The following filter layer thickness must be regarded as minimum
requirements (for fair grain-size distribution throughout the layer) for a
filter construction in dry conditions:
sand, fine gravel 0.05–0.10 m;gravel 0.10–0.20 m;stones 1.50–2 times the largest diameter.ALTERNATIVE FILTER CONSTRUCTIONS
These include (a) single rip-rap or concrete blocks with open joints filled
with spawls (broken stones) on a nylon filter, (b) nylon–sand mattresses
and (c) gabions on fine gravel, etc.
The filter is subject to damage at either end as the subgrade may be
washed out at these end joints (upstream end with the concrete structure
and downstream end with unprotected channel) if no special measures are
taken. The following examples (Figs 9.13 and 9.14) show the recommenda-
tions that the thickness of the filter construction be increased at these
joints.
At the head regulator waterway,
Qcanal100 m^3 s^1.WEIRS AND BARRAGES 387
Fig. 9.13 Filter joints