Graded Filter Design cont.

[cdale]

This is an addtional question/discussion to the original post...

The reference materials proved quite useful in the previous thread. I was wondering though, these manuals refer mainly to granualar soil. Does anyone know if the same pricipals should be applied to layering aggregate/rock materials?

For example I have a native soil base, then a con sand layer, then a layer of no. 57 aggregate, and I want to put a layer of larger drain rock on top of the no. 57...do I have to run the same calculations I ran for the granualar soil interfaces on the rock/aggregate interface?

I don't know if anyone actually will know the answer to that one, but any input would be appreciated? Thanks.

 

[jdonville]

cdale,

This is one of those "engineering judgement" situations. It will depend, I think, on which way the hydraulic gradient runs. You definitely don't want sand boiling up into openwork gravel. But if the hydraulic flow will be downward, then I would tend to think that placing open gravel on top of sand shouldn't be as big a deal. Potential for flow to run both directions indicates that care must be taken to look at both flow regimes. However, it's been quite some time since I worked on earthfill dams, so I will happily listen to any dissenting opinion.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[BigH]

As far as 57 aggregate is concerned, I would consider it a soil even though it is made of rock fragments. My experience is that even when using large rockfill and fine rockfill (say 18 inch minus), the designers would still stick to the filter gradings. To digress on to the other side - viz., fine grained soil. Ripley says concrete sand is good for silts and clays. Work by Kenney and Sherard give credence to different filter (than traditional) for these soils.