Gravel may be made more compact and will subside if water is pumped upon it and allowed to filter through, and in making an artificial foundation of gravel, it is not reliable without water percolation and consolidation by ramming.
All earth consisting of particles having rounded surfaces is liable to become loose, and upon weight being placed upon it the grains are inclined to roll and become detached, but if they are angular fragments, which seldom is the case, this tendency will be lessened, and the angle of repose will be steeper.
With regard to the slopes necessary in sand and gravel, the more angular, rough, hard, and clean the particles, the steeper the inclination.
Earth that can be properly called gravel seldom requires a flatter slope than 1½ to 1, and usually a less inclination is sufficient, but if loose it will not stand vertically even for a depth of a few feet.
Solid indurated masses of gravel will stand perpendicularly and as rock.
If the gravel consists of quartz or sandstone boulders, or is very coarse with stones of considerable size, or like a clean pebbly beach, 1 to 1 TO 1¼ to 1.
Ordinary clean gravel of uniform size at about 1 to 1.
Thoroughly compressed, hard, clean sand, about 1 to 1.
Looser sand and light gravel, 1¼ to 1 TO 1½ to 1.
Irregular beds of sand, gravel, clay, and fragments of rock, 1¼ to 1 TO 1½ to 1.