![]() Vertical Stress Distribution on a Horizontal Plane : It is also possible to draw isobars of stress intensity 0.2Q, 0.5Q, and 0.75Q using a similar procedure these isobars are shown in Fig. ![]() The left-hand side of the pressure bulb can be drawn using the same value of r at different depths but with a negative sign. It then decreases with further increase in depth and becomes zero again at a depth of 2.185. The radial distance r attains a maximum value of 0.938 when z = 1.25. It may be observed from the pressure bulb that r is zero directly below the point load and increases with the increase in depth below the ground surface. The resulting coordinates of r and z are plotted to get the pressure bulb of intensity 0.1Q, as shown in Fig. Thus,īy substituting the different values for z, the corresponding values of r/z and hence r can be obtained. The coordinates of the points on the pressure bulb can be obtained by substituting σ z = 0.1Q in the Boussinesq equation for vertical stress due to a point load. ![]() A pressure bulb is an isobar of stress intensity 0.1Q, where Q is the magnitude of the concentrated load. For a concentrated load acting at the ground surface, the isobar is symmetrical on either side of the axis of the load. Isobar is a spatial curved surface in the shape of an electric bulb or an onion. The soil outside the pressure bulb is assumed to have negligible stresses.Īn isobar is a curve joining points of equal vertical stresses in the soil mass. The zone of soil within which the vertical stress is significant and causes significant deformation of the soil, affecting the safety and stability of the structure, is known as pressure bulb. It also decreases with the increase in the distance from the axis of the load in the lateral direction at any depth. ![]() Pressure Bulb :įrom the Boussinesq equation for vertical stress, it may be observed that the vertical stress decreases with the increase in depth from the ground surface. The load is applied at the ground surface. The soil is weightless and unstressed before application of the load.Į. The soil may not be elastic, but obeys Hooke’s law that is, the stress-strain relationship is linear.ĭ. The soil mass is semi-infinite that is, it extends infinitely in all directions below a level surface.Ĭ. The following are the assumptions in the Boussinesq theory:Ī. ![]()
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