Scouring Around Foundation

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  S_U  as a function of Reynolds number Re is (Herbich, Schiller and Dunlap 1984)

S_U=0.00073R_e^0.619      
          

where  S_U  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.