Global slope stability

The global slope tool calculates a Bishop simplified factor of safety for circular slip surfaces through a two-dimensional soil section. It is separate from the pile-row resistance tool.

Method and inputs

Define terrain coordinates, horizontal soil layers, total unit weights and drained strength parameters. A piezometric line specifies pressure head at each slice base. Vertical strip surcharges add weight to the affected slices. A prescribed empty tension crack can truncate the uphill side; water-filled cracks are outside this implementation.

The calculation follows USACE EM 1110-2-1902 Appendix F equations F-4 and F-5. Each slice satisfies vertical force equilibrium, and the complete sliding mass satisfies moment equilibrium. Bishop simplified does not impose full horizontal interslice equilibrium.

Search and numerical checks

Supply individual trial circles or a bounded grid of circle centres and radii. Each candidate is solved again with twice as many slices; differences over the declared numerical gate reject that candidate. The result retains the lowest computed valid factor of safety, its slice audit, and the status of every trial circle.

A rejected or unresolved trial circle leaves the search incomplete. The displayed minimum belongs to the evaluated candidates; it is not proof that no lower surface exists outside or between them.

Interpreting the result

Supported geometry has terrain descending from left to right and contiguous horizontal strata. This static circular method excludes seismic acceleration, noncircular failure, reinforcement and water-filled tension cracks. Use the separate pile-row tool for mobilized reinforcement resistance; its output is not automatically included in this factor of safety.

USACE Slope Stability manual provides the governing equations and comparison examples. The report records search bounds, rejected candidates, water conditions and numerical refinement so the calculation can be reproduced.