Infiltration Tests

[martin888888]

We have a 6' deep retention pond we need some infiltration tests for. Is it normal when doing the tests for the geotech to dig down to the 6' level and conduct the test, or is it done at the surface?

 

[msquared48]

Better check the 2005 EPA requirements for infiltration rate testing, the infiltration spec. There is a minimum size for the pit.

Also, you need to check for the maximum seasonal watertable depth. I don't klnow how you can do that by only testing at the top layer of soil.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[martin888888]

Thanks. We have boring samples so we know water table depth. Per the boring samples we also know we have soil consistency down to a clay layer at a depth of 12'.

What I am trying to just get some information on is, since we have this consistency, can the infiltration test pit be dug at surface level or do we need to get a backhoe and dig down 6' to the elevation at the bottom of the bottom and conduct test there.

 

[msquared48]

After reading the EPA requirements, I would dig down to the 6' level. I am involved in one now where we are digging down to 16 feet for testing.

However, if there is an adjacent project with an approved infiltration test, you might be able to use those results if you want to limit testing expenses.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[fattdad]

Food for thought. . .

In Virginia I've done a few infiltration tests and we used "The Fairfax Method." I don't have all these requiremeetns in front of me, but will try to capture the essence in this post.

Drill a hole that extends 5 ft below the intended base grade of the infiltation pond. Take samples and get the most restrictive sample for texture classification (i.e., hyddrometer/USDA soil classification).

Set a 4- or 5-in diameter PVC pipe into this restrictive layer (providing that it's greater than 2 or 4 ft above the water table - can't recall. . .). Use bentonite to seal the outer rim of the PVC pipe (I'd first use a few inches of rock so the bentonite doesn't get into the exposed bottom).

Soak the test zone for 24 hours.

Fill the pipe (and there is some limit to the extent of hydraulic head) and take readings for 4 hours.

Get the infiltration rate.

I just want to point out I personally have a huge problem with these tests. The principal flow gradient is vertically downward, but in reality infiltration over thousands of square feet requires a significant horizontal gradient. Huge scaling effects are in play when you consider the potential for vertical flow in a 4-in diameter pipe and the potential for vertical flow in a 10,000 sf pond. These items are not addressed by the infiltraiton test and the potential for system failure seems great to me.

f-d

¡papá gordo ain’t no madre flaca!

 

[CarlB]

You should consider using the ASTM method for a field infiltration test-
ASTM D3385 - 09 Standard Test Method for Infiltration Rate of Soils in Field Using Double-Ring Infiltrometer

http://www.astm.org/Standards/D3385.htm

Use of the double ring is to attempt to determine just the verticaal infiltation rate, by screening out the lateral/boundary effects.

 

[cvg]

unless you are meeting EPA CWA permit requirements or it is required for your grading and drainage permit, I'm not sure why the EPA method would be preferred. I agree with CarlB, double ring infiltrometer is the standard around here. Perhaps there is a regional standard that needs to be followed for this project also.

 

[fattdad]

The scaling effects of these tests remains whether you use a pipe-type infiltrometer or a double-ring-type infiltrometer.

Let's say you have 5 ft of unsaturated sand and it's sitting on top of a clay layer. You set your double ring (or pipe) infiltrometer and get 30 minutes per inch. For vertical flow under a gradient of 1 (unity), that'd correlate to a hydraulic conductivity of 1.4E-3 cm/sec. Not bad for infiltration!

Now let's say you have a circular area of 2,000 sf that's being considered for infiltration (i.e., r= 25 ft). What's the potential for 24 inches of standing water (i.e., storm level) to infiltrate at a rate of 30 minutes per inch? (After all it worked on the double-ring device.)

Consider that 48 inches of standing water would require 8 ft of unsaturated subsoil considering an effective porosity of 0.25 and as stated there's only 5 ft of subgrade soil available.

Well, the water is not going through the underlying clay layer - not at that rate at least! So, how's it leaving? It has to be going horizontally away from the circumfrence. Let's consider this rate:

two inches per hour would equate to 41.5 gpm. Now, the question is, can an equivalent well with a radius of 25 ft and a head of 2 ft produce 41.5 gpm of yield?

Look at figure 4-11 at the following link:

http://www.army.mil/usapa/eng/DR_pubs/DR_a/pdf/tm5_818_5.pdf

For fully-penetrating conditions, gravity flow and considering that the phreatic surface is at the subgrade, you'd get the following:

Qw=(pi)(k) (H^2-h^2)/ln(R/r) = 3.28 cfm or 24.5 gpm.

Doesn't work. Busted by a factor of 2.

This happens alot and is worthy of note. Check the big picture when you do one of these tests.

f-d

¡papá gordo ain’t no madre flaca!

 

[beej67]

Doesn't work. Busted by a factor of 2.

Not to belittle your analysis, which is quite good in my opinion, but the safety factors inherent to the retention pond design process usually more than account for being off by a factor of two or more with the infiltration rate.

A stormwater retention pond is not a watch. The "design storm" will never fall on your watershed. The "first flush" is an extremely arbitrary runoff volume that in truth should vary widely by location, but regulations usually don't acknowledge the variance. The whole field of stormwater hydrology is built on one conservative assumption after another.

So lets say your regulation requires that the retention volume in your pond recovers in 72 hours after a storm. I think that's typical in Florida, but I'm pulling that from memory and haven't done a Florida job in over a year, so please look it up before trusting me on that. You design it by the simplified methods presented to you in your regulatory guideline, which basically use darcy law, (incorrectly) assume a constant head instead of the falling head you'd actually see in a pond leaking out, use your infiltration rate over the pond bottom, divide, show the regulator some math that says 72 hours or less. Well that's not how it's actually going to happen when it rains. For one, you're not allowed to take credit for any infiltration through the banks, just along the flat bottom, so that's conservative. Another thing, as the head in the pond drains down, you've got less head driving the flow. Third, if you're down near the water table as in much of Florida, the water table itself changes during the course of the infiltration.

Lets continue to monkey with the discussion. In SFWMD, your pervious SCS runoff curve numbers vary by depth to the water table instead of pulling them out of the book. Well if the groundwater is varying, then your runoff number is varying too, which changes how much water should come in to your retention pond, but we pretty much ignore that during design. Add in seasonal fluctuations in depth to the water table, just for fun, or the idea that your neighbor might have a retention pond that's crowding out the volume you'd like to infiltrate into. Then lets say the contractor sods the thing instead of seeding it, and sod is grown on muck from a sod farm that's got less infiltration capacity than your soil, whoops. Or the owner dumps his grass clippings into it, because why not, it's just a hole out back. Etc.

By the time you're done with it all, even if you did have a 100% correct and accurate infiltration rate, you'd need to do some sort of flownet model combined with a stormwater hydrology model just to figure out exactly what's going on. And by the time you'd done all that, then answer would be this:

"Hrmm, yeah, that hole seems about right."

So instead of diving down that rabbit hole, we pick a infiltration test and stick with a simple design method to go with it. And if those two methods doesn't give us good results, we don't go searching for the perfect infiltration test in combination with the perfect computer model, we just change the regulation to say "48 hour recovery time" instead of 72, and call it a day. It's not like anyone's sitting around these things with a stopwatch.


Sorry for the rant. I don't do a lot of wellpoint design, and only very cursory work designing temporary dewatering operations, so I don't encounter many applications where accurate infiltration rates are crucial. For stormwater hydrology, if you can get it within a factor of 2 you're doing pretty good. As long as it's not 10 times off, it'll probably work in the end.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[fattdad]

beej67: I like your post and agree - what's a safety factor for infiltration design? I note that your first comment uses "your analysis" referring to my well analogy. Your post then continues to use the word "your," but I'm certain you are then referring to you in general - not to get all linquistic on you or such. . .

There is so much vague "science" that goes into infiltration design that it irritates me no end. Thankfully I'm not in our civil engineering group, so I'm can keep my head from hitting the desk.

good luck and continued success to you all that do this as it's a field where greater reason is needed - mostly in the proper characterization of soil properties and ground water flow. Extrapolation from septic drainfield design seems alll wrong to me.

f-d

p.s., I acknowledge that I did a constant head quantity calculation assuming that the 2 ft of driving head would be available for some duration. In practice, I'd look at the flow from the system at 2 ft of head, 1.5 ft of head, 1 ft of head, etc. - not that hard. That said, if the system is busted at the full 2 ft of head, imagine how much water can leave the system when it's down to 1 ft of head.

¡papá gordo ain’t no madre flaca!

 

[gbam]

beej67 - good rant! That is why hydrologists have chicken bones and a bottle of rum in our desk.

 

[beej67]

There is so much vague "science" that goes into infiltration design that it irritates me no end. Thankfully I'm not in our civil engineering group, so I can keep my head from hitting the desk.

Hahaha, this one had me rolling. Hehe.

You're right of course.

I'm working on my desk divot right now, actually. XP-SWMM keeps crashing on a stormwater model I should have had buttoned up earlier today. Wonderful activity for a beautiful Friday evening.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com