Geotechnical Engineering

DHARM

522 GEOTECHNICAL ENGINEERING

c = 12 kN/m^2

γ = 17.0 kN/m^3

∴ zc =

212

17 0

× ×1 428

.

.m

= 2.00 m
Example 13.12: A retaining wall with a smooth vertical back retains a purely cohesive fill.
Height of wall is 12 m. Unit weight of fill is 20 kN/m^3. Cohesion is 1 N/cm^2. What is the total
active Rankine thrust on the wall? At what depth is the intensity of pressure zero and where
does the resultant thrust act?

H = 12 m γ = 20 kN/m^3 φ = 0°

Nφ = tan^2 (45° + φ/2) = 1

c = 1 N/cm^2 = 10 kN/m^2

zc =

2210

20

c 1

γ

= × = m

Wall Pa

12 m

240

3.67 m

220

+

• zc=1m

20 kN/m^2

(a) (b)

Fig. 13.58 Retaining wall with purely cohesive fill (Ex. 13.12)

∴ The intensity of pressure is zero at a depth of 1 m from the surface.

γ

φ

H

N

=

20 12×

1

= 240 kN/m^2

2210

1

c

Nφ

= × = 20 kN/m^2

The net pressure diagram is shown in Fig. 13.58 (b).
The total active thrust may be found by ignoring the tensile stresses, as the area of the
positive part of the pressure diagram.

Pa =

1

2

××220 11 = 1,210 kN/metre run

This acts at a height of 11/3 m or 3.67 m from the base of the wall.