Nonabsorbing Ground : Earth or fill that is not readily permeable or absorbent to large quantities of flammable or combustible liquid or water, or both
NFPA 15 Standard for Water Spray Fixed Systems For Fire Protection, 2001 ed. https://law.resource.org/pub/us/cfr/ibr/004/nfpa.2 5.2002.pdf, (Std: Wtr Spray Fixed Sys for Fr Prac - Amer) {183 NFC}