Seismic Site Class
Seismic Site Class
(OP)
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.





RE: Seismic Site Class
RE: Seismic Site Class
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.
RE: Seismic Site Class
RE: Seismic Site Class
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.