fattdad
Geotechnical
- Sep 7, 2006
- 2,790
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!
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!