Permeability Test Method? - Old Gravel Pit 1

You are asking about two conceptually different characteristics.

For infiltration abilities of standing water into this soil, permeability will have to be run on the undisturbed layer/strata at whatever depth infiltration will be designed on...a 2' deep bed will have different transmissivity than a 4' bed. This will require a perc test at that depth.

To deduce a Curve Number or other runoff coefficient, you don't need to find permeability. For runoff and abstraction concerns, you just need to know the absorptive and flow resistance of the surface layer. I suppose you could do it empirically, but I would just use the SCS Curve Number for the HSG of the soil type...they range from 76 in HSG A to 91 in HSG D. Flow resistance is reflected in the Mannings "n" of 0.011 in SCS, for Tc.

Hmmm... My wording was poor. I'd like to determine the permeability of the surface crust, to support a determination of a relatively high curve number. Typically a gravel material would have a fairly low curve number and little runoff. My site has considerable runoff, implying a high curve number. I would like to determine the permeability of the surface layer, not a subsurface layer. I am not looking to infiltrate water, at this time, just how to accurately set up a test at the surface of the gravel, as opposed to within the gravel.

Thanks for the response.

As the designer you can make a judgement about the surface characteristics. If you observe surface ponding due to fines of the surface crust that impede percolation I would suggest using Hydrologic Soils Group C or D curve numbers.

While there is typically a relationship between the soil grain size - soil moisture storage capability - soil infiltration rate, the actual CNs are intended/computed to reflect the soil storage capacity of the soil/land cover combination and is related by the standard SCS equation:
S = 1000 / CN - 10 where S is storage in inches.

For a gravel pit, you probably don't have much vegetation, so your storage is primarily shallow surface depressions and storage in the soil voids. If the voids are "crusted over" so as to be non-effective, then estimate the average storage over the site before runoff occurs and add some nominal amount for water within the voids of the crust layer.

Example: Assume 1/2 inch average ponding storage and 0.2 inches within the soil crust layer, so net storage is 0.7 inches, corresponding CN is 93.

NOTE: once you remove the crust, I would recommend using the established curve numbers for the mapped soil, likely hydrologic group A or B.

Depending on the application a simple rational method may be simpler and within the range of the assumptions, e.g.:
Runoff volume = site area * 90% * (Precipitation - 0.2 inches)

Hope this helps!

You might try a shelby tube pushed through the crust only. I use an engineer that has modified a permeameter so he can pull a shelby tube from pond liners, cut a two inch section out of the middle and run a perm test on it. It might work for you as well. Be advised, you will want to take more than one sample. Permeability will most likely vary throughout your pit. Running tests this way will give reasonable results and is probably about the only way to do lab testing and keep the sample undisturbed.