Illustrates about the effect of wall movement on earth pressure?

Effect of wall movement on earth pressure

About fifty years ago Terzaghi conducted a series of tests on large scale model retaining walls to ascertain the variation of magnitude of earth pressures with the movement of the walls. While the wall is rigid and unyielding, no deformations and the soil mass are there in a state called earth pressure at rest. This is represented by the point A in figures (a). When the wail rotates about its toe, moving away from the soil as in figure (b), the soil mass expands, resulting in a decrease of earth pressure. Any element of soil will then behave just like a specimen in a tri-axial test in which the confining pressure is decreased while the axial stress remains constant. When the horizontal stress is decreased to a certain magnitude the full shear strength of the soil will be mobilised. No further decrease in the horizontal stress is possible even with further movement of the wall. This is called active earth force. This is represented by point B in the figure (a).

If the wall is pushed towards the wall as in figure (c) the soil is compressed and the soil offers resistance to this movement by shearing resistance. Any element of the soil can now be considered to be in the condition of a triaxial specimen being failed by increasing the confining pressure while holding the vertical stress constant. The horizontal $tress cannot be increased beyond a magnitude called passive stress. At this stress failure occurs arid the passive earth force is denoted by the point C in figure (a). Active and Passive earth pressures develop corresponding to two limiting slates of equilibrium. The soil mass is said to be in a state of plastic equilibrium ill these two stages. A small increase in stress at this stage will cause the plastic flow condition-a continuous increase in the corresponding strain.

Wall Movement and Development of Active and Passive Pressure

It is to be noted that for stands very little horizontal strain, less than 0.5% is required to reach the active state while little horizontal compression, about 0.5% is required to reach one-half the maximum passive resistance. However about 2% of horizontal compression is needed to reach the full maximum passive resistance in dense sands while m loose sands horizontal compression needed to reach full passive resistance may be as large as 15%.