---
title: "Stability: HS 175/52, BP 150/48"
subject: "Engineering Geology II"
chapter: 6
key: "semester_4/engineering_geology_ii:stability-hs175-52-bp150-48"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/stability-hs175-52-bp150-48
site: BCT Engineering
---

# Stability: HS 175/52, BP 150/48

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

- Asked in: 2078 Poush, 6 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/stability-hs175-52-bp150-48
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6

## Question

Analyze the stability condition of rock slope from the following data by using stereographic projection method. Give your suggestion, if necessary, for maintaining the stability of the rock slope.
Hill slope: 175°/52°
Bedding plane: 150°/48°
Friction angle: 38°

## Short answer

The slope is marginally stable with potential plane failure along the bedding plane (48/150): it is 25° from the face, daylights in the 52° slope, and 48° exceeds the 38° friction angle. Cut back to 45° or less, or use rock bolts/shotcrete, provide drainage, and avoid undercutting the toe.

## Answer

**Method (lower-hemisphere stereonet, equal-angle):** plot the great circle of the slope face and of each discontinuity, draw the friction circle of radius $\phi$, and mark intersections of great circles. Then apply the kinematic tests below.

#### Data (dip direction/dip)
Hill slope 175°/52°, bedding plane 150°/48°, $\phi = 38^\circ$. In dip/dip direction: HS 52/175, BP 48/150.

| Plane (dip/dip dir) | Diff. of dip dir from slope | Daylights ($\psi_p<\psi_f$) | $\psi_p>\phi$ | Result |
|---|---|---|---|---|
| BP (48/150) | 25° | yes | yes | Marginal (dip dir. off by 25°) |

#### Stability analysis
- Dip direction of BP is $25^\circ$ from the slope direction: just outside the usual $\pm 20^\circ$ limit.
- $\psi_p = 48^\circ < \psi_f = 52^\circ$ and the apparent dip of the face in the BP direction is $49.2^\circ$, so the bedding plane just daylights.
- $\psi_p = 48^\circ > \phi = 38^\circ$, so the block can slide if it daylights.

So the great circle of BP lies close to the edge of the safe zone: **the slope is marginally stable, with a potential for plane (bedding) failure** which becomes real if the toe is cut, or if water pressure or weathering lowers the friction.

#### Suggestions
1. Cut back the slope to $\leq 45^\circ$ (less than the bedding dip) so that bedding does not daylight.
2. Alternatively install rock bolts or anchors across the bedding and shotcrete the face.
3. Provide surface and sub-surface drainage (weep holes) to prevent water pressure.
4. Avoid undercutting the toe; provide a catch ditch or retaining wall.

## Check yourself

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

1. Why is the slope only marginally stable rather than clearly failing by plane failure?
   A. The BP dip direction differs by 25°, just outside the ±20° limit
   B. The bedding plane dips into the hill
   C. The BP dip is less than phi
   D. The face is flatter than BP
   Answer: A. 175° − 150° = 25°, slightly beyond the usual 20° limit, so plane failure is only marginal. (Key point: Marginal stability reason)

2. Why can the bedding block slide if it daylights?
   A. psi_p = 48° is less than phi = 38°
   B. psi_p = 48° exceeds phi = 38°
   C. Because phi equals zero
   D. Because psi_f is 90°
   Answer: B. Sliding requires dip greater than friction angle: 48° > 38°. (Key point: Friction check)

3. What can make the marginal slope fail?
   A. Drainage and bolting
   B. Reducing the slope angle
   C. Cutting the toe, water pressure or weathering lowering friction
   D. Adding a catch ditch
   Answer: C. Toe cutting, water pressure and weathering all reduce stability. (Key point: Triggers)

4. Which suggestion makes the bedding plane stop daylighting?
   A. Steepen the slope to 60°
   B. Undercut the toe
   C. Remove drainage
   D. Cut the slope back to 45° or less (below the bedding dip)
   Answer: D. A face flatter than the bedding dip (48°) means bedding does not daylight. (Key point: Stabilisation)

## Related in Engineering Geology II

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