Slope stability in soil mechanics: Along what type of surface does failure of natural or engineered slopes generally occur?

Introduction / Context:

Understanding the typical geometry of failure surfaces in slopes is fundamental for choosing an appropriate analysis method. While some idealized problems assume planar failure, most real soil slopes, especially in homogeneous clays and sands, fail along curved surfaces due to the distribution of stresses and the nature of soil strength envelopes.

Given Data / Assumptions:

• Homogeneous slopes without distinct weak planes or geologic discontinuities.
• Undrained clays and drained sands commonly analyzed by circular (Bishop/Janbu) methods.

Concept / Approach:

Classically, the Swedish circle method and Bishop’s simplified method model the slip as a circular arc—i.e., a curved surface. This geometry captures the rotation and translation typical in soil mass failure. Planar failure is more associated with rock slopes along pre-existing joints or bedding planes.

Step-by-Step Solution:

Verification / Alternative check:

Back-analyses of landslides and embankment failures frequently reveal near-circular slip profiles in homogeneous soils, validating curved-surface assumptions.

Why Other Options Are Wrong:

• “Slip plane” implies planar failure; not the general case for soils.
• Horizontal or vertical surfaces are special and uncommon for soil slope failures.
• “All the surfaces” is too broad and not supported by typical soil behavior.

Common Pitfalls:

• Using planar wedge analysis for homogeneous soil slopes without justification.

Final Answer:

a curved surface