Where is the site? Soil/groundwater details? Your preliminary design?

Please see FAQ731-376  by VPL for tips on how to make the best use of Eng-Tips Fora.

Site is Sunnyvale,Ca. in Santa Clara County. Ground water is 110 ft down. Top 8 ft. is top soil, then 15 ft of Sandy Clay (ML), 10 ft. of sandy clay with gravel (CL), and then 40 ft. of gravelly silty clay(ML-CL). I have 11,000 sf of pavement area and an occasional 1500 gal load of water from a vacuum truck. My conceptual design is 925 sf by 2 ft deep for storage with 1 ft. each of sand and then drain rock below. The storm outlet will be in the rock portion. Filter fabric will be utilized as material barrier. Any guess at the infiltration rate? I would like to limit standing water to 5-7 days max.

It doesn't sound like a good site for percolation.  The CL material, barring secondary inclusions (sand lenses, gravel pockets, etc.), will probably have a permeability of about 10^-7 to 10^-8 cm/sec.  Landfill liner material.

Depending on the gravelly silty clay's (ML-CL) properties, you might be able to use that stratum as your sink.  I did something like that on a site in the L.A. area for Roadway Express around 1990.  It worked like a charm - but there was one difference: the soils at that site were silty fine sands.

I'd suggest hiring a pier rig to drill a 36 inch hole, and run falling head permeability tests (i.e. "perc tests") at various depths to evaluate the infiltration rate.  Be sure the rig can reach to 60 feet - just in case.

Please see FAQ731-376  by VPL for tips on how to make the best use of Eng-Tips Fora.

or a cheaper method, do a shallow pit perc test 3 feet below the proposed bottom or the retention basin.  This is a standard requirement for retention basins in this area.  Deeper perc tests might be done where dry wells are being considered.

5 - 7 days is a long time.  Our requirement generally limits it to 36 hours, partially to prevent health and safety problems such as mosquitoes breeding, children playing in contaminated stormwater, drownings, etc.

Your design should be the result of a geotechnical investigation firstly, and secondly, in consideration of the recommendations provided by the geotechnical report, which would be specific to that site.  There are no "cookie cutter" designs for infiltration pits.  The geotechnical engineer can provide the design of the pit in accordance with your flow specifications.

Even tile disposal fields require a minimum of a geotechnical investigate and perc. tests before they can be signed off.

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