Site access and topography constraints — trees, fences, utilities, driveways, easements, rock.

Topography is a pass/fail criterion: uniform slopes under 30% are suitable for standard subsurface absorption, slopes over 30% and complex slopes are unsuitable, slopes under 2% require added surface drainage, and excavations must run parallel to the contour (30 TAC §285.31(c)(1) and §285.91(5)). Rock and caliche show up as "restrictive horizons" — rock-like material an auger will not penetrate — and a restrictive horizon intersecting the sidewall or within 24 inches below the excavation makes a standard system unsuitable, while a rock horizon can strip sidewall credit from the absorptive-area calculation and force a larger or non-standard field (30 TAC §285.30(b) / §285.91(5); Mills County OSSF standards). Easements and improvements constrain layout: underground and overhead easements require 1-foot separation (no setback for overhead if the easement holder grants permission), spray may reach the edge of an easement but not into it with sprinkler heads 1 foot back, tanks and absorption systems must be 5 feet from foundations, property lines, pools, and other structures, and pipe crossing a drainage easement must be sleeved in Schedule 40, buried at least a foot deep (or encased in concrete), taped, and marked (30 TAC §285.91(10) and §285.32(a)(8)). Owners may not allow driveways, storage buildings, barns, sidewalks, or patios over the treatment or disposal area, or drive/park on it (30 TAC §285.39(c); TCEQ, Advice for an OSSF Owner); brush must be cleared and wells, property lines, and buried utilities marked before the evaluator can even dig test holes, and remote or uncleared sites raise evaluation cost (Texas Septic Guide — industry source).

Reviewed August 2026. Links go to the sources this answer was checked against.