seismic base shear for substation equipment foundation

[structural-eng]

I am designed a small mat foundation to support some substation electrical equipment in an area that has moderate seismic loads. The lateral seismic load applied to the center of gravity of the equipment is determined per IEEE 693 and is similar to the load resulting from ASCE 7. Do I need to include the weight of the foundation in the lateral seismic load? The foundation is 2 ft thick and the top of it is 6" above finished grade. I started by conservatively including the weight but because the lateral seismic load is 0.33*W and the coefficient of friction that I was given by the soils engineer is 0.32 the safety factor for sliding does not even equal 1.0. If the foundation weight does have to be included, it seems I would have to use passive pressure to resist the force. Is it common to use passive pressure instead of friction for foundations like this or is the weight of the foundation neglected from the foundation stability analysis. Thanks.