The chief conditions of a safe foundation upon pure sand, namely, that it cannot escape laterally or be undermined, are obviously not to be attained in either cuttings or embankments, as the lateral support is removed, and the slopes are liable to be undermined and unequally charged with water, and the influence of water on sandy soils is the principal cause of their instability, for in excavating cuttings the face will frequently stand at a very steep slope if dry, but upon its becoming saturated the sand may flow, and in the case of gravel and sand, although the stones forming gravel do not change, the whole subsides.
As gravel is found in various conditions, it may be well to classify it as it is herein regarded.
Clean gravel is considered as that which nearly approaches the condition of a pebbly beach. If an appreciable quantity of sand is present, it is sandy gravel. If loam, or marl, or clay, it is loamy, marly, or clayey gravel.
Gravel hills are large accumulations of water-worn rocks, and may have boulders in them intermixed with the freshwater deposits of sands and marls, and by means of natural cementing material between the particles seem to be firmly set and to be so conglomerated as to appear to be in a similar condition to weak concrete; but there is always a chance of the matrix becoming dissolved, therefore it is advisable to test a mass by the application of water and to expose it to the atmosphere before relying upon its permanent stability, and with the view to determine whether it is hard cemented gravel or not.
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.