Page 215 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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5.6 Structural Requirements 177
Figure 5.9 The Dual 1600-mm
Desalinated Water
Transmission Line in Abu
Dhabi, UAE. The pipes are
250 km-long. The pipes are
ductile iron laid above
ground level with bitumen/
zinc coating.
(Source: Abu Dhabi Water
and Electricity Authority,
http:// www.water-technology
.net/projects/shuweihat/shuwe
ihat4.html)

Twin lines generally cost 30% to 50% more than a single line of equal capacity. If they
are close enough to be interconnected at frequent intervals, gates should be installed in the
bridging pipes to keep most of the system in operation during repairs to affected parts.
However, if failure of one line will endanger the other, twin lines should not be laid in the
same trench. Thus, cast-iron pipe can fail so suddenly that a number of pipe lengths will be
undermined and pulled apart before the water can be turned off. Another reason for having
dual lines traverse different routes is to have them feed water into opposite ends of the distri-
bution system.

5.5 CROSS-SECTIONS
Both hydraulic performance and structural behavior enter into the choice of cross-section.
Because hydraulic capacity is a direct function of the hydraulic radius, and the circle and
half circle possess the largest hydraulic radius or smallest (frictional) surface for a given
volume of water, the circle is the cross-section of choice for closed conduits and the semi-
circle for open conduits whenever structural conditions permit. Next best are cross-sections
in which circles or semicircles can be inscribed. Examples are (a) trapezoids approaching
half a hexagon as nearly as maintainable slopes of canals in earth permit; (b) rectangles
twice as wide as they are deep for canals and flumes of masonry or wood; (c) semicircles for
flumes of wood staves or steel; (d) circles for pressure aqueducts, pressure tunnels, and
pipelines; and (e) horseshoe sections for grade aqueducts and grade tunnels.
Internal pressures are best resisted by cylindrical tubes and materials strong in tension;
external earth and rock pressures (not counterbalanced by internal pressures) by horseshoe
sections and materials strong in compression. By design, the hydraulic properties of horseshoe
sections are only slightly poorer than are those of circles. Moreover, their relatively flat invert
makes for easy transport of excavation and construction materials in and out of the aqueduct.
As shown in Fig. 5.7, four circular arcs are struck to form the section: a circular arc rising from
the springing line of the arch at half depth, two lateral arcs struck by radii equaling the height
of the crown above the invert, and a circular arc of like radius establishing the bottom.

5.6 STRUCTURAL REQUIREMENTS
Structurally, closed conduits must resist a number of different forces singly or in combination:

1. Internal pressure equal to the full head of water to which the conduit can be subjected
2. Unbalanced pressures at bends, contractions, and closures