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
11. What is the importance of trench width for the installation of
flexible pipes? (DT1)
Granular bedding is commonly used as bedding for flexible pipes. Granular
material is self-compacting and it helps to reduce the pressure acting on
the wall of pipe trench. Granular materials have higher modulus of soil
reaction than adjacent in-situ soils and this allows it to carry higher loads
than in-situ soils without deformation.
However, for poor in-situ soil conditions, the in-situ soils may not provide
adequate lateral support when the flexible bends deform vertically and
push the sides of pipe outwards the wall of pipe trench. As such, additional
trench width is required so that thicker granular bedding on the sides of
trench can be backfilled. As such, the thicker layer of granular bedding
enhances the spreading of the force over a larger area so that the strength
of poor soils is adequate to support it without failure.
Fig. Bedding material of flexible pipes
12. What is the relation of the angle of contact between pipe invert
and bedding material to the load resisting capacity of pipe?
Minimum crushing strength is a commonly adopted parameter for
describing the strength of rigid pipes like concrete pipes. This value is
determined in laboratory by subjecting the test concrete pipe to a line load
diametrically along the pipe length while the pipe invert is supported on two
bearers for stability reason. This test is called three-edge bearing test and
the load at failure of pipes is expressed in terms of kN per length of test
pipes (called minimum crushing strength).
Bedding factor of a pipe is defined as the failure load for the pipe laid in
actual ground with bedding to the failure load under three-edge bearing
test. The bedding factor is largely related to the angle of contact between
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