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The ordinary method of slices assumes that the resultant of the side forces acting on a slice act parallel to the base of the slice. By resolving forces normal to the base of the slice, the side forces are eliminated and the following equation results:

(3)

Moment equilibrium around the center of the circular slip surface is the only condition of equilibrium satisfied by this method. The value of P given in Equation 3 is a low approximation, which leads to a conservative value of F. In cases of flat slopes with high pore pressures, the error in the value of F may be as much as 50%. In total stress analyses the error is typically not more than 10%.