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in construction, the subsidence of a structure as a result of the compression of its foundation or a decrease in the vertical dimensions of a structure or its parts. Settling is caused by the properties of the supporting ground and the effective loads; the type, dimensions, and design of the foundations; and the rigidity of the structure.
Usually an uneven process, settling is described in terms of the absolute amount of settling at various points and in terms of average values. Absolute settling must be less than the permissible maximum based on the building’s structural features and intended use. The expected settling is calculated on the basis of soil tests and is compared with the permissible maximum for the particular structure.
Uneven settling of a foundation produces strains in the structure. The added forces that accompany these strains can disrupt structural stability or interfere with normal use of the building. The design process takes these distorting forces into account by specifying vertical through joints (called keyed expansion joints), which divide structures into sections that settle independently. Building plans also provide for foundations with higher rigidity and strength to withstand the added forces without damage.
Settling usually begins immediately after construction is started. It continues while a structure is being erected and the bearing load is increasing, and it goes on for some time after construction has been completed. On clayey soils a foundation settles very slowly, and in some cases the movement never ceases entirely. When the loads on the soil approach the limit of its load-bearing strength, an abrupt settling may be observed as a result of the soil being squeezed out from under the footings. In earthen dams, embankments, and similar structures, settling may occur when there is consolidation of the mass of earth fill caused by the forcing out of water through pores in the soil and by viscous deformation of the soil’s primary structure.
Unlike settling, when the ground sags under a foundation because of a radical change in its structure, this sagging is caused by the compaction of loesslike soils as they become wet, of frozen soils when they thaw out, or of friable sandy soils that are subjected to vibration. Leaching out of the soils and the undermining of the building site can also cause sagging. Decrease in the volume of a mass of soil as it dries out is known as shrinkage.
A reduction in the vertical dimensions often occurs in masonry structures. This is most evident when masonry that is laid in winter and allowed to freeze contracts when the mortar thaws out. Settling occurs in wooden walls because of shrinkage in the wood and tightening of the joints. The settling of the walls must be taken into account at the time of construction. Plastering, in particular, should not be done until the settling has been completed.
Settling is inspected mainly by geodetic methods, which employ a fixed triangulation network.
M. V. MALYSHEV
the slow separation under the influence of gravity of a liquid disperse system, which can be a suspension, emulsion, or foam, into its components—the dispersion medium and the dispersed phase. The particles in the dispersed phase either precipitate to the bottom of the container or float to the surface of the liquid during settling. The combination of settling with decantation is called elutriation. The concentrated layer that is generated during settling and that consists of separate drops at the surface is called the cream. The sediment, which accumulates at the bottom, consists of particles of suspension or droplets of emulsion. Whether settling will result in the accumulation of sediment or cream is determined by the laws of sedimentation. Settling in highly disperse systems is frequently accompanied by the enlargement of particles owing to coagulation or flocculation. The form of the sediment depends on the physical characteristics of the disperse system and on the conditions of settling. Coarsely disperse systems produce dense sediments, while poly-disperse suspensions of finely divided lyophilic products give loose, gel-like sediments.
Settling is a widely used method of purifying liquids to remove coarsely disperse mechanical impurities. It is also used in the purification of water for industry and human consumption, in the treatment of sewage effluents, to dewater and desalt crude petroleum, and in numerous processes in chemical technology. Settling is an important stage in the spontaneous purification of natural and man-made water reservoirs. It is also used to separate various industrial and natural products that can be dispersed in liquid mediums.
REFERENCESKasatkin, A. G. Osnovnye protsessy i apparaty khimicheskoi tekhnologii, 8th ed. Moscow, 1971. Page 185.
Planovskii, A.N., and P. I. Nikolaev. Protsessy i apparaty khimkheskoi i neftekhimicheskoi tekhnologii, 2nd ed. Moscow, 1972. Pages 49, 370.
L. A. SHITS