A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
Level One (Core FAQs)
Part II: Thrust Blocks
1. Why are hydrodynamic forces not considered in the design of
thrust blocks? (DTB1)
Liquids in motion produce forces whenever the velocity or flow direction
changes. The forces produced by changes in direction of fluid is called
hydrodynamic forces and is equal to (density of fluid x discharge x change
in flow velocity).
In underground pressurized pipelines, the configuration of pipelines causes
unbalanced forces of hydrostatic and hydrodynamic and joint separation
shall result if these forces are not properly balanced. In general, the
unbalanced hydrostatic and hydrodynamic forces are called thrust forces.
In normal applications of pressurized pipelines in wastewater works and
waterworks, it is observed that the range of fluid velocity and discharge is
quite limited. As such, the resulting unbalanced hydrodynamic forces
induced are insignificant when compared with unbalanced hydrostatic
forces and they are often neglected in the design of thrust blocks.
2. How can thrust blocks resist unbalanced force in horizontal bends
in watermain? (DTB2)
Thrust block resists the unbalanced force in two common approaches. In
the first approach, thrust block serves as gravity block which makes use of
its own dead weight to resist the thrust forces. An example of this
application is vertical down bends.’
The second approach of thrust block to resist unbalanced forces in
watermain involves providing a larger bearing area so that the resulting
pressure against the soils does not exceed the bearing capacity of soils.
Therefore, the function of thrust block in this case is to make use of
stiffness of concrete to spread the thrust force into larger area. An example
of this application is horizontal bends in watermain.
3. How can restrained joints resist thrust forces in pressurized
pipelines? (DTB3)
The unbalanced thrust forces in pressurized pipelines cause the line to
move and joints to separate unless the unbalanced force is
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