Liquefaction and Seismic Site Class

[JennM]

I am looking for input on determining the seismic site class for a municipal water treatment facility. The subsurface conditions consist of medium dense sand to 22 feet, clean & loose sand to 33 feet, dense to very dense sand below. The groundwater is at 3 feet.

The loose sand between 22 to 33 feet is susceptable to liquefaction. We are recommending ground improvement to ~10 feet beyond the building to mitigate the liquefaction potential.

Following the ground improvement work, would the the corresponding IBC Seismic Site Class be D because the weighed average N value is 16 bpf?

 

[fattdad]

How did you determine that the 10-ft layer of loose sand is prone to liquifaction?

f-d

¡papá gordo ain’t no madre flaca!

 

[JennM]

The exploration program consisted of cased borings with STP testing. Several lab grain size tests were performed to determine the percent fines. A groundwater observation well had also been installed to measue the static water level. We used this data in a liquefaction assessment following Youd et. al (2001) which is based on Seed and Idris.

 

[civilperson]

Without shear wave velocity measurements, site class D is appropriate. If the soil improvement is extensive, the measured post improvement velocity may allow site class C.

 

[muuddfun]

How to determine Fa and Fv?
thread487-212388

check out this thread.