The PML boundary has to be placed at least two elements away from the INTERFACE_SSI, where the LOAD_SEISMIC is applied. It's up to you to decide where to put the INTERFACE_SSI - if the foundation pad is not sliding on the soil, you can even place it at the pad-soil interface. Else, you can define the INTERFACE_SSI at a small distance from the pad, so that it encloses any non-linearity near the pad.
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