I use an average of one third the trench volume. Why? Specific gravity of most stone is 2.75. The weight of a cubic foot of the stone should weigh about 120 pounds. The volume of air will be about 1/3 cubic foot. If you are using a clean stone graded from 1" to 1/4" you will be fine with storage volume of 1/3. If the stone is contaminated by fines all bets are off. You will have to do a voids test on it. What is the gradation of '57' stone?
You could build a one cubic foot box, fill it with your stone sample and then pour in water. Measure the water as it goes in. Your solution for your stone and you can save the box to test other stone. There may be a cheaper gradation in your vacinity.
To answer waterguy, 57 is a grading classification for concrete aggregate. It consists of:
100% passing a 1 1/2 sieve
95 to 100% passing a 1 in. sieve.
25 to 60% passing a 1/2 in. sieve.
0 to 10% passing a #4 sieve.
0 to 5% passing a #8 sieve.
I wouldn't expect something with such an even gradation to be really good for holding liquid. But if that is what you have, you're stuck.
Why would you use a concrete gradation rock when most aggregate manufacturers have a "drain rock" available.
100% passing 2"
75% passing 1 1/2"
50% passing 1"
25% passing 3/4"
10% passing 1/2"
0 passing #4
What you are searching for is the void ratio or void % of the stone mass in place. A few years ago I contacted the North Carolina DOT materials tesing lab and asked the same question. I was told a good average void ratio for #57 washed stone is 40%. It will vary slightly from sample to sample depending on how the material is handled, whether it is vibrated, shaken, etc., but 40% is a good average number to use.
