- It should be permeable (to relieve uplift pressures), i.e.
∆Hn/Dn∆Hn 1 /Dn 1 ∆Hn 2 /Dn 2 ...where∆Hnis the head loss over the nth layer of thickness Dn(Fig.
9.11). This is satisfied if(a) d 15 filter/d 15 base 5 40, where the filter and the base are two
adjacent layers, the filter being the top layer, and
(b) d 5 of any layer0.75 mm (to avoid clogging of filter layers) if
possible.- It should be soil tight (to prevent the loss of fine material) – two con-
ditions must be satisfied:
(a) d 15 filter/d 85 base5, and
(b) d 50 filter/d 50 base 5 60.WEIRS AND BARRAGES 385
Fig. 9.11 Filter construction
The following ranges of the shape and gradation of the grains are recom-
mended:
Shape of grains d 15 (filter)/d 15 (base) d 50 (filter)/d 50 (base)
Homogeneous round grains
(gravel) 05–10 05–10
Homogeneous angular grains
(broken gravel, rubble) 06–20 10–30
Well-graded grains 12–40 12–60
Provided that the sieve curves of the top layer and subgrade are known,
the sieve curves for the intermediate layers can be plotted so that their
bottom tails (small-diameter grains) run about parallel to that of the sub-
grade; however, it is more economical to use local materials that have a
reasonably suitable grain-size distribution rather than composing a special
mixture. A typical example of plotting the sieve curves of a filter construc-
tion is shown in Fig. 9.12.