Hydraulic Structures: Fourth Edition

112 EMBANKMENT DAM ENGINEERING

Therefore

Nd3/0.05554.5, say 55.

The relative rate of head loss, cut-off and alluviumk/kcut-off

20.0/1.0.

Thus,ndin cut-off zone, nd 20 nd(16.5 nd)55. Therefore nd2.02,

say 2. However, the flownet mesh dimension on the transform scale3m,

6 m on the natural scale, and hence the cut-off thickness, t 2  6 12 m.

EHh/H,H    2  20 H/Nd 40/20Nd.

Therefore

h 40 20/5514.5 m and EH14.5/2073%.

3. H h5.5 m head loss in the alluvium. The seepage path in
   alluvium 11620 t84 m. Therefore i5.5/84 and the upstream
   head loss(485.5)/843.1 m; the downstream head loss2.4 m;
   hence the hydraulic grade line in Fig. 2.11, and porewater pressure at
   X 9.3 m head.

Worked Example 2.2

The profile of a zoned earthfill embankment founded on impervious clay is

illustrated in Fig. 2.12, and steady-state piezometric levels recorded within

the embankment are indicated.

DATA

Measured seepage at the V-notch, q0.1 m^3 day^1 per m length. The core

zone consists of soil A. The shoulders are of soil B downstream and coarse

rockfill upstream. The drains are of fine gravel.

1. Interpret the piezometric levels and construct a flownet representat-
   ive of the steady-state seepage régime.


2. From the flownet estimate the effective permeability k, of soil A
   and the head efficiency, EHof the core.

Solutions

1. See the flownet in Fig. 2.12, noting (1) the interpretation of zone

0.5 105



2.5 107