Gravel

GRAVEL, is a product of the comminution of various hard rocks, formed mainly under the influence of the prolonged action of flowing water, as a result of which individual gravel pebbles always have more or less rounded edges. Depending on the location of the deposits, a distinction is made between river gravel, found in the beds of watercourses and on their banks, and ravinal or mountain gravel. Pure gravel consists of an accumulation of more or less small rounded pebbles, ranging from 3 to 20 mm in diameter. According to size, gravel can be divided into the following grades: 1) fine gravel - 3–7.5 mm, 2) medium - 7.5–12 mm, 3) coarse - 12–20 mm. Large quantities of gravel located in depressions on dense impermeable layers often serve as sites for large accumulations of pure underground water, which serve as sources of water supply for large settlements. The occurrence of gravel in the form of interlayers, which is found in some cases, can also have a negative sanitary significance due to the high permeability of gravel, which under appropriate local conditions can cause the penetration of polluted

wastewater into the water-bearing horizon from which water is used for drinking. Depending on the mode of occurrence, gravel is mined in various ways. River gravel, lying in layers in the beds of rivers and streams, is mined using special wire nets mounted on a pole. The gravel captured by the net is washed right there in the river from the sand and silt adhering to it by repeated movements of the net in the water. Mountain gravel is mined by quarrying. For practical use, gravel must be sorted in addition to washing. Sorting is performed on screens or using special mechanical devices. Gravel has two important properties upon which its practical application is based: the first property is significant permeability and water capacity, and the second is the ability to form very strong bonds with binding mortars, especially with cement. A mixture with sand, cement, and water produces a durable building material—concrete (see), and with the simultaneous use of an iron frame—reinforced concrete. In connection with these properties, gravel has a wide and diverse application in construction, road building, and sanitary-technical engineering. In the latter field, gravel has found a particularly important application in filtration structures, where it is used both for charging preliminary filters in complex filtration systems (for example, Puech filters) and as a sub-base layer in English and some American sand filter systems (see American and English filters).

V. Muscat.