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The user has the option to specify the initial value of the factor of safety. Convergence is somewhat sensitive to the choice of the initial value of the factor of safety. If you are analyzing a slope which you know is marginally stable, start with an initial value of 1, for instance, else use a higher value such as 3.

There are two other things the user can do to facilitate convergence. One is always specify a tension crack and then check the initial output to make sure that the inter-slice forces are not negative at the top of the slope. If they are, increase the depth of the tension crack. Likewise, if the effective normal forces on the slip surface are negative this is not physically realistic but may occur if high pore pressures, pseudo-static seismic forces or unusual line loads and pressures are specified. We have considered automatically eliminating negative effective normal forces but for the time being have left this for the user to deal with. See Ching and Fredlund (1983) for further discussion on this point.

When the geometry of the slip surface is such that the Spencer assumption of a constant angle of inclination of inter-slice forces is unreasonable, convergence can often be facilitated by using the Morgenstern and Price option. Such a situation is illustrated in Figure 13. The inter-slice forces on the right hand side of the sliding mass will tend to have inclinations opposite to those on the left hand side, so that forcing a constant angle of inclination is artificial. This may inhibit convergence, especially if too coarse a spacing of slices is used.

Figure 13 Natural variation of angle of inclination of interslice forcs.

The convergence problem can be minimized by increasing the number of slices, especially in the vicinity of abrupt changes in the direction of the slip surface, but the inter-slice forces may still look artificial. For this reason we have added an option for TSLOPE to use the Morgenstern and Price method with an automatically specified Taylor/Lowe distribution of the angle of inclination of the inter-slice forces. Under this option the distribution f(x) (which multiplies on tan ) in the Morgenstern and Price method is taken to be proportional to the tangents of the angles which are the average of the angle of the ground surface and the angle of the base of the slice within each slice. This procedure seems to provide reliable convergence and a reasonable line of thrust under normal gravity loads and static pore pressures, but converges is less reliable if pseudo-static forces or unusual line loads and pressures are specified. In these cases it is better to use Spencer's method even if the constant angle of inclination of the inter-slice forces looks a bit odd. If Spencer's method fails to converge or gives an unreasonable line of thrust, the Morgenstern and Price option with a user specified distribution of the angle of inclination of the inter-slice forces can be tried.

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