seismic site class

[structSU10]

Having a general discussion - A colleague of mine argues that if you meet the shear wave velocity requirements for site class A or B but have more than 10' of soil between the bottom of footing and the rock, the site class A or B is still applicable. I take the statement in ASCE 7-16 (20.1) to be a hard limit where if you have 10' of soil below your footing you cannot be A or B. What have others seen on this issue?

 

[BridgeSmith]

The way we interpret the AASHTO provisions on this subject, the class is determined by the average shear wave velocity in the 100 feet of material below the foundation. I don't believe AASHTO has the provision about the 10' of soil you referenced, so that may change the answer for you. The shear wave velocity is an indication of the magnitude of the ground motions for a given magnitude of earthquake.

The addition of the site class parameter was primarily due to the disastrous 'Mexico City earthquake', where despite being farther from the epicenter than the west coastal areas, the lateral ground motions were much larger, with much more devastating results. It takes a lot more energy to move massive rock than it does to move silt, and Mexico City is built on hundreds of feet of silt, so the lateral ground movements were huge.

 

[emmgjld]

What is the soil layer?
What is the ground water condition?
I could go either way, depending upon the conditions.

 

[structSU10]

The particular site has pretty decent stuff - glacial till - from about 5 feet below current grade. There is anywhere from 17-32' of this until weather rock.

My main query though is whether the 10' below the footing is a hard stop as far as defining a site as A or B. They have the same provision in ASCE 7-22, and they have changed methodology to be entirely shear wave velocity based with more classes.