Foundations such as drilled shafts and piles constructed in flowing water will cause the flow to divert around the foundation. The velocity of flow will increase around the foundation and can cause the flow to separate from the foundation. A wake develops behind the foundation and turbulence can occur. Eddy currents contrary to the stream flow are the basic scour mechanism. The foundation must be constructed at a sufficient depth beneath the maximum scour depth to provide sufficient bearing capacity.
(1) Scour
Around Drilled Shafts or Piles in Seawater. The scour depth may be estimated
from empirical and experimental studies. Refer to Herbich, Schiller and Dunlap
(1984) for further information.
(a) The maximum scour depth to wave height ratio is £ 0.2 for a medium to fine sand.
(b) The maximum depth of scour
SU as a function of Reynolds number Re is (Herbich, Schiller and Dunlap 1984)
(b) The maximum depth of scour
SU as a function of Reynolds number Re is (Herbich, Schiller and Dunlap 1984)SU=0.00073Re0.619
where
SU is in feet.
(2) Scour
Around Pipelines. Currents near pipelines strong enough to cause scour will
gradually erode away the soil causing the pipeline to lose support. The maximum
scour hole depth may be estimated using methodology in Herbich, Schiller, and
Dunlap (1984).
(3) Mitigation
of Scour. Rock-fill or riprap probably provides the easiest and most
economical scour protection.
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