Questions of Filter Criteria for a Very High Dam

[BigH]

A question for my learned mates concerning filter requirements, i.e., gradation of filter to base. I am keeping this to a close knit group.

Does anyone know as to what level of confining pressures that the filter criteria covers? Of course, gradations are based on “atmospheric” pressures in the laboratory and the various “rules” have been developed – again, from my understanding, at “atmospheric” conditions.

It seems reasonable to me, for a dam filter, that the gradation “rules” might be applicable to 20, 30, or, say, 50 m high dams, say up to 1 MPa of confining pressure. Would these same rules apply when the confining pressures increase to say 4 to 8 MPa?

One issue, of course, is a breakdown of the filter materials due to the high overburden/confined pressures. The gradation at “atmospheric” pressure might change drastically (say, for example by 10% breakage) on breakdown when applying 8 MPa leading to a significant change in the material gradation. This could lead to where the “rules” are not met any longer – and do the “rules” actually still apply at such high confining pressures. What is sad is that tests for “settlement” were done to 10 MPa but no gradation of material after the test was carried out.

A second issue is the use of crushed fine filters which may not “flow” into hydraulic fractures that could develop on the upstream side of the core. As well, crushed fine filters might even be more susceptible to breakdown of gradation and the high confining pressures.

Any thoughts or references would be appreciated. Yes, this is with respect to a dam in excess of 250 m in height.

 

[fattdad]

drinking coffee and thinking of this interesting question. . .

You are not asking about whether there is settlement over time, changing the void ratio and affecting permeability.
You are asking if intergranular forces are high enough to cause contact breakage and materially affect the grain-size distribution and filter effectiveness.

I don't know. To the extent that it may happen, now I have to wonder if it'd be a bigger factor for the coarse-grained materials or the finer-grained filter materials?

Can we learn from aquifers? In many instances, the same aquifer formation resides at widely-varying elevations (confinements). Just how much does an aquifers transmissivity vary owing to confinement? Perhaps not that much?

I'll keep pondering this one, BigH!

f-d

ípapß gordo ainÆt no madre flaca!

 

[BigH]

Fattdad. It is also very important for the filter to be able to flow and fill in any fractures at the upstream face to "anneal" the fracture. Send me an email and I can provide some info if interested.
[Cheers]

 

[GeoEnvGuy]

I believe the standard NCRS filter design adopted by ferc and USace is based on laboratory testing from sherard with subsequent laboratory studies. There is a significant discussion in Geotechnical engineering of Dams regarding Internal erosion and piping as well as embankment dam zoning and design for internal erosion.

For really tall embankments I am concerned about the compaction of the filters at the base and if they are truly dense or have they transitioned to loose with the high confining pressure to pose a liquefaction hazard.

 

[fattdad]

. . . now do I still have your email?

I'm wondering about permeability studies related to confinement. I did a landfill project where we used shredded tires for leachate collection layer. I actually did permeability as a function of strain at confinement to learn more about that. Wow! You're at a lot of confinement! Like 500 to 600 psi (not sure where the design phreatic surface may be)?

Love to see the site for a 750-ft tall dam! You got coordinates?

Here in Virginia, our big pile of fill is 320-ft for an embankment and below all that there are triple-box culverts!

Still pondering. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[BigH]

fattdad - the Dam will actually be 1100 ft (335 m) high. See, for instance,

https://www.britannica.com/topic/Rogun-Dam.

https://www.intellinews.com/tajikistan-s-massive-rogun-hydropower-dam-a-blessing-or-a-curse-169691/

Interestingly, the 4th Ed of Hydraulic Structures says the dam was completed in 1985. HA! ( bohicafries at the rate of gmail )

Please do not share photos . . .

I am taking the picture looking upstream. I am standing at about el 1300 m (the crest of the dam) at the core area. The "cofferdam" holding back the reservoir is at el 1075; water level is 1065. Base of the dam is about el 980

Looking from left to right bank - standing at el 1300 (again) - if you look closely you can see a few ants on the slope - these are large excavators - pushing down an access road.

 

[fattdad]

Wow!

Now I'm thinking elastic theory. . . You have some filter at some depth within the embankment. It's confined by some K-naught, based on some sigmaV'. But, the ground above is not flat - it's an embankment slope.

Just what is the state of stress in the filter? It may not be gammaZ*K-naught (i.e., something less)?

Such an interesting project!

f-d



ípapß gordo ainÆt no madre flaca!

 

[fattdad]

Hi BigH:

It's behind a paywall, but speaks to your concern. Seems the research is from the oilfields; however. And their, "Unconsolidated" is not quite like ours!

https://onepetro.org/SJ/article-abs...-Characterization-and?redirectedFrom=fulltext

f-d

ípapß gordo ainÆt no madre flaca!

 

[BigH]

Following the above, I ran across a paper I thought I would share that discusses crushing of sand - unfortuanately it is on sand that is 1 mm and smaller . . . but Fig 14 is quite iteresting about the breakdown under strain and creep . . . - of course this filter sand will also be "operative" over a hundred years . . . so time effects are important.

I will be asking the contractor to carry out some testing of his proposed sand - get a gradation and then use a large vibrator hammer and see what we can get afterwards . . .

 

[GeoEnvGuy]

Seen this and thought it may be of use to you.

https://www.researchgate.net/public..._Engineering_Properties_of_Granular_Materials

 

[BigH]

THanks GeoEnvGuy - saw this myself a few days ago This is really quite an interesting subject. I am trying to get the contractor to obtain a sample of their crushed sand (which I don't like), obtain a gradation and then use the exact same sample (all of it) by packing into a mould using a vibratory hammer (in thin lifts and packing the 'ell out of the material. Then after completely packing in the mould to obtain a gradation on the material which will give an "exact" comparison. Also would like to obtain k values of the super compacted sand.

If anyone is interested eamail me and I'll send out a draft (still a lot of work to do) of an outline to address the low plastic core and filters. Reactions/comments would then be appreciated.

Hope all are well!

 

[oldestguy]

I'd not compact in the usual way. I'd use a loading machine and push it down, no disturbance like compacting. leave the load on when checking the head loss with water going thru, etc.

 

[BigH]

fattdad - send me an email!