Usually infeasibility is proven based on the underlying soil's conductivity . We have to do infiltration tests as part of the geotechnical investigation for the site prior to design. If the volume of water from the 85th percentile storm can't be infiltrated within a 36 hour period it's infeasible. There are other criteria like nearby utilities, steep slope, contaminated soil etc that can preclude infiltration as well. 85th percentile is based off long term (80 years) local rain gauge data. Never encountered a new fault being discovered, but most geotech investigations don't go deep enough for that.