permeability of clay barrier

[spags]

Question - I have to design a 0.5m thick cap on a waste rock dump. Criteria is 10-9. My materials are 10-8 What extra thinned do I need to meet same infiltration rate ?

 

[oldestguy]

I assume the rate is cm/sec. In any case,I'd get back to the designers and ask for their past experience requiring that rate. Then, I'd go there and examine the actual installation. If we could handle bets here I'd put my money on you finding a site full of shrinkage cracks in temperate weather, I was once in that position with a state regulatory agency and upon visiting one of heir landfill sites, I showed them how you can drop a tape measure in the crack and it goes all the way to the bottom. So much for a tight speck on hydraulic conductivity. Instead look to providing a low shrink-swell well graded material likely not meeting the spec, but not shrinking either.

 

[BigH]

OG - it is probably m/s which seems to have overtaken cm/s these days (our only link to metric when we were all in English units!). Our dam core was 1x10-9 m/s. OG hit the nail on the head about the "capping" . . . any clay capping, and at that coefficient of permeability it must be clay, will crack on drying - the only way would be to continually keep the capping wet which is probably prohibitive.

 

[fattdad]

I'm taking the OP as a permeability requirement for compacted clay. The 10-9 criterion is likely (as BigH points out) in units of meters per second, which would be the 10-7 cm/sec threashold I'm used to (and often used for landfill liners. Mitchell, Hooper and Companella (ASCE, Vol.91, SM4, July 1965) shows interesting correlation between compaction moisture content and saturated permeability. Please note that if you compact the sample on the dry side of optimum you will obtain a saturated permeability that's likely to be one, two or three orders of magnitude greater than the exact same soil compacted wet of optimum.

Now, what's optimum? Bear in mind if the requirement is 95 percent compaction, the optimum moisture content for 95 percent compaction is not the same value as the optimum moisture content for 100 percent compaction. The ASCE journal article talks about that too. Basically, for the lowest possible permeability value you need to compact the sample at about 90 percent saturation.

f-d

p.s., try to avoid using the words, "Infiltration" and "permeability" interchangably. They mean different things!

ípapß gordo ainÆt no madre flaca!

 

[spags]

Thanks for the responses. My problem is relating that to the EPA !.
We have a spec of 10-9 in the clay barrier, but I have 10-8 materials - so how do I convince them that we can construct a cap with the same rate of infiltration from rainfall ?. Do we increase then depth ?