---
title: "Condition of toppling failure"
subject: "Engineering Geology II"
chapter: 6
key: "semester_4/engineering_geology_ii:toppling-failure-conditions"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/toppling-failure-conditions
site: BCT Engineering
---

# Condition of toppling failure

Engineering Geology II · Chapter 6: Measurement, analysis and interpretation of structural geological data

- Asked in: 2068 Bhadra, 3 marks
- Video: [Tutorial 5: Kinematic Analysis of Toppling in Rock Mass in Civil Engineering | Procedure Explained](https://www.youtube.com/watch?v=swYSosPn4Mk) — Soil Mechanics and Engineering Geology
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/toppling-failure-conditions
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6

## Question

Describe the condition of toppling failure.

## Short answer

Oppling failure requires steep discontinuities dipping into the slope (dip direction opposite to the face) with dip angle ≥90° minus face dip plus friction angle. The rock columns must be slender, have a free toe, and their centre of gravity must lie outside the base.

## Answer

**Toppling failure** is the forward rotation (overturning) of columns or slabs of rock about a fixed base. It occurs in steep slopes with closely spaced discontinuities that dip steeply **into** the slope.

#### Conditions (Goodman and Bray)
1. **Discontinuities dip into the hill:** the dip direction of the discontinuity set is opposite to the dip direction of the slope face, $\alpha_p \approx \alpha_f \pm 180^\circ$ (within about $10^\circ$ to $30^\circ$); the strike is nearly parallel to the slope face.
2. **Steep dip:** the planes dip steeply, with
$$
\psi_p \geq (90^\circ - \psi_f) + \phi
$$
so that interlayer slip occurs and the layers can flex and rotate.
3. **Slender columns:** the slabs are thin compared with their height, with a continuous set of cross joints that form the base of the columns.
4. **Steep slope face** with the toe free to move, and the centre of gravity of each block falling outside its base.

```text
        \  |  |  |  |
         \ |  |  |  |  columns tilt
          \|  |  |  |  out of slope
           \__|__|__|__
```

**Stereonet test (pole plot):** the poles of the planes plot on the same side as the slope dip direction (within about $10^\circ$ to $30^\circ$ of it), at a distance of at least $(90^\circ - \psi_f + \phi)$ from the centre, i.e. below the slip-limit great circle that dips at $(\psi_f - \phi)$ in the slope direction.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How do discontinuities dip in toppling?
   A. Out of the slope, parallel to face
   B. Flat
   C. Randomly
   D. Steeply into the hill, opposite to the slope face dip direction
   Answer: D. Dip direction is about 180° opposite the slope's. (Key point: Geometry)

2. Which condition on dip is required?
   A. psi_p >= (90° - psi_f) + phi
   B. psi_p <= phi
   C. psi_p = psi_f
   D. psi_p < 20°
   Answer: A. Steep dip allows interlayer slip and rotation. (Key point: Dip condition)

3. What rock form topples?
   A. Massive blocks
   B. Slender columns or slabs with thickness small compared to height
   C. Loose soil
   D. Horizontal beds
   Answer: B. Slender columns rotate forward about a base. (Key point: Columns)

## Related in other subjects

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- [Khosla: barrage HGL and floor thickness](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akhosla-barrage-hgl-thickness-pond1135) — Irrigation and Drainage Engineering, chapter 5