OAR 340-071-0360: Absorption Trenches in Saprolite

(1) General conditions for approval. An onsite system construction-installation permit may be issued for a single-family dwelling on a site with soil shallow to saprolite if requirements in either paragraph (a) or (b) of this section can be met. (a) If slope does not exceed 30 percent: (A) The saprolite is sufficiently weathered so that it can be textured, crushed, or broken with hand pressure to a depth of 24 inches and can be dug from a test pit wall with a spade or other hand tool to a depth of 48 inches; and (B) Clay films or iron coatings with moist values of 5 or less and moist chromas of 4 or more, organic coatings with moist values of 3 or less and moist chromas of 2 or more, or both occur on fracture surfaces of the saprolite to a depth of 48 inches. (b) If slope exceeds 30 percent but not 45 percent: (A) The saprolite is sufficiently weathered so that it can be textured, crushed, or broken with hand pressure to a depth of 24 inches and can be dug from a test pit wall with a spade or other hand tool to a depth of 60 inches; and (B) Clay films or iron coatings with moist values of 5 or less and moist chromas of 4 or more, organic coatings with moist values of 3 or less and moist chromas of 2 or more, or both occur on fracture surfaces of the saprolite to a depth of 60 inches. (c) For saprolite derived from granite or other deposits where clay films or iron coatings are not present, a soil absorption test and the degree of consolidation may be used to predict hydraulic conductivity of the saprolite. Agents may approve sites where conductivity is sufficiently high to ensure adequate drainage. Test methods must be acceptable to DEQ. (2) Construction Requirements. (a) Standard absorption trenches must be installed where slope does not exceed 30 percent. (A) The trenches must be installed at a minimum depth of 24 inches and a maximum depth of 30 inches below the natural soil surface and contain 12 inches of filter material and a minimum of 12 inches of native soil backfill. (B) The trenches must be sized at a minimum of 100 linear feet per 150 gallons sewage flow. (b) Seepage trenches must be installed where slope exceeds 30 percent but not 45 percent. (A) Seepage trenches must be installed at a minimum depth of 30 inches and at a maximum depth of 36 inches below the natural soil surface and contain a minimum of 18 inches of filter material and 12 inches of native soil backfill. (B) Seepage trenches must be sized at a minimum of 75 linear feet per 150 gallons sewage flow.