Science of Water : Concepts and Applications

Water Hydraulics 53
FIGURE 3.4 Thrust block.
FIGURE 3.5 Thrust anchor.
There are two types of devices commonly used to control thrust in larger pipelines: thrust blocks
and thrust anchors. A thrust block is a mass of concrete cast in place onto the pipe and around the
outside bend of the turn. An example is shown in Figure 3.4. These are used for pipes with tees or
elbows that turn left or right or slant upward. The thrust is transferred to the soil through the larger
bearing surface of the block.
A thrust anchor is a massive block of concrete, often a cube, cast in place below the fi tting to be
anchored (see Figure 3.5). As shown in Figure 3.5, imbedded steel shackle rods anchor the fi tting to
the concrete block, effectively resisting upward thrusts.
The size and shape of a thrust control device depends on pipe size, type of fi tting, water pressure,
water hammer, and soil type.
HEAD
Top view
Thrust
Thrust
direction
Couplings
Shackle
rods
Head is defi ned as the vertical distance through which water must be lifted from the supply tank
to the discharge, or as the height a column of water would rise due to the pressure at its base. A
perfect vacuum plus atmospheric pressure of 14.7 psi would lift the water 34 ft. If the top of the
sealed tube is opened to the atmosphere and the reservoir is enclosed, the pressure in the reservoir
is increased; the water will rise in the tube. Because atmospheric pressure is essentially universal,
we usually ignore the fi rst 14.7 psi of actual pressure measurements and measure only the difference
between the water pressure and the atmospheric pressure; we call this gauge pressure. For example,
water in an open reservoir is subjected to the 14.7 psi of atmospheric pressure, but subtracting this