Line of Thrust
Review the line of thrust for rigorous methods.
Review the line of thrust for rigorous methods.
The line of thrust traces the location of the interslice normal forces' resultant through the sliding mass. It is a check on solution quality: a physically reasonable solution keeps the line of thrust inside the sliding mass, roughly in the lower third of each slice. A line of thrust that climbs out of the mass or oscillates wildly signals that the interslice force assumptions are being strained.
Availability
The line of thrust is only meaningful for methods that solve the interslice forces explicitly. In JW Slope it is available for GLE (Morgenstern–Price) and Spencer only.
Enable it with Show Line of Thrust in the Compute panel display controls. It is off by default and the control is enabled only when:
- a GLE or Spencer method is selected in the Analysis Type filter, and
- the critical slip surface is shown.
Thrust diagnostics
Alongside the drawn line, the results readout reports two diagnostics for the critical surface:
| Readout | Description |
|---|---|
| Thrust Residual | Residual in the thrust-line force balance; small values indicate a well-converged solution. |
| Thrust Line Error | Error measure for the computed line of thrust. |
FS vs Lambda plot
For GLE / Spencer results, the readout also includes an FS vs Lambda plot. The interslice force function scaling factor, lambda (λ), is varied along the horizontal axis, and two curves are drawn:
- Force equilibrium — factor of safety satisfying force equilibrium at each λ.
- Moment equilibrium — factor of safety satisfying moment equilibrium at each λ.
The GLE / Spencer solution is the point where the two curves cross: the value of λ (and the corresponding factor of safety) that satisfies both force and moment equilibrium simultaneously. The lambda value at the solution is also printed in the readout.
Reading the crossing
If the two curves are nearly parallel or cross at a steep angle, the solution is sensitive to the interslice force function. Curves that do not cross within the searched λ range indicate the method could not satisfy both equilibrium conditions for that surface.