In the stability analysis of slopes, particularly those related to earth dams, it is necessary to estimate the location of the phreatic surface or the line of seepage. In the case of an existing slope, the phreatic surface can be determined from the subsurface investigation with adjustments for seasonal changes. If the slope has not been constructed and is quite complex in configuration, the easiest way to determine the phreatic surface is by drawing a flow net, as shown in Fig. 4.1. For a homogeneous and isotropic cross section, if the flow net satisfies the basic requirements that the flow lines and equipotential lines are perpendicular and form squares or rectangles of the same shape and that the vertical distance between equipotential lines along the phreatic surface are the same, the assumed phreatic surface is correct; otherwise, the phreatic surface must be changed until a satisfactory flow net is obtained. For an anisotropic cross section, a transformation based on the ratio between vertical and horizontal permeabilities must be made so that square flow nets can still be constructed. For a nonhomogeneous cross section, the flow nets must satisfy the continuity and interface conditions; as a result, the flow nets in some regions must be rectangular instead of square. Methods for constructing flow nets can be found in most textbooks in soil mechanics and also in Cedergren (1977).
KeywordsPore Pressure Slope Stability Pore Water Pressure Failure Surface Failure Plane
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