under the downstream shoulder. This is frequently supplemented by deep
relief wells under or near the toe. Both drainage features are identified in
Figs 2.9(a)–2.9(d).
Seepage can also be moderated by a continuation of the core
upstream as a horizontal impervious blanket extending over the bed of the
reservoir (Fig. 2.9(d)). The blanket is carried upstream for a distance, L,
sufficient to lengthen the seepage path, and hence reduce flow, to the
required degree. The thickness or depth of compacted fill required may be
taken as c.1.00.1H, where H (m) is the embankment height. The
efficiency of an upstream blanket can be low relative to the considerable
construction costs involved.
PRINCIPLES OF EMBANKMENT DAM DESIGN 69
hydraulic grade line access cut-offFig. 2.9 Cut-offs and control of underseepage