Mass Movement And Slope Safety: Paper 2

Mass Movement and Slope Safety: Paper 2

2026-11-11

Tutor Prep Guide · Mass Movement and Slope Safety

Mass Movement and Slope Safety: Paper 2

Ten items on measurement and engineering judgement: rainfall increase, moisture percentage points, driving force ratio, cut-off drains, soil nails, the 1977 system, vegetation clearance, drainage priority, the angle of repose and a retaining wall misconception.

Direct answers　Items 1-3: 50 per cent, 30 percentage points, about 1.5 times. Items 4-6: cut-off drains, soil nail anchorage, the 1977 office. Items 7-9: three effects of vegetation, drainage first, an angle of repose near 35 degrees. Item 10: earth pressure rises with wall height.

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	Intermediate

02　Questions (10 items)

FoundationTwenty-four hour rainfall rises from 100 mm to 150 mm. What is the increase?
FoundationSoil moisture rises from 60 per cent to 90 per cent. How many percentage points is that?
FoundationGradient rises from 25 to 30 degrees. By what factor does the downslope driving force (proportional to tan of the angle) grow?
IntermediateWhat is the main purpose of a cut-off drain above a slope?
IntermediateHow does soil nailing strengthen a slope?
IntermediateAfter several severe slope failures in the 1970s, how did the Hong Kong government respond institutionally?
ChallengeWhy does clearing vegetation raise slope risk?
ChallengeWhy is drainage usually preferred to building a taller retaining wall?
ChallengeWhat is the angle of repose, and how does it relate to stability?
Error redoError clinic: “The taller the retaining wall, the safer the slope.” What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	50.0 per cent	The extra 50 mm divided by the original 100 mm.
2	30 percentage points	Percentage points subtract directly; the relative increase would be 50 per cent.
3	about 1.24 times	Tangent of 30 degrees divided by tangent of 25 degrees, or 0.577 over 0.466.
4	to intercept runoff from upslope and stop water entering the slope body	If water stays out of the slope, pore water pressure is less likely to rise.
5	steel bars or tubes are inserted to anchor the unstable mass to the more stable ground behind, raising overall resistance	Unlike a retaining wall, nailing relies on anchorage rather than wall weight.
6	the Geotechnical Engineering Office was set up in 1977, creating slope safety control and audit	The shift was from case-by-case approval to systematic slope registration and risk management.
7	canopy interception and transpiration fall, runoff and erosion rise, and root cohesion at the surface is lost	Three effects act together, so the risk is not down to one cause.
8	removing groundwater lowers pore water pressure and so reduces the driving force, at lower cost	A taller wall only adds resistance, while water pressure behind it grows as well.
9	the steepest angle at which loose material still stands, about 35 degrees for sand; beyond it, movement starts	It is the natural limit for uncemented loose material.
10	wrong: earth pressure behind the wall rises with height, so raising a wall without better drainage and foundations can be more dangerous	Stability is a balance of resistance and driving force, not a matter of size alone.

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.

Compare measures on both resistance and driving force.Tutor Registration →

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