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<br />In some cases, It may be necessary to provide for both grout bag. and rock r1prap to <br />provide the desired degree of scour protection. A. a general rule, however, It I. <br />preferable to provide either riprap or grout bag. but not both at anyone pier or <br />abutment. <br /> <br />For small structures such as bridges or "bottomless" culverts with spans in the range of 4.6 <br />m to 7.6 m (15 to 25 feet), there are essentially two choices for the design of the bags: <br /> <br />Place the bags full width under the structure <br />Place the bags along each abutmenV wingwall, leaving the center of the channel <br />unprotected. <br /> <br />If the center channel is unprotected, it can be expected to scour. This may result in <br />undermining and displacement of the bags next to the channel or possibly of the whole <br />installation. As an interim guide, it is suggested that consideration be given to lining the <br />entire channel if more than half of the channel would be covered by grout bags placed along <br />the abutments. If the bags extend across the entire channel, attention need. to be <br />given to the treatment of the upstream and downstream end. of the bag to avoid <br />undermining and displacement. <br /> <br />Filter Cloth <br /> <br />The following interim guidance is provided with regard to use of filter cloth: <br /> <br />Filter cloth should generally be used at locations where the bags are placed in a single layer <br />along a level plane on the channel bed or flood plain. The filter cloth provides for additional <br />support and stability in the event that the bags are subjected to undermining or movement <br />as a result of scouring and hydraulic forces. <br /> <br />Where grout bags are placed in layers in a trenched condition, such as might occur in a <br />scour hole, there is probably less need to provide for the filter cloth. At this point. however, it <br />is recommended that the decision to eliminate filter cloth be made on a case by case basis. <br />The general rule should be to place filter cloth under the grout bags. <br /> <br />Undermined Foundations <br /> <br />Grout bags provide for an efficient, cost effective means of undepinning foundations that <br />have been scoured down below the bottom of the footing. General guidance on placement <br />of bags and procedures for grouting the voids under the footing has been developed by <br />MSHA in standard drawings. <br /> <br />Appearance <br /> <br />If grout bags are placed underwater, they are barely noticeable. A well designed and <br />installed grout bag mat exposed to view under a bridge can be expected to have a <br />streamlined and pleasing appearance. At some sites, the mats become covered with silt <br />and are barely distinguishable from the channel banks or bed. Grout bags placed along <br />win9walls are usually exposed to the sun. Bags in these locations are likely to be covered <br />by vegetation, especially when they have been covered by silt during high water events. <br /> <br />7.7 <br />