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Hydraulic conductivity

Hydraulic conductivity

(OP)
We have a segmental block wall spec that includes a section for low permeability soil for drainage blanket behind the wall and footing scour protection (in combination with riprap) for situations when the walls will be well within the 100-year floodplain. We have tried to be conservative with our spec and design, as we are never provided with detailed drainage and flow analysis, and I do not feel comfortable trying to get into that analysis myself. Thus, my section on the low perm soil requires not only low plasticity clay, but that D85 passes #200.

On a recent job that involves a wall within a stormwater management pond, we have gotten resistance to that requirement. Of course, the contractor didn't notice until the work got started. My feeling is that we could cut back but I don't want to go any less than the minimum definition for clay. I guess I am wondering where the diminishing return really kicks in for particle size, and how I can remain reasonably conservative when not getting into computations. The special inspector for the job gave me feedback that he thought that we were being a bit stringent, but I was still interested in getting some other thoughts.

"It is imperative Cunth doesn't get his hands on those codes."

RE: Hydraulic conductivity

you can specify clay as follows and it would be low permeability:

Quote (EPA)

The most common type of compacted soil liner is one that is constructed from naturally occurring soils that contain a significant quantity of clay. Soils are usually classified as CL, CH, or SC soils in the Unified Soil Classification System (USCS) and ASTM D-2487.

If you want to verify permeability, than you will need to test it. You do not need to specify a D85 in order to meet the above. The presence of less than 50% sand or gravel will not have an appreciable affect on the permeability.

If the borrow soils are not sufficiently clayey to be suitable for direct use as a soil liner material, a common practice is to blend natural soils available on or near a site with bentonite. you might need to test a mix to determine the ratio of bentonite needed.

If you want a commercial product that might be easier to install, try a GCL.

http://nilex.com/sites/default/files/nilex-geosynt...

RE: Hydraulic conductivity

(OP)
Thanks, cvg. We don't specify to verify permeability in the field because we don't have a spec requirement that defines the limit. The product sheet you attached sounds pretty fancy for my situation. They aren't using anything like that for the liner of the stormwater pond itself - I believe that are using any inorganic soil meeting at least D30 passing #200. That seems to be close to being inline with the EPA quote. At the end of the day, the borrow clay soil onsite ended up testing to CL with D65, which I felt I could live with for our walls.



"It is imperative Cunth doesn't get his hands on those codes."

RE: Hydraulic conductivity

I have a lot of piping &nd problems with clays and silty clays which have dispersive LIKE properties. I have noticed a couple of walls in Detention/Retention Ponds and in some over-irrigated landscape situations which appear to be 'loosing' backfill and support soils.
For the last 20 years, I have also been looking at Skempton's Activity for some guidance. This does require the hydrometer as part of the classification process, which can cause a lot of complaining from most of my competition.

RE: Hydraulic conductivity

You can get as fancy as you want with CH clays required, but are you aware of shrink-swell activity then with varying weather, including long dry periods. This possibility was amply demonstrated many years ago when the Wisconsin DNR guys wanted to prove to me that high plasticity clays were needed for proper landfill cover. So I went with them to their "best" example of what to do. The site had been closed for a couple of years. So we walked out on the site and I took along a folding rule, just in case. Sure enough the site had numerous big shrinkage cracks. There was no problem at all poking that folding rule down the full three feet into the garbage. After that I had no problem getting approval for landfill cover that was lower plasticity, a little lower permeability, but minimal shrinkage potential.

RE: Hydraulic conductivity

(OP)
I am aware of the shrink-swell with fat clays. That is why I am specifying lean clays.

"It is imperative Cunth doesn't get his hands on those codes."

RE: Hydraulic conductivity

Flow nets are independent of permeability.

The permeability of compacted clay can vary by three orders of magnitude depending on whether the compaction moisture content is greater than or less than the line of optimums. Refer to Mitchell, Hooper and Campenella.

Shrinkage cracks typically result from mud drying out. If the compacted clay is properly compacted (i.e., void ratio below the critical state), I doubt there'd be much of a problem in the absence of smectite.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Hydraulic conductivity

Quote (macgruber)

I believe that are using any inorganic soil meeting at least D30 passing #200. That seems to be close to being inline with the EPA quote

that spec would allow all of the following without limitation (not really inline with EPA which only allows clay):
silty gravels (GM)
clayey gravels (GC)
silty sands (SM)
clayey sands (SC)
inorganic silts (ML)
inorganic clays, low plasticity (CL)
inorganic silts (MH)
inorganic clays, high plasticity (CH)

some of these would be barely suitable and some would be problematic. your drainage blanket spec is better. just get rid of the D85 requirement unless you have some good reason for it.

RE: Hydraulic conductivity

(OP)
CVG - I agree. My intent in my words was not to accept less than D50, but to recognize that my spec may be overboard.

"It is imperative Cunth doesn't get his hands on those codes."

RE: Hydraulic conductivity

I've put in some sites that might be of interest to you . . .

http://www.murrayrix.co.uk/murrayrix/Selection%20o...
http://igsl.ie/wp-content/uploads/clay_liners_for_...
http://www.gov.mb.ca/conservation/envprograms/live... (see section 5.3.2 gives grain sizes)

From a practical side - it is difficult to "confirm" compliance to a permeability specification (such as 1x10-9 m/s - or 1x10-7 cm/s for US). when you are placing 40k m3 of fill a day - how can you wait, each layer - for a permeability test - or if a spec says 1 test every 100k of placed material - and it takes only 2 days to place and a week to test the permeability . . ??

As noted, many specs call for gradation limits and some on the Atterberg limits. In one province in Canada, they typically use glacial till - usually from moraines for the core - with the following typical gradation limits:
#4 sieve: 50 to 95; #40 sieve: 30 to 70; #200 sieve: 15 to 50/55 . . . maximum size 300 mm; no mention of plasticity - but morainic fills will have low plasticity indices.

I know of another dam whose core - used the following:
4.75 mm: 85 to 100; 0.425 mm: 58 to 100; #200: 35 to 80 with PI between 6 and 50 and LL between 30 and 70 - permeability was to be less than 5x10-9 m/s (or 5x10-7 cm/s)
Inner shells (also somewhat impervious)
4.75 mm: 84 to 100; 0.425 mm: 48 to 81; #200: 20-61 - LL between 30 and 70 no mention of PI. Again, permeability was to be less than 5x10-8 m/s (5x10-6 cm/s).

RE: Hydraulic conductivity

(OP)
BigH - thank you very much. The second link is broken for me, though.

Quote (BigH)

From a practical side - it is difficult to "confirm" compliance to a permeability specification (such as 1x10-9 m/s - or 1x10-7 cm/s for US). when you are placing 40k m3 of fill a day - how can you wait, each layer - for a permeability test - or if a spec says 1 test every 100k of placed material - and it takes only 2 days to place and a week to test the permeability . . ??

That kind of gets at my main point of being reasonably conservative. The contractor is already complaining about minimum testing requirements for the reinforced backfill, among other things that are "slowing his progress". It is my experience that contractors experienced in building segmental block walls are super fast. If I had added an in situ permeability test, their heads would have popped off.

"It is imperative Cunth doesn't get his hands on those codes."

RE: Hydraulic conductivity

stockpile the material, test gradation and plasticity for compliance and approve the material before starting. than if you want to also confirm permeability, conduct a test fill, verify the rolling pattern and test permeability. this will establish an approved method for the placement. this is how most dams are done and it is not that big of a deal. if the contractor had a spec before bidding the job than he is trying to weasle out of the spec requirement and save a few bucks.

RE: Hydraulic conductivity

(OP)
CVG - I think that is a great line to have in the spec, but given that I do not specify permeability limits, forcing that testing seems logically inconsistent. Now, if I got to the point where I calculated or knew what was the minimum permeability required to do the job at hand, I would definitely want that sort of instruction in my specs.

Quote (cvg)

if the contractor had a spec before bidding the job than he is trying to weasle out of the spec requirement and save a few bucks.

Everyday of my life, I am put in situations where I am asked to accept something less than specified. That is the nature of engineering, and more often than not, the owner is convinced by the contractor to encourage me to sharpen the pencil and see if I can indeed accept something less. And more often than not, we entertain trying to sharpen that pencil - sometimes the results are good, sometimes not.

Thank you for your feedback. :)

"It is imperative Cunth doesn't get his hands on those codes."

RE: Hydraulic conductivity

From a practical standpoint, and particular considering low plasticity, anything with greater than 50% passing a #200 sieve would probably meet your requirements.

RE: Hydraulic conductivity

cvg - that might be possible for some jobs - but when you are digging into a tropical hillside and are using the residual soil for fill - you will run into zones in various stages of weathering and also composition. As on one job in which I am aware - digging 25 m into the hillside was still producing inner shell and core material gradations - much different than the geotechnical investigation showed. And, to stockpile 3 million cubic metres of material is not easy - where does one have the room?

RE: Hydraulic conductivity

(OP)
Thanks, guys.

"It is imperative Cunth doesn't get his hands on those codes."

RE: Hydraulic conductivity

you are right. current job is stockpiling a million yards and it takes a lot of room. but to build a dam you do need a lot of material. as for any borrow job goes, there should be adequate geotechnical investigation to characterize the material at the proposed excavation depths and locations and better prior to digging than after starting the job. otherwise it is a crap shoot on what you will get and a guaranteed change order.

RE: Hydraulic conductivity

(OP)
To be fair, I my job only has about 150 CY of this material and the lot is huge.

"It is imperative Cunth doesn't get his hands on those codes."

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