SAND AND GRAVEL. In fine sand-cuttings springs may be expected, and the earth become in a semi-fluid state if there is water at a higher level to filter through it; also in the case of all porous and open soils. Any drawing away of the sand must be prevented, as it will induce a slip, and cause the earth to become running sand, especially dangerous near buildings, for its egress must be prevented, or subsidence will ensue, and serious erosion. The excavation in such cases should be in as short lengths as practicable, so that the surfaces are not unsupported, and walls and structures should be quickly erected. The sands that are met with in estuaries are frequently in such a condition, that a slight obstruction to the tidal flow will cause movement, the equilibrium being easily destroyed. Should there be a break in the continuity of a clay stratum, overlying light loose soil, the latter will probably boil up, and in determining the depth of a cutting, care should be taken that this impervious stratum is not broken or injured. Marl, clay and sand beds are likely to slip when they are superimposed, and there are some districts in which sandy soil is so charged with water that, unless the drainage of the slopes and formation suffices to drain for some distance the land outside a cutting, the sand will become overcharged with moisture and will act as a fluid and slip, the lateral support being removed by the act of excavation and its normal condition altered. Being so delicately balanced the least additional disturbing force, such as a spoil bank being tipped upon the surface, or the inducement or acceleration of a flow of water, will set it in motion and make it a quicksand. For instance, small sand islands have been removed by making cuts in them from 15 to 20 feet in width, and by men shaking bars, &c., inserted in the soil; the sand along the edge becomes loose, falls, and the current sweeps it away. As an example of the changeability of the condition of sand may be named that in sinking pits by congelation in loose sand it has been found that the grains during the freezing of the water, by means of tubes containing a freezing mixture, were additionally separated about 5 to 7 per centum. As the sand met with in public works is seldom in very deep beds, it has not been subject to the steadying forces which many earths have undergone, and it may have been constantly moving until its final deposition, and therefore it is easily set in motion; and although sand will subside less from a load after it is saturated with moisture, the water in it trying to escape may cause it to slip upon an unsupported surface such as a slope.