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# How Much Void in RipRap

## How Much Void in RipRap

(OP)
I am trying to estimate how many tons of riprap per neat cubic yard of placement.  I am thinking along these lines:

1) Determine riprap density from source...assume 155 lbs/ft3 in this case.  That's about 2.09 tons/cy of 'rock'

2) Now the part I need someones experience on is how much void to factor in.  If voids account for 33% of the total volume then this reduces to 1.5 tons per 'neat' cy.  I realize this can vary, but what we have to use something.  I think the void ratio would also vary by what size of riprap is being placed. What void ratio would you use for the
a) 1/4 ton riprap
b) 1 ton riprap
c) 3 ton riprap
d) 6 ton riprap

3) I would also use this information to determine the quantity of concrete used to fill the voids if placing concreted riprap.

### RE: How Much Void in RipRap

Your assumed RR density is too high.  155 pcf would be high if it were solid concrete!  Considering the gradation and the large interstitial voids characteristic of rip rap, you are probably looking at significantly less unit weight.  If the particles are cobble-sized or less, your unit weight will likely be 100 pcf or less (typical concrete aggregate is about 90 pcf or less).

### RE: How Much Void in RipRap

(OP)
Ron,

Thanks for your interest.  The specification on my job is a minimum 2.4 specific gravity.  That's 150 pcf.  The source I am looking at is 2.6, 162 pcf.  That is the density of the "solid rock", higher than concrete.  I assume (I hate using that word, but that is what brings me to write this question in the first place..) that there is 20-33% voids in the same volume that "stacked" riprap is placed.  If there is 20% that amounts to 120 pcf for the minimum specified scenario. A 90 pcf volume amounts to 40% voids.  That sounds reasonable to me too.  Does anyone have actual field verified data on this topic?

Thanks!

### RE: How Much Void in RipRap

Your void ratio can be as high as 60% or so. The void ratio for rip rap can be quite high.  I would suggest building a box, a 2-foot cube, and checking it.  Do this a couple of times and you'll have a reasonably accurate answer.

Using a 1 cubic foot bucket as you would for concrete aggregate will likely not be adequate as your particle size is fairly large.  Make sure that the box you build has dimensions of at least 3 times the maximum particle size on each side.

### RE: How Much Void in RipRap

(OP)
I am dealing with 4 ton riprap.  That is about a 2 cy chunk of rock,  pretty big.  It is for a coastal eddy.  The rock will be placed with barge derricks.  I dont have a practical way to  test this scenario until it actually happens.  Any contractors out there with experience?

### RE: How Much Void in RipRap

I'm not a marine engineer but about six or seven years ago I assisted in a material take off of rip-rap! The drawings showed the rip-rap as near circular so I worked on that basis. I can't remember the exact stacking arrangement but I recall only about 20% voids. For my project it was better to overestimate...

### RE: How Much Void in RipRap

justbuildit - As a college senior, in summer of 1969, I worked with my father on placement of large rip rap (about 10 ton/each), by barge mounted crane, at the mouth of a coastal inlet. I made estimates of individual stone weight to give the project superintendent guidance on how to handle each stone. Because of ocean wind & wave conditions, stone placement was a "best efforts" performance. The Corps of Engineers was well satisfied with the results - they had no real way to verify what was "going on" underwater.

However with that said, perhaps this US Army Corps of Engineers document "Engineering and Design - Hydraulic Design of Flood Control Channels" may be of help. Here is a link
http://www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-2-1601/toc.htm
be sure to check out "Chapter 3" and "Appendix H"

### RE: How Much Void in RipRap

For what it’s worth...I was an inspector on a bridge construction job where we used riprap that was between 1.5’ and 2’ min diameter (Ohio DOT type B rock channel protection).  From what I saw in the field, I would estimate 20-30% voids in the riprap.

### RE: How Much Void in RipRap

I would take a trip to your quarry and take a look at the rock.  (If you haven't located a suitable quarry, you should locate one first)  Talk to the operator and ask what the swell factor is.  You should also be able to get an idea of the specific gravity of the material from this trip. This will also give you an idea of the distance for hauling the rock.

Also, look at your spec because if you allow a certain percentage of smaller fragments, they will fill the gaps and could amount to a significant weight.

(OP)
Thanks everyone

### RE: How Much Void in RipRap

What you are looking for is the bulking factor and shrinkage factor.  These are commonly used to estimate costs for hauling and earthwork calcs.  The bulking factor is (Volume after excavation)/(Volume before excavation).  For hard solid rock, the value is usually 1.5 to 1.6. The shrinkage factor considers compaction.  Shrinkage is (Volume after compaction)/(Volume before excavation).  Shrinkage factors for hard solid rock would probably be in the range of 1.3 to 1.4.

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