How soil class (Class I–IV, clay, rock, caliche) drives allowable system type and drainfield sizing.

Texture determines the class — Ia = sandy soils with more than 30% gravel, Ib = sand/loamy sand with ≤30% gravel, II = sandy loam and loam, III = silt through sandy clay, IV = silty clay and clay (30 TAC §285.30) — and the class sets the long-term loading rate Ra used to size the field: Ia >0.50, Ib 0.38, II 0.25, III 0.20 and IV 0.1 gallons per square foot of absorptive area per day (Bandera County OSSF Technical Information Packet, Table I; TCEQ RG-472). Class Ia (too coarse) and Class IV (clay) along the sidewall or within two feet below the excavation make a standard absorption system unsuitable, a restrictive horizon or seasonal groundwater within 24 inches below the excavation is likewise unsuitable, and slopes over 30% or complex slopes are unsuitable (TCEQ RG-472, Table V criteria); clay, rock and plugged laminar soils are the rule's named restrictive horizons (30 TAC §285.30). Rock and clay push the design toward lined/unlined ET, LPD, mound, drip, soil substitution or surface application (soil substitution is prohibited in Class IV; drip on Class IV is allowed with secondary treatment; where solid rock is within 12 inches of a drip emitter the Class IV rate governs) (30 TAC §285.33; TCEQ, Choosing a Septic System). Caliche has no separate rule class — a TCEQ-funded study covering the roughly 9% of Texas underlain by caliche soils recommended caliche-specific guidelines, backhoe exposures, and relaxing Class II/III gravel and Class IV restrictions, but TCEQ states "there were no changes made to TCEQ rules as a result of this research project" (TCEQ caliche final report, 2007).

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