Table IV gives the voids in broken stone as determined by various engineers; it requires no explanation. Table V, taken from Feret's tests, shows the effect of changes in granulometric composition on the amount of voids in both broken stone and gravel. Considering the column giving voids in stone it is to be noted first how nearly equal the voids are for stone of uniform size whatever that size be. As was the case with sand a mixture of coarse and fine particles gives the fewest voids; for stone an L1M0F1 mixture and for gravel an L8M0F2 mixture. Tamping reduces the voids in broken stone. Mr. Geo. W. Rafter gives the voids in clean, hand-broken limestone passing a 2½-in. ring as 43 per cent. after being lightly shaken and 37½ per cent. after being rammed. Generally speaking heavy ramming will reduce the voids in loose stone about 20 per cent.
It is rare that gravel has less than 30 per cent. or more than 45 per cent. voids. If the pebbles vary considerably in size so that the small fit in between the large, the voids may be as low as 30 per cent. but if the pebbles are tolerably uniform in size the voids will approach 45 per cent. Table V shows the effect of granulometric composition on the voids in gravel as determined by Feret. Mr. H. Von Schon gives the following granulometric analysis of a gravel having 34.1 per cent. voids:
| Retained on 1-in. ring, per cent. | 10.70 |
| Retained on ⅜-in. ring, per cent. | 23.65 |
| Retained on No. 4 sieve, per cent. | 8.70 |
| Retained on No. 10 sieve, per cent. | 17.14 |
| Retained on No. 20 sieve, per cent. | 21.76 |
| Retained on No. 30 sieve, per cent. | 6.49 |
| Retained on No. 40 sieve, per cent. | 5.96 |
| Passed a No. 40 sieve, per cent. | 5.59 |
| Passed a 1½-in ring, per cent. | 100.00 |
As mixtures of broken stone and gravel are often used the following determinations of voids in such mixtures are given. The following determinations were made by Mr. Wm. M. Hall for mixtures of blue limestone and Ohio River washed gravel:
| Per cent. Stone. | Per cent. Gravel. | Per cent. Voids in Mix | |
| 100 | with | 0 | 48 |
| 80 | " | 20 | 44 |
| 70 | " | 30 | 41 |
| 60 | " | 40 | 38½ |
| 50 | " | 50 | 36 |
| 0 | " | 100 | 35 |
The dust was screened from the stone all of which passed a 2½-in. ring; the gravel all passed a 1½-in. screen. Using the same sizes of gravel and Hudson River trap rock, the results were:
| Per cent. Trap. | Per cent. Gravel. | Per cent. Voids in Mix. | |
| 100 | with | 0 | 50 |
| 60 | " | 40 | 38½ |
| 50 | " | 50 | 36 |
| 0 | " | 100 | 35 |
The weight of a cubic foot of loose gravel or stone is not an accurate index of the percentage of voids unless the specific gravity is known. Pure quartz weighs 165 lbs., per cu. ft., hence broken quartz having 40 per cent. voids weighs 165 × .60 = 99 lbs. per cu. ft. Few gravels are entirely quartz, and many contain stone having a greater specific gravity like some traps or a less specific gravity like some shales and sandstone. Tables VI and VII give the specific gravities of common stones and minerals and Table VIII gives the weights corresponding to different percentages of voids for different specific gravities.