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Should riprap be constructed by dumping or by hand-placing?

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Riprap by dumping involves the dumping of graded stone by dragline or crane in such a way that segregation would not take place. Dumped riprap is a layer of loose stone so that individual stones independently adjust to shift in or out of the riprap. The dumped riprap is very flexible and would not be damaged or weakened by minor movement of the bank caused by settlement. Moreover, local damage or soil loss can be readily repaired by placement of more rock.

Riprap by hand-placing involves laying of stones by hand and by following a pattern with the voids between the large stones filled with smaller stones and the finished surface is kept even. The interlocking riprap produces a tidy appearance and decreases flow turbulence. Also, owing to the interlocking nature of riprap it allows the formation of riprap on steeper bank slopes. The thickness of riprap can usually be reduced when compared with dumped riprap. However, it requires much labour for installation of riprap. Another drawback is that the interlocking of individual rocks produces a less flexible revetment so that a small movement in the base material of the bank could cause failure of large portions of the revetment.

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This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

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