One of the most significant forces in the marine environment is that of wave action. Wave action, coupled with strong currents, is the cause of the erosion of seabed material around marine structures. Protection against scour forces falls into two general categories:
• Riprap placement
• Sheet pile driving
An economical method of scour protection is the placement of riprap at the base of the structure or at points where wave/ current action is the strongest. Riprap, which consists of stones dumped randomly into place, is quite usefiil in this application because its mass and surface toughness make it highly resistant to movement under the effects of wave and current action.
Where riprap is not available or suitable, several alternatives may be used. Bags of almost anv maHf nf cvntViftif fiVior woven into water-permeable fabric are commercially available under a variety of trade names (manufacturers are listed in Appendix A). These bags are placed in the area to be protected and filled with concrete. The bags should be provided with a self-closing inlet valve to accommodate the insertion of the concrete pump hose. Refer to Section 2.10 for a discussion of placing concrete underwater.
When large mass units are not required or other alternatives are not available, cement bags may be used. The stability of the bags may be increased by placing them in interlocking stretcher-header fashion and driving reinforcing bars through them before the cement sets. Further discussion of sacked concrete techniques is given in Section 2.10.4.
On sandy seabeds, a filter fabric may be required tinder the riprap or bags to prevent scour through the individual units. The materials most commonly used are commercially available, synthetic fiber, nonwoven fabrics, weighing between 8 and 12 oz/yd2. Woven synthetic fiber fabrics weighing between 5 and 7 oz/yd2 also perform well. Figure 5-24 illustrates a scour repair and prevention using fabric bags and a filter fabric.
The driving of sheet piling, particularly steel sheet piling, in front of a structure subject to scour, is sometimes used as a remedial measure. The increased surface area of the sheet piling diminishes the effect of the wave/current action, and, if necessary, allows any lost foundation material to be replaced in back of the sheet piling. Usually the equipment required to drive sheet piling is not directly available to the UCTs; thus, this type of repair procedure would be carried out in conjunction with other construction groups.
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