be classed as a C automatically. Others have classed site as D. Many rockfill sites for buildings are constructed with fines included to create a dense matrix and as well to get rid of overburden cheaply. How would you treat such a site as opposed to a site that has clean rockfill.

In the final analysis, seismic site class can have an impact on costs associated with the structure and ensuring that proper classification by the geotecnical engineer becomes important.

 

[msucog]

the average is for the upper 100'. you could always performs shear wave velocity measurement. well compacted fill is typically better than residuum so if rock is within say 40-50', class c is certainly not out of the question. there are some surface methods that would have no problem measuring the shear wave velicty even through a thich rock fill layer. again, the key is having well compacted fill.

 

[moe333]

I wouldn't "automatically" assign a site class C to the site. Rather, I would attempt to justify it so that you have a sound basis if questioned.

For the site you describe, I would estimate the depth to rock and assign a conservative shear wave velocity for rock to that portion of the 100 feet of the profile. I would then calculate the minimum shear wave velocity required for the remaining thickness of rock fill that would result in a combined site class C for the total 100 foot thickness. I would then compare the required shear wave velocity of the rock fill to what you would realistically estimate it at, and if it appears reasonable, then you will have a good basis for your estimate.

To be conservative, you may want to choose the shear wave velocity of the rockfill to be at about the middle range for a site class D material.