Dry Well / Infiltration Trench Sizing

Clean crushed stone holds water only in its VOIDS (~30-40%), so the excavation is larger than the stored water by 1/void ratio: storing 200 ft^3 of runoff in 0.35-void stone needs a 571 ft^3 pit, a 143 sf footprint at 4 ft deep. It empties by infiltration; a rough draindown is the void-water column (depth x void) over the soil rate, so a 4 ft pit at 0.35 void over 0.5 in/hr soil drains in ~34 hr (a good system empties between storms, ~24-72 hr). Runoff comes from the design storm/area (a rational/detention calc), the void from the actual aggregate, and the infiltration rate from a field PERC test -- not a default. An overflow/bypass is required. A sizing screen; the perc test, the stormwater code, and the AHJ/geotech govern.

excavation_volume_ft3 = runoff_volume_ft3 / void_ratio; footprint_sf = excavation_volume_ft3 / trench_depth_ft; draindown_time_hr = 12 x trench_depth_ft x void_ratio / infiltration_rate_in_hr.

Dry well / infiltration trench (soakaway) sizing by void-ratio storage and soil infiltration, standard stormwater / on-site infiltration practice, by name; the field percolation test, the stormwater code, and the AHJ / geotech govern.

The storage and draindown relations are public first-principles (voids store water; infiltration empties it); the runoff, void ratio, depth, and infiltration rate are the design, aggregate, and field-test values.

Estimate. AHJ and licensed professional govern.

Field names used by the API: runoff_volume_ft3, void_ratio, trench_depth_ft, infiltration_rate_in_hr, excavation_volume_ft3, footprint_sf, draindown_time_hr

• Void-ratio storage excavation = runoff / void ratio (stone ~0.35); footprint = excavation / depth; stone holds water only in its voidson-site infiltration practice
• Draindown + perc test draindown ~ 12 x depth x void / infiltration rate (target < ~24-72 hr); the infiltration rate MUST come from a field perc test, not a default; an overflow is requiredfield percolation test / stormwater code