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The geometry of the problem is described in an X-Y coordinate system. The X-coordinates (horizontal) increase from the top to the toe of the slope (either left or right) and the Y-coordinates (vertical) increase downward or upward as specified in the control variables.

The ground surface and each slip surface are specified by a series of points. The points defining the slip surface also define the slice boundaries. A tension crack may be included in the analysis by specifying that the first point on the slip surface is below the ground surface. Fluid in the tension crack may have a density different from water if desired.

The user should normally begin by drawing the problem to scale on gridded paper. The ground surface should extend beyond any slip surface at each end of the slope as shown in Figure 10. If distinct stratigraphy is known, layer boundaries should be drawn. In choosing the specified slip surface points, that is the slice boundaries, the following guidelines should be followed:

1. There should be a slice boundary at each change in slope of the ground surface
2. The first and last specified slip surface points should be on the ground surface, unless there is a tension crack in which case the first slope point should be specified below the ground surface.
3. In order to obtain reasonable results at least ten slices should be used, and slight gains in accuracy will be achieved with increasing number of slices. Very little gain in accuracy will be achieved by specifying more than forty slices.
4. Slices should be relatively uniform in width throughout the slope. Wide slices should be avoided, but very narrow slices may be used when geometry dictates.
5. If the user has drawn distinct layer boundaries, the base of each slice should be within one layer. The average unit weight for each slice should be determined by hand; this calculation being most easily performed by "counting squares."

Figure 10 Description of TSLOPE geometry.

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