Soil Stability Analysis: Limit Equilibrium Methods

Limit equilibrium methods are used for soil analysis, particularly for determining the material factor of safety. These methods analyze a finite number of potential sliding surfaces.

For each surface, equilibrium is formulated using the maximum stresses available along the sliding surface, with soil strength reduced by an unknown factor F. The value of F is obtained from the solution of the resulting equations.

In soils, commonly selected surfaces are straight lines, circles, or log spirals. In rocks, sliding is governed by weakness planes, and hence blocks or wedges with planar faces are considered.

Corrective Measures for Soil Instability

Corrective measures typically involve either reducing unfavorable actions (weight and/or water flow) or increasing resistant forces.

Reducing Unfavorable Gravity Forces

To reduce unfavorable gravity forces, geometry changes are needed:

• Regrading the slope angle
• Excavation at the upper part
• Creation of berms

Reducing the Unfavorable Effects of Water

To reduce the unfavorable effects of water:

• Surface water: Capture and diversion of all springs and water courses in the slope and its surroundings. Cut-off ditch (collector drain) at the crest and berms to avoid rainfall infiltration. Re-vegetation of the slope face.
• Phreatic water: “Horizontal” drain holes (drenes californianos). Draining shallow trenches. Rockfill. Deep wells (gravity or pumped). Drainage galleries.

Increasing Stabilizing Forces with External Elements

To increase the stabilizing forces with external elements:

• Retaining structures at the foot of the slope, founded on stable ground (Concrete walls, Rockfill walls or gabions, Embedded walls).
• Elements in tension: anchors and bolts. All the active length must be in stable ground. Very frequently used in rock slopes. In soils, the usual configuration is with anchored concrete blocks or walls.
• Elements in shear: They must extend well below the unstable area.
• Piles (or barrettes), anchored or not, tangent, secant or isolated.
• Micropiles, vertical or inclined (can be anchored).
• Jet-grouting columns.
• Steel beams (or rail sections), for temporary solutions.