§ 11.3.7. Internal Drainage

Retaining wall backfill must be free-draining, non-expansive material that is non-aggressive to external structural or drainage components. Underdrains must be provided to prevent hydrostatic loading caused by local groundwater seepage, surface water infiltration, floodwater inundation or by water and wastewater line breaks. Geotextile fabric or graded granular filters must be provided as necessary to prevent migration of fine-grained soil particles from the surrounding soils into the backfill and drainage media. The fabric or granular filter must be designed not only to prevent migration of fine-grained soil particles but also not to become clogged by those particles. Underdrains must not discharge where drainage can flow onto adjacent sidewalk or into the street.