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soil mechanics[′sȯil mi‚kan·iks]
a branch of engineering geology that studies soils with a view to the possibility of erecting various structures on them. Soil mechanics is divided into three fields, which have independent scientific value and are taught as separate disciplines: general soil mechanics, the study of the technical improvement of soils, and regional soil mechanics.
General soil mechanics studies the composition, structure, texture, and properties of the most widespread and typical rocks and soils in relation to the needs of engineering construction and is the theoretical foundation for the other fields of soil mechanics. Modern soil mechanics proceeds from the assumption that the properties of soils are determined by the genesis of the rocks and by subsequent epigenetic processes. Therefore it is first necessary to study the processes of formation and strength of rocks using geological-petrographic and physicochemical methods. Soils are studied in the laboratory and under natural conditions. The determination of the strength of massifs is very important, especially in the case of rocky soils where the strength of individual samples may be very high but that of the massif negligible because of tectonic disintegration and weathering of the rock. Electron microscopy, electronography, roentgenography, and other modern techniques are widely used in soil mechanics to study changes in the composition, structure, and texture of rocks and in their properties as a result of man’s engineering activities.
The study of the technical improvement of soils is concerned with the theoretical and experimental development of methods for artificially improving the properties of soils in accordance with the needs of different kinds of construction. The practical significance of this field of soil mechanics increased after World War II in connection with the difficult problem of artificially changing rocks.
Regional soil mechanics studies the main engineering and geological characteristics of specific rocks and soils and the laws of their spatial distribution. Soil mechanics is closely connected with engineering geodynamics, regional engineering geology, and related geological and other disciplines.
Soil mechanics arose in the USSR in the 1920’s in connection with the tasks of socialist construction, especially road building and hydraulic-engineering projects. The studies of P. A. Zemiatchenskii, M. M. Filatov, V. V. Okhotin, F. P. Savarenskii, V. A. Priklonskii, N. Ia. Denisov, and B. M. Gumenskii played a major role in the development of Soviet soil mechanics. Soil mechanics is also being developed in other socialist countries. In most capitalist countries, however, it has not been recognized as a separate discipline and is included in geological engineering, a science concerned with both the mechanics of soils and the construction of foundations.
REFERENCESPriklonskii, V. A. Gruntovedenie, 3rd ed., part 1. Moscow, 1955.
Gruntovedenie, 3rd ed. Edited by E. M. Sergeev. Moscow, 1971.
Trudy soveshchaniia po inzhenerno-geologicheskim svoistvam gornykh porod i metodam ikh izucheniia, vols. 1–2. Moscow, 1956— 57.
Voprosy inzhenernoy geologii i gruntovedeniia. Moscow, 1963.
E. M. SERGEEV
the scientific discipline that deals with the stress-deformed state of soils, conditions of soil strength, pressure on barriers, and the stability of soil masses.
Soil mechanics considers the relationship between the mechanical properties of soils and their structural and physical state, investigating general soil compressibility, structural-phase deformability, and shear strength at the interface. The results obtained are used in planning the bases and foundations of buildings and industrial and hydraulic engineering structures, in road and airport construction, for installing underground utility lines and laying pipes, and for predicting deformations and the strength of slopes and retaining walls. The methods of soil mechanics are also used in dealing with the problems of blasts and vibrations in industrial processes involving ground development.
The basic type of soil deformation is compaction under pres-sure. Compaction is a result of normal forces applied to a soil element, primarily through the relative displacement (shifts and turns) of solid mineral particles. Compaction leads to a reduction
in soil porosity. Indexes of soil deformability are the coefficient of relative compressibility (or its inversely proportional general deformation modulus) and the relative lateral deformation coefficient. These are analogous to the modulus of elasticity and to Poisson’s ratio for elastic bodies, with the difference that loading the soil is assumed to be a single process (without subsequent unloading); in addition, soil is far from destructible. Soils are typically deformable over periods of time, both when water is forced out of their pores and a redistribution of pressures results between pore water and soil skeleton (the process of filtration consolidation) and as a result of the viscous movement of soil particles (soil creep).
The principal type of soil strength impairment is the displacement of one of its parts in relation to another as a result of a sustained shift that turns into a shear. The resistance of unbound (loose) soils to shear is a result of the forces of internal friction that develop at the points of contact between soil particles during relative shift. In clayey soils, cementation and water-colloid bonds impede shift and determine resistance to shear.
The indexes of soil strength (the angle of internal friction and specific cohesion, which depend on the physical state of the soil) are the only parameters of the shear diagram necessary to calcu-late strength. For clayey soils, the magnitude of the forces of internal friction depends on the share of the external load taken by the mineral skeleton. If part of the load is transferred to the pore water, the soil has less resistance to shearing as a result of friction. The speed of movement of the water in the pores is described by Darcy’s law; the speed of deformation of viscous-plastic interparticle bonds is described by an integral equation of the Boltzmann-Volterra theory of hereditary creep, the core of which is established by experiment. In the case of vibrations, the mechanical properties of soils (especially unbound) vary with the intensity of vibration. Under certain conditions, weakly cohesive soils affected by vibrations take on the properties of viscous fluids.
In making predictions, soil mechanics draws on data from engineering geology and engineering hydrogeology and the basic relationships of continuous medium mechanics (in particular, the theories of elasticity, plasticity, and creep and loose medium statics).
The most important tasks of soil mechanics are investigating stresses and deformations of soil masses under the effect of both external forces and soil weight and working on questions of the effect of soil strength and stability and soil pressure on barriers and on underground structures set at shallow depths. The solutions to these problems for various cases of loading find direct applications in construction.
Two basic methods are used in dealing with the problems posed in soil mechanics. The first is theoretical calculation, based on the mathematical solution of precisely formulated problems and involving the experimental (laboratory or field) determination of the values of the initial parameters. The second method is modeling, used in cases where the complexity of the problem makes it impossible to obtain a “closed” solution or anything but a very cumbersome result. The theoretical calculation method is developing intensively, thanks to the use of computers. Modeling, which was first proposed in the USSR by G. I. Pokrovskii and N. N. Davidenkov, is developing in two directions: physical, for problems in which mass forces are not taken into account, and centrifugal, which meets the requirements of similarity theory and does consider mass forces.
Using solutions based on equations for a continuous linearly deformable medium and applicable to soils only under certain conditions makes it possible to consider many problems of soil mechanics where the state of stress is less than the limit. In a number of cases, only the stress state is established by the linearly deformable medium theory; the transition to deformations is made by means of experimentally determinable relationships.
The distribution of stresses obtained by solving the problem for a continuous linearly deformable medium is used in considering problems of soil deformation over periods of time according to the theories of filtration consolidation or creep.
The theory of limit equilibrium of loose mediums is used in soil mechanics for problems related to determining critical loads on bases, the limit equilibrium of a ground slope of a given profile, the outline of maximally stable slopes in the absence of additional loads or in the presence of certain additional loads from above, the active and passive ground pressures on inclined retaining walls, and the stability of soil vaults.
Some structurally unstable types of soil (thawing permafrosts, sagged loesses under moisture, weak structural soils) have special deformation characteristics associated with sharp changes in their physical state and structure. Special methods have been developed in modern soil mechanics for calculating the subsidence of permafrost soils upon thawing and of loesses under moisture. The maximum loading rates are established for weak clays and peats with regard to the preservation of their structural strength. On the basis of scientific advances in soil mechanics, the USSR has created a new method of designing bases and foundations according to limit deformations. The important tasks of modern soil mechanics include the further refinement of methods for determining the physicomechanical properties of soils under laboratory and field conditions, for comprehensive studies of the combined work of structural foundations and base soils, and for designing pile foundations.
The first basic work on soil mechanics was the French scientist C. Coulomb’s investigation of the theory of cohesionless materials (1773). A number of Coulomb’s results are still used successfully today in calculating ground pressure on retaining walls. In 1885 the French scientist J. Boussinesq solved the problem of stress distribution in elastic semispace under concentrated force; this served as the basis for the determination of stresses in linearly deformable bases. An important stage in the development of soil mechanics was the research of the American scientist K. Terzaghi.
Important contributions to soil mechanics have been made by Russian (V. I. Kurdiumov and P. A. Miniaev) and, in particular, by Soviet scientists. Soviet scientists have worked out the latest theory of the limit equilibrium of soils (V. V. Sokolovskii, V. G. Berezantsev, S. S. Golushkevich, M. V. Malyshev), formulated and solved problems of the theory of the consolidation of two-phase and three-phase soils (N. M. Gersevanov and D. E. Pol’shin, V. A. Florin, N. A. Tsytovich, N. N. Maslov, lu. K. Zaretskii), and investigated questions of the combined work of structures and their bases using a theory of beams on an elastic foundation (A. N. Krylov, M. I. Gorbunov-Posadov, V. A. Florin, B. N. Zhemochkin, A. P. Sinitsyn, I. A. Simvulidi). Soviet scientists have played an important part in solving a number of problems related to the mechanics of particular regional soil types, including structurally unstable settled soils (lu. M. Abelev, N. la. Denisov, and R. A. Tokar’) and permafrosts (N. A. Tsytovich, S. S. Vialov, M. N. Gol’dshtein). Among studies in slope stability, the best known are by V. V. Sokolovskii, N. N. Maslov, and M. N. Gol’dshtein. The works of I. P. Prokof ev and G. K. Klein on retaining walls are the most prominent.
Among foreign scientists in soil mechanics, the most well known work has been done by J. Kerisel (France), J. Brinch Hansen (Denmark), R. Gibson and A. Bishop (Great Britain), and M. Biot and W. Lamb (USA).
Research in soil mechanics is carried out at a number of scientific establishments and higher educational institutions in the USSR, including the N. M. Gersevanov Scientific Research Institute of Foundations and Underground Structures and the V. V. Kuibyshev Moscow Institute of Construction Engineering.
The International Society of Soil Mechanics and Foundation Engineering (ISSMFE) was established in 1936 on the initiative of Terzaghi. The USSR has been a member since 1957, and the society held its eighth congress in Moscow in 1973. The society’s journal, Geotechnique, has been published in London since 1948. In 1959 the USSR began publication of the journal Osnovaniia, fundamenty i mekhanika gruntov (Bases, Foundations, and Soil Mechanics). Periodicals are also published in the USA, France, and Italy.
REFERENCESProkof ev, I. P. Davlenie sypuchego tela i raschet podpornykh stenok, 5th ed. Moscow, 1947.
Gersevanov, N. M., and D. E. PoPshin. Teoreticheskie osnovy mekhaniki gruntov i ikh prakticheskie primeneniia. Moscow, 1948.
Florin, V. A. Osnovy mekhaniki gruntov, vols. 1–2. Leningrad-Moscow, 1959–61.
Sokolovskii, V. V. Statika sypuchei sredy, 3rd ed. Moscow, 1960.
Terzaghi, K. Teoriia mekhaniki gruntov. Moscow, 1961. (Translated from German.)
Tsytovich, N. A. Mekhanika gruntov, 4th ed. Moscow, 1963.
Tsytovich, N. A. Mekhanika gruntov: Kratkii kurs, 2nd ed. Moscow, 1973.
Klein, G. K. Raschet podpornykh sten. Moscow, 1964.
Gol’dshtein, M. N. Mekhanicheskie svoistva gruntov, 2nd ed. [vols. 1–2]. Moscow, 1971–73.
N. A. TSYTOVICH and M. V. MALYSHEV