RANKINE'S THEORY OF EARTH PRESSURE AND THE COULOMB'S THEORY OF EARTH PRESSURE, AND THE GRAPHICAL REPRESENTATION - REBHANN'S CONSTRUCTION

THEORIES OF EARTH PRESSURE  :-- The theories of the earth pressure, Rankine's theory of earth pressure and Columnb's theory of earth pressure, are used for computation of earth pressures. RANKINE'S THEORY OF EARTH PRESSURE :-- In computation of pressure by this theory it is assumed that the Retaining wall yields by sufficient amount so that state of the plastic equilibrium exists in the soil mass in immediate vicinity of wall .The remaining soil is in the state of elastic equilibrium. In this theory it is further assumed that there is no friction between the soil and wall .(a) ACTIVE PRESSURE :--When the retaining wall deforms outwards , ie soil is allowed to stretch by about two percent of the height of wall , the earth pressure exerted is active earth pressure.The active earth pressure at a depth h in a retaining wall with horizontal ground surface acts at h/3 of the base and is inclined at an angle alpha to the horizontal. (2) COULOMB'S  THEORY OF EARTH PRESSURE:-- It is assumed in the Rankine's theory of earth pressure that the back of the retaining wall is smooth and thus there is no friction between the wall and the earth .This assumption is however seldom justified and as the back of the retaining wall is never smooth and thus the earth pressure computed on the basis of the Rankine's theory leads to the erroneous results. Coulomb's theory can be applied to any boundary conditions.In this theory it is assumed that the retaining wall yields by sufficient amount and causes state of plastic equilibrium in soil. This is also the assumption made in Rankine's theory. In the Coulomb's theory it is further assumed that the surface of sliding is plane though in the reality the surface of sliding is slightly curved .The resultant earth pressure acts at lower middle third and is inclined at an angle delta to the normal to the face of the walls .