Permeability of Aggregate Base Course Stone

[Mickney]

Does anyone have information on the permeability of Aggregate Base Course Stone? We are in the process of running a constant head test for a specimen set up at 95 percent of Standard Proctor compaction. Any information would be helpful. Thanks.

 

[fattdad]

Are you referring to a dense-graded aggregate with some measure of fines (i.e., 5 to 12 percent)? Something on the order of 21A or 21B dense-graded aggregate?

Alternately, are you referring to something with out any fines - something more like #57 aggregate?

f-d

¡papá gordo ain’t no madre flaca!

 

[Ron]

The permeability will depend on the gradation, obviously. If dense graded with high stability (CBR>80), you would expect the permeability to be low, whereas a graded aggregate with a high void ratio would have a much higher permeability.

We generally like to see base courses with non-plastic, low permeability as moisture gain tends to lower stability.

If drainage is so poor that you need significant permeation of the base, then you have more problems than just permeability.

 

[Mickney]

I probably should have been more clear for the reason behind the question.

We are analyzing a situation where the primarily spillway pipe through an embankment dam was bedded on a 3 to 4 feet thick layer of ABC stone. The embankment dam is mostly a clayey SILT and silty CLAY soil. Pressure head is approximately 10 feet. The embankment dam forms a permanent detention pond for a residential development. Seepage loss is critical to the water balance of the dam.

The gradation (per NCDOT standards) is as follows:
Nominal Aggregate size is 25.0 mm.
37.5 mm sieve 100% passing
25.0 mm sieve 90-100% passing
12.5 mm sieve 90% passing
2.36 mm sieve 23-49% passing
.075 mm sieve 3-8% passing

 

[cvg]

it sounds like what you have is a filter diaphragm around the principal spillway pipe. It is designed to pass water without allowing piping or internal erosion to occur around the pipe. With only 10 feet of hydraulic head, you probably will not see high seepage velocities through this material. Your material is a mixture of gravel and sand and permeability should be moderate to high, in the range of 10 - 1000 gallons per square foot per day. Anything more accurate than that will require testing. You can use Darcy's law to determine the flow.

 

[Mickney]

cvg: Actually, the first geotechnical firm undercut poor foundation soils beneath the primary spillway pipe. In lieu of backfilling with soil, they elected to use ABC stone. Seepage collars are present along the length of the pipe.

 

[cvg]

too bad. seepage collars are generally not used in dams any more in favor of the filter diaphragm. However the diaphragm is supposed to encapsulate the pipe on top and bottom. Yours is just on the bottom, so it would only be partially effective. It would have been good if they had brought the aggregate up to at least the springline.