- flow efficiency,
EQ 1 Q/Q 0 (2.19)whereQandQ 0 are, respectively, the seepage flows with and without
the core or cut-off in position.EHmay be determined from the piezometric levels upstream and
downstream of core or cut-off, and EQfrom flow measurements. Both may
be approximated from flownet studies (Section 2.6.1).
For the special case of a fully penetrating cut-off and a parallel
boundary flow régime efficiencies EHandEQare related thus:
(2.20)
wherek 1 andk 2 are the coefficients of permeability of natural foundation
and cut-off zone respectively. Efficiencies EHandEQrecorded for well-
constructed and effective cut-offs are normally both in excess of 50–60%.
2.6.3 Filter designThe design of filters and transition layers to prevent seepage-induced
migration of fines is discussed in soil mechanics texts and, in greater detail,
in Mitchell (1983) and Sherard and Dunnigan (1985). Reference may also
be made to ICOLD Bulletin 95 (ICOLD, 1994). They are required to be
sufficiently fine to prevent migration of the protected soil (piping crite-
rion) while being sufficiently permeable to freely discharge seepage (per-
meability criterion).
The essential principle of design is that any change from fine to coarse
material must be effected gradually in staged filter or transition zones, e.g.
clay core →sand→coarse sand →pea gravel →coarse shoulder, i.e.
[protected soil →transition or filters →drain]multistage if requiredA widely employed empirical approach to defining appropriate filter
material grading envelopes is given in the form of ratios for specified
particle passing sizes as typified by the expressions
5, (2.21a)D 15 (filter)
D 85 (soil)k 1
k 1 k 2EH
EQ