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.
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
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
"It is imperative Cunth doesn't get his hands on those codes."
RE: Hydraulic conductivity
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
RE: Hydraulic conductivity
"It is imperative Cunth doesn't get his hands on those codes."
RE: Hydraulic conductivity
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
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
"It is imperative Cunth doesn't get his hands on those codes."
RE: Hydraulic conductivity
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
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
RE: Hydraulic conductivity
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
RE: Hydraulic conductivity
RE: Hydraulic conductivity
http://citeseerx.ist.psu.edu/viewdoc/download?doi=...
RE: Hydraulic conductivity
"It is imperative Cunth doesn't get his hands on those codes."
RE: Hydraulic conductivity
RE: Hydraulic conductivity
"It is imperative Cunth doesn't get his hands on those codes."