Hello-
At my office we need to specify the size of a rock going into a small channel for filtration. The void ratio has been specified for us at 0.4, so how would we determine the rock size?
Thank you,
Kevin

As I remember my admittedly long-ago soils classes, the size will do little to influence the void ratio.  Imagine simple spheres, like ball bearings or bowling balls.  Pack them as tight as possible, and you will have an identical ratio of voids to solids.  More important is the range of sizes.  Add ball bearings to the bowling balls, and they fill up a portion of the original void space.  

Of more importance for filtration is the grain size of adjacent materials, to minimize migration into your filter material.  You can find reference material to determine relative grain size distributions, once you have the data on the neighboring material.

Most details that I have seen call for 1" or 1.5" clear stone.  I generally use 1.5" clear stone.  The Minnesota Stormwater Manual has a lot of good details for filtration and infiltration basins.  Follow this link http://www.pca.state.mn.us/water/stormwater/stormwater-manual.html.  The file is quite large, but very useful.  You might want to download it onto your computer for future reference.  Go to page 721 for a filtration basin detail.

I suspect you mean 40% VOIDS, which is the maximum value that can generally be obtained by using clean, uniform, angular stone.

The void RATIO is the ratio of voids to solids.  So 40% voids would correspond to a ratio of 40/60 = 2/3.
 

Peter Smart
HydroCAD Software
www.hydrocad.net
 

Something is missing in what you are asking for.  For filtration of the base material (the soil upon which the channel liner sits), you start with a fine granular material, such as ASTM C-33 fine aggregate or a mix of sand and gravel, etc, and then coarser layers on top.  The top layer probably would be sized for the erosion resistance needed in the channel.

If someone wants a void ratio of 0.4, say for that top layer, then they are asking for a single sized material, possibly 3" clear rock.

On the subject of void ratio, if you have one size spheres, the maximum void ratio you can get is 0.92 with them stacked one on top of the other, as loose as possible.  When they are nested as close together as possible, the void ratio is 0.35.  

So you can see you need a rock material that is single sized, not necessarily spheres, that when in its tightest position, results in a void ratio of about 0.4.   The diameters don't matter for this as long as they all are the same.
There still must be some relationship between the grain size distributions of each layer so that the finer layers don;t come washing out through the voids of the coarser layers.  References for how to do this can be found at NAVFAC manuals on line, under the subject of "filter design".

psmart-
I mean a void ratio of 0.4, what you have suggested is a void ratio of 0.67.

Stone storage is commonly designed with a best-case value of 40% voids, hence my concern that the terms may have been swapped.  As long as everyone is using the same parameter...

Your void ratio or 0.40 is equivalent to .4/1.4 voids = 29% voids, which is probably more realistic than the common 40% value.
 

Peter Smart
HydroCAD Software
www.hydrocad.net
 

He has said in the beginning "for filtration". That is not for storage, so he hangs with "void ratio", which is what he was to provide. Enuff said.

I appreciate everyone's input. Upon further clarification from our client, psmart was exactly right! The situation calls for a single sized rock with 40% voids. We have no fines, just a constant grain size. Intuitively I would say that this is only dependent on the shape of the rock just as oldestguy said. But still, I'm kind of stumped as to what to specify in the detail...

You could specify the voids and let the contractor select the stone.  Or you could specify 1.5" to 3" clean stone, as others have suggestes, but the actual voids will be subject to variation depending on the exact shape of the rock.  You would have to test the actual stone to be sure.

BTW, this is a common problem.  Many stormwater designs are done with a "standard" value of 40% voids, which may not be achievable with local materials.
 

Peter Smart
HydroCAD Software
www.hydrocad.net
 

Thanks again everyone.

Haha interesting twist here:
1.5"-3" stone will work from an engineering perspective, however the local municipality has specified that the stones must be "too large for someone to pickup and throw".  

It might not hurt to ask them who is going to do the picking up and throwing, so you can size them.  That ought to answer your very first question.  It's beginning to look like you might want to let some other outfit carry on from there.

Explain that arranging 18" diameter rocks one beside the other won't do much filtering.  OK, reduce it to 10" diameter for 5 year old kids doing the job of throwing.  Can't be satisfied with just picking them up.  Gotta throw also.

Whoops! Ask how far the throw has to be.  Gotta be precise about this.

Go get some No.3's or #57 stone, fill up a 5 gallon bucket and measure how much water you have to add to find the brim.  You'll have your answer!  Otherwise, you'll have to rely on us.  I'd get my own perspective.

Recalling an old quote, "Love many, trust few, always paddle your own canoe."

f-d

¡papá gordo ain't no madre flaca!