Civil Engineering Reference
In-Depth Information
or use of excessive backwash flow rates. The gravel layer can be stabilized by using
3 inches (75 mm) of garnet or ilmenite as the top layer of the gravel bed. This coarse,
very heavy material will not fluidize during backwash and provides excellent stabili-
zation for the gravel. It also prevents the fine garnet or ilmenite used in a mixed-media
filter from mixing with the gravel support bed.
Gravel layers are used with several of the commercially available underdrain sys-
tems, such as the traditional block filter bottoms (see Fig. 12-10) and Wheeler bottoms.
The gravel serves to prevent the migration of the filter media downward to the un-
derdrains, where the underdrain can become plugged or the filter media can be lost.
Gravel depths and gradations vary for these underdrain systems. For example, one
manufacturer recommends the gradation listed in Table 12-2.
Gravel should be hard and rounded with an average specific gravity of not less than
2.5. Not more than 1 percent by weight of the material should have a specific gravity
of 2.25 or less. Not more than 2 percent by weight of the gravel should consist of
thin, flat, or elongated pieces (pieces in which the largest dimension exceeds five times
the smallest dimension). The gravel should be free from shale, mica, clay, sand, loam,
and organic impurities of any kind. The porosity of gravel in any layer should not be
less than 35 percent or more than 45 percent. Gravel should be screened to proper
size and uniformly graded within each layer. Not more than 8 percent by weight of
any layer should be coarser or finer than the specified limit.
Filter tanks must be thoroughly cleaned before gravel is placed, and kept clean
throughout the placing operation. Gravel made dirty in any way should be removed
and replaced with clean gravel. The bottom layer should be carefully placed by hand
to avoid movement of the underdrain system and to ensure free passage of water from
the orifices. Each gravel layer should be completed before the next layer above is
started. Workers should not stand or walk directly on material less than
1
⁄
2
inch in
diameter, but rather should place boards to be used as walkways. If different layers of
gravel are inadvertently mixed, the mixed gravel must be removed and replaced with
new material. The top of each layer should be made perfectly level by matching to a
water surface at the proper level in the filter box.
Coarse Garnet
A 3-inch (75-mm) layer of high-density gravel (garnet or ilmenite) can be used be-
tween the gravel bed and the fine media. This coarse, dense layer prevents disruption
Fig. 12-10.
Leopold Type S Technology Underdrain can be used for air-scour backwash with a
porous-plate or gravel underdrain (Courtesy of The F. B. Leopold Company Inc.)
