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<br />Selection of Factor of Safety <br /> <br />The designer must determine what factor of safety should be used for a particular design. <br />Some variables which should affect the selection of the factor of safety used for final design <br />are: risks associated with a failure of the project, the uncertainty of hydraulic values used in <br />the design, and uncertainties associated with installation practices. Typically, a minimum <br />factor of safety of 1.5 is used for revetment design when the project hydraulic conditions are <br />well known and variations in the installation can be accounted for. Higher factors of safety <br />are typically used for protection at bridge piers, abutments and at channel bends due to the <br />complexity in computing shear stress at these locations. Research is being conducted to <br />determine appropriate values for factors of safety at bridge piers and abutments. <br /> <br />Design Guidelines <br /> <br />The selection of an appropriate mat size can be computed by applying the methodologies <br />discussed above given the appropriate data from the manufacturer. Guidelines on the <br />selection, design, and specifications of filter material can be found in Brown and Clyde <br />(HEC-11) (1989) and Holtz et al. (FHWA HI-95-038) (1995). The following recommendations <br />reflect experience from the Oregon Department of Transportation (ODOT) and Arizona <br />Department of Transportation (ADOT). Research reports from an ODOT installation of an <br />articulating block mat erosion control system on Salmon Creek in Oakridge, Oregon also <br />provide experience and insight to the use of these mats. <br /> <br />1. Both upstream and downstream ends of the mat should be trenched (see Figure 5.2). <br />The use of tension anchors can increase the stability of the mattress at the edges. <br /> <br />2. All edges should be keyed in and protected to prevent undermining and flow behind the <br />mat. <br /> <br />3. At abutments, the mat can be wrapped around the abutment and buried to provide <br />anchorage and to control flanking. <br /> <br />4. It is recommended that weep holes be cut into the fabric at the seam to allow for proper <br />drainage. <br /> <br />5. The mattress should be filled with portland cement slurry consisting of a mixture of <br />cement, fine aggregate, and water. The mix should be in such proportion of water to be <br />able to pump the mix easily, while having a compressive strength of 17 243 kPa (2500 <br />psi). <br /> <br />6. Fabric mats have been installed on slopes of 1 V: 1.5H or flatter. <br /> <br />7. Large boulders, stumps and other obstructions should be removed from slopes to be <br />protected to provide a smooth application surface. <br /> <br />8. Use sand and gravel for any backfill required to level slopes. Silty sand is acceptable if <br />silt content is 20% or less. Do not use fine silt, organic material or clay for backfill. <br /> <br />5.5 <br />