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Mass Movement and Slope Safety: Paper 3
Mass Movement and Slope Safety: Paper 3
Ten items moving into calculation and evaluation: infinite slope safety factor, groundwater effects, the drop, return period probability, cumulative movement, the definition of risk, assessment criteria, the weathering profile, climate change and a stability misconception.
Direct answers Items 1-3: 1.43, 1.19, 16.8 per cent. Items 4-6: 2 and 10 per cent, 50 mm and 80 mm, probability times consequence. Items 7-9: criteria first, the weathering sequence, trigger and material effects. Item 10: below one is unstable.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 20 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: DSE Geography Mass Movement and Slope Safety |
| Level | Challenge |
02 Questions (10 items)
- FoundationFor a homogeneous infinite slope with c = 10 kPa, unit weight 18 kN/m3, z = 3 m, beta = 30 degrees and phi = 30 degrees, what is the factor of safety?
- FoundationAfter the water table rises, cohesion falls to 5 kPa and unit weight rises to 20 kN/m3. What is the new factor of safety?
- FoundationThe factor of safety falls from 1.43 to 1.19. What is the percentage drop?
- IntermediateWhat are the annual exceedance probabilities of a 50 year and a 10 year storm?
- IntermediateA slope moves 5 mm a year. What is the ten year cumulative movement, and the total if one storm adds 30 mm?
- IntermediateHow is risk usually defined in assessment?
- ChallengeWhy fix the assessment criteria before comparing slope risks?
- ChallengeWhat is the usual sequence of a Hong Kong weathering profile from the surface down?
- ChallengeWhat are two main effects of climate change on slope risk?
- Error redoError clinic: “A factor of safety below one means the slope is stable.” What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | about 1.43 | Substitute into the infinite slope formula: 10 plus 23.38, divided by 23.38. |
| 2 | about 1.19 | Halving cohesion while raising unit weight cuts the numerator and lifts the denominator. |
| 3 | 16.8 per cent | The difference divided by 1.43 gives the relative fall. |
| 4 | 2.0 per cent and 10.0 per cent | Probability is one divided by return period, so a longer period means a lower chance. |
| 5 | about 50 mm over ten years; about 80 mm with a 30 mm event | Multiply the annual rate by years, then add discrete events; monitoring cares about acceleration more than a single reading. |
| 6 | the probability of occurrence multiplied by the severity of consequences | At equal probability, different consequences change the ranking. |
| 7 | otherwise differing scales for probability and consequence make the ranking reflect method rather than danger | Setting criteria first is basic to any evaluation answer. |
| 8 | completely weathered material, then highly, moderately and slightly weathered zones, and finally fresh rock | Grades are assigned by weathering degree because strength and permeability change with depth. |
| 9 | more intense and frequent extreme rainfall raises triggering chances; wet dry cycles accelerate weathering and crack growth | One changes the trigger, the other changes the material itself, and the two combine. |
| 10 | wrong: below one the driving force exceeds resistance, which is unstable; only above one is there net resistance | The denominator is the driving force, so reversing the direction reverses the conclusion. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
List the formula and units first, then judge whether the result is plausible.Tutor Registration →
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