Relationship of void ration to rock size/shap
Relationship of void ration to rock size/shap
(OP)
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
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





RE: Relationship of void ration to rock size/shap
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.
RE: Relationship of void ration to rock size/shap
RE: Relationship of void ration to rock size/shap
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
RE: Relationship of void ration to rock size/shap
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".
RE: Relationship of void ration to rock size/shap
I mean a void ratio of 0.4, what you have suggested is a void ratio of 0.67.
RE: Relationship of void ration to rock size/shap
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
RE: Relationship of void ration to rock size/shap
RE: Relationship of void ration to rock size/shap
RE: Relationship of void ration to rock size/shap
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
RE: Relationship of void ration to rock size/shap
RE: Relationship of void ration to rock size/shap
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".
RE: Relationship of void ration to rock size/shap
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.
RE: Relationship of void ration to rock size/shap
Recalling an old quote, "Love many, trust few, always paddle your own canoe."
f-d
¡papá gordo ain't no madre flaca!