Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 6 SLOPE STABILITY
ii.
The height of each slice is set off below the centre of the base, and the
normal and tangential components, h cos α and h sin α respectively are
determined graphically as shown in Figure 3.3. Thus:
W cos α
W sin α
=
=
30h cos α
30h sin α
iii.
The pore water pressure at the centre of the base of each slice is taken to be
γwzw, where zw is the vertical distance of the centre point below the water
table (Fig 3.3 refers). [Note: This procedure slightly overestimates the pore
water pressure, which strictly should be γwze, where ze is the vertical
distance below the point of intersection of the water table and the
equipotential through the centre of the slice base. The error involved is
however, on the safe side].
iv. From Figure 6.13, the overall arc length, La is calculated as 14.35m. v.
The results are summarised in Table 6.2 below.
Table 6.5 Summary of Results
Slice No.
1
2
3
4
5
6
7
8
h cos α
(m)
0.75
1.80
2.70
3.25
3.45
3.10
1.90
0.55
h sin α
(m)
- 0.15
- 0.10
0.40
1.00
1.75
2.35
2.25
0.95
u
(kN/m 2 )
5.9
11.8
11.2
18.1
17.1
11.3
0
0
l
(m)
1.55
1.50
1.55
1.10
1.70
1.95
2.35
2.15
u.l
(kN/m)
9.1
17.7
25.1
29.0
29.1
22.0
0
0
17.50 8.45 14.35 132.0
vi.
Hence:
Σ W cos α = 30 x 17.50
Σ W sin α = 30 x 8.45
Σ (W cos α - ul) = 525 – 132
= 525 kN/m
= 254 kN/m
= 393 kN/m
F = c' L a +tan' ∑Wcosα-ul
∑ Wsinα
= 10x14.35+(0.554x393)
254
= 143.5+218
254
= 1.42
6A-2 March 2009