Hydraulic Structures: Fourth Edition

WORKED EXAMPLES 111

Worked Example 2.1

The outline profile of an earthfill embankment is illustrated in Fig. 2.18.

ROCK grout cut-off

k (see part (b))

Fig. 2.18 Worked example 2.1 (Fig. 2.11)

GEOTECHNICAL DATA

The foundation alluvium has a permeability (antisotropic) of kh 4 kv
1  10 5 ms^1. The design effective permeability of the grouted cut-off
zone,kc2.5 10 7 ms^1.

1. Draw the foundation flownet without a grouted cut-off in position
   and calculate seepage flow, Q 0 , in m^3 m^1 day^1.


2. Determine the width, t, of grouted cut-off zone required to achieve a
   designEQof 70% and compute the corresponding value of EH.


3. Determine the corresponding porewater pressure at point X in m
   head.

Solution

1. Transform factor1/2. See flownet on Fig. 2.11: NF/Nd3/16.5
   0.18.k(alluvium)(kvkh)1/20.5 10 5 ms^1. Therefore

Q 0 (0.5 105 )20(0.18)(243600) (2.15)

1.55 m^3 m^1 day^1.

2. With EQset at 70%, Q0.30Q 0 0.47 m^3 m^1 day^1. Therefore

‘equivalent’Nf/Nd0.055.

(0.18)0.47



1.55