1. Po Shan Road, Hong Kong, 18 June 1972
ASSIGNMENT
QUESTION - 1
A landslide occurred in Po Shan Road, Hong Kong, on 18th June, 1972 which caused massive destruction and loss of lives and properties. The site was investigated and it was found that during dry season suction which developed in the soil, enabled the slope to be stable at an angle greater than the critical state effective friction angle (ϕ'crit). However, during monsoons, rainwater percolated into the soil slope resulting in release of suction. This resulted in a catastrophic failure as effective normal stresses developed within soil was reduced suddenly. Keeping this in view, answer the questions in next slide.
Po Shan Road, Hong Kong, 18 June 1972
(Source:

2. Temporary Sheet Pile Wall
ASSIGNMENT
QUESTION – 1 (contd…)
Given: Angle of the slope with the horizontal = 38°
Critical state friction angle of the soil (decomposed volcanic rock) = ϕ' = 30°
Unit weight of decomposed volcanic rock γ = 19 kN/m3 5m
Temporary Sheet Pile Wall
Decomposed
Volcanic Rock
Failure Plane
38°
100 m
γ = 19 kN/m3
ϕ' = 30 °
c' = 0
a) For the given slope, estimate the amount of suction required for the slope to be stable neglecting stability contribution from the sheet pile wall.
b) In the absence of negative pore water pressure after heavy rainfall, what should have been the minimum value of ϕ' if failure was to be avoided assuming steady seepage occurs parallel to slope?
c) Estimate the force the sheet pile wall should have resisted in order to avoid failure for the given soil for steady seepage parallel to slope in absence of suction?

3. ASSIGNMENT QUESTION – 2 Solution : GL SOIL: Depth of layer = 6 m, 25°
GWT
ROCK GL z
SOIL:
Depth of layer = 6 m,
γ = 18 kN/m3
ϕ' = 30 °
c' = 0
Position of GWT = 3 m below GL
ROCK:
γ = 22 kN/m3
ϕ' = 40 °
c' = 200 kPa
Locate the critical slip plane that is most susceptible to failure.
Solution :
The slip surface that gives the minimum factor of safety is called the critical slip surface.
Since, it is an infinite slope, slip plane is likely to be parallel to the slope.
Factor of safety corresponding to any slip plane located at z ≤ 3 m is independent of z as we assume the soil above the GWT to be dry.
Corresponding Factor of Safety for any slip plane located at z ≤ 3 m

4. ASSIGNMENT QUESTION – 3 y (m) y (m) x (m) Circular Slip Surface1 2 3 4 5 6 7
y (m)
O (5,12)
Circular Slip Surface V VI
VII
y (m)
x (m)
(2.5, -0.76)
(5, -1)
(7.5, -0.76)
1.22 m
(7.5, 1.75)
(5, 0)
(2.5, 0)
ϕ' = 25°,
c' = 10 kPa,
γ = 18 kN/m3 both above and
below water table.
The details of a partly completed stability analysis of the slope shown in the above figure using Swedish method [Fellenius (1927)] is given in Table 1 (see next slide). The configurations of slices 5, 6 & 7 are also shown in the figure (right side). Complete the analysis to obtain the operational factor of safety (Fs). Given: ϕ' = 25°, c' = 10 kPa, γ = 18 kN/m3.

5. Slice i αi (°) Wi (kN/m) li (m) ui (kN/m2)
ASSIGNMENT
QUESTION – 3 (contd…)
TABLE 1
Slice i
αi (°)
Wi (kN/m)
li (m)
ui (kN/m2) 1
57.80
65.0
3.88 2
42.30
165.5
3.41 3
28.74
167.0
2.86
4.52 4
16.77
135.0
2.62
10.49 5
5.52 6
-5.52 7
-16.77