Field Infiltration Testing - problems and peril?

[fattdad]

Dear Fellow Engineers:

I have just completed a field study for the design of "bio-retention" facilities in Central Virginia. The local county requires the design be based on a field infiltration test where a 4-in diameter PVC pipe is embedded 30 inches below the base grade of the retention facility. After a 24-hour soak, the infiltration rate is measured over a 4-hour period beginning with a 24-in water column using the equation:

IR = ?h/?t

(one inch drop in one hour would yield a value of 1.0 in/hr)

If the infiltration rate is less than 0.5 in/hr, then the bio-retention facility design must include an underdrain.

Here's the subject of this thread: How meaningful is this test - I mean really?

Consider the Hvorslev equation for permeability using the same data set:

Km = [pi*d/11(t2-t1)]ln(h1/h2), where

d = diameter of the PVC casing (4-in)
t2 = time at end of interval
t1 = time at beginning of interval
h1 = height of water column at beginning of interval
h2 = height of water column at end of interval

(one inch drop, i.e., from 24 to 23 inches, in one hour would yield a permeability value of 0.049 in/hr)

Using the Hvorslev equation (which takes into account geometry and radial flow), you calculate a value for "permeability", which is 20 times smaller than the simple infiltration rate.

My concern is that the infiltration rate (i.e., a perc test type value) is wrongly being applied to this problem when permeabilty (with a hydraulic gradient of 1.0) should be used instead. Bear in mind that simple infiltration test does not account for radial flow.

I'm looking for comments on this. . . . .

f-d



¡papá gordo ain’t no madre flaca!

 

[fattdad]

Note: In the original message IR = Delta H/Delta T

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

As far as fundamental soil parameters like k, you are correct that the test doesn't mean a whole lot. For that, you would need a double-ring infiltrometer (to approximaate 1-d flow), or something like the Hvorslev formula. There might, however, be a base of local experience that shows things are OK with 0.5 in of drop per hour, regardless of what k really is.

0.5 in of drop in an hour doesn't sound like very much permeability to me, regardless of the actual k numbers. If I understood right, the minimum required 0.5 in/hr of drop corresponds to a k of 0.025 in/hr. I'm calibrated in cm/sec, so I convert and get 2x10^-5 cm/sec (which looks to me like an aquiclude, or a good dam core). Does this facility need to have much drainage below it? If so, the county's criterion doesn't give you much.

BTW, what are "bio-retention" facilities? Is that a euphemism for storage of fecal matter, which is in turn a euphemism for animal waste lagoons, which is in turn a euphemism for ...?

Happy New Year, fattdad.

 

[fattdad]

dgillette: Thanks for the thoughtful reply. I am familiar with the double-ring infiltrometer and their accuracy of use. Notwithstanding the method of determining the permeability, I remain flumoxed over the local jurisdictions use of the "infiltration rate". Here is where I think the body is buried.

The jurisdiction wants the water out of the bio-retention facility within 48 hours. The also require that the base grade be at least 4 ft above the seasonal high ground water table (i.e., 48 inches). I'm betting they just say at 0.5 inches per hour (and presumably a void ratio of 0.5) there would be somewhere for the water to go in 48 hours (i.e., into the unsaturated soil column). I bet this stems from the use of infiltration rate in drainage trenches.

Back to the body. . . .

For the case of septic trenches, there is a component of radial flow (i.e., not all vertical) as there is a trench to trench spacing. For the case of a bio-retention facility, this is not the case. It's vertical downward to the water table - period. I'm confident that these things will fail if the design is just based in the infiltraiton rate. Actually, they may fail if the design is just based on the vertical permeability.

In my mind the infiltraton rate is akin to the "specific yield" of a well in the early time of a pumping test. It just doesn't account for the horizontal flow required in the aquifer. You can almost look at bio-retention facilities as a large diameter well where after mounding occurs the governing flow is determined by the horizontal flow in the "aquifer" (like 10e-5 is an aquifer).

I'm rambling, but appreciate any discussion on this matter.

Just to complete the thread: A bio-retention facility is used to control stormwater quality, via infiltration through some biomass. Typical design is to design a recession area (i.e., to receive stormwater) and then undercut the native soils to include sand covered by mulch. Woody shrubs (not trees) can be grown in the mulch. Some level of annual maintanence should be anticipated.

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

So they think they can get rid of a lot of water through 1-d flow in 2x10^-5 cm/sec? That sounds like material I would accept if I wanted to LIMIT infiltration. Now that I understand what the application is, I believe more firmly that you are correct about the criterion being inappropriate.

Will you get crosswise with your client if you recommend a drainage system the county doesn't require?

I suspect the county's engineering staff hasn't actually given the thing a lot of detailed thought. My (admittedly limited) experience with such people is that their experience is broad but not deep (with due acknowledgement of the well-rounded skills of a certain Mr. Phil P. who is an ex-Army Ranger about twice my size, should he happen to read this). The same person may have to do pavement design, construction scheduling, stormwater hydrology, and bridge inspections.