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<br />00'(21.80 -{ 0.223) <br />-\ 0.076 <br />SF = 1.53 (for Block Size 2) <br />(0,223) . ( OL ( 0) 0.122(39.7)00.(60.440)+0.223(39.7) <br />0.156 - + Sin 21.8 fOS 27.33 + ( ) <br />0.076 0.076 148 <br /> <br />Steo4: <br /> <br />Block Size 1 exhibits a factor of safety slightly less than the minimum value of 1.50. <br />Recommend Block Size 2 <br /> <br />It can be seen that the consideration of projecting blocks has a significant effect on the <br />factor of safety. In this example, a projection of 12.7 mm resulted in a reduction in the <br />factory of safety by approximately a factor of 2. If the effect of projecting blocks is not <br />considered in the development of design charts or the factor of safety equations, then <br />increasing the factor of safety used for final design may be appropriate. <br /> <br />Application 2: Design Guidelines for ACB's for Pier Scour <br /> <br />The hydraulic stability of articulated block systems at bridge piers can be assessed using the <br />factor of safety method as previously discussed. However, uncertainties in the hydraulic <br />conditions around bridge piers warrant increasing the factor of safety in lieu of a more <br />rigorous hydraulic analysis. Experience and judgment are required when quantifying the <br />factor of safety to be used for scour protection at an obstruction in the flow. In addition, <br />when both contraction scour and pier scour are expected, design considerations for a pier <br />mat become more complex. The following guidelines reflect guidance from McCorquodale <br />(1993), Minnesota Department of Transportation (MnDOT), and the Maine Department of <br />Transportation (MOOT) for application of ACB's as a countermeasure for pier scour. <br /> <br />Hydraulic model studies were conducted for cable-tied articulated block systems at the <br />University of Windsor, Canada (McCorquodale 1993). Laboratory testing gave rise to a <br />method of quantifying the suggested revetment extent around circular bridge piers as shown <br />in Figure 4.6. <br /> <br />The pier scour protection dimensions shown in Figure 4.6 are defined by: <br /> <br />Width of the scour protection mat, <br />Upstream extent of scour protection, <br />Downstream extend of scour protection, <br />Estimated unprotected scour depth, <br />(using the CSU pier scour equation) <br /> <br />WS=2.5Y.+D <br />X1 = 1.25 Y. <br />X2 = 3 Y. <br />Y .=(2.Kl.~.K..(Yl/a)O.3S.F""'43).a <br /> <br />These dimensions are intended to reduce the amount of material required as compared to a <br />rectangular mal. The concept is based on observations of greater pier scour occurring at <br />the upstream end of a pier. The extent of protection at the upstream end of the pier is wider <br />than the extent at the downstream end of the pier. Actual field applications of articulated <br />block systems for pier protection have been installed as rectangular mats. The technique <br />illustrated in Figure 4.6 has not been applied in the field. <br /> <br />4.13 <br />