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<br />erosion protection treatments. The research was concluded in July 1989, with the finall two <br />years of testing concentrating on the performance of ACB's. Testing methodologies and <br />results for embankment overtopping conditions are published in Clopper and Chen (1988) <br />and Clopper (1989). <br /> <br />The tests provided both qualitative and quantitative insight into the hydraulic behavior of <br />these types of revetments. The mechanisms contributing to the hydraulic instability of <br />revetment linings were identified and quantitatively described as a result of this research <br />effort. Threshold hydraulic loadings were related to forces causing instability in order to <br />better define selection, design, and installation criteria. Concurrently with the FHWA tests, <br />researchers in Great Britain were also evaluating similar erosion protection systems at full <br />scale. Both groups of researchers agreed that an accurate, yet suitably conservative, <br />definition of "failure" for articulated revetment systems can be described as the local loss of <br />intimate contact between the revetment and the subgrade it protects. This loss of contact <br />can result in the progressive growth of one or more of the following destabilizing processes: <br /> <br />1. Ingress of flow beneath the armor layer, causing increased uplift pressure and <br />separation of blocks from subgrade. <br /> <br />2. Loss of subgrade soil through gradual piping erosion and/or washout. <br /> <br />3. Enhanced potential for rapid saturation and liquefaction of subgrade soils, causing <br />shallow slip geotechnical failure (especially in silt-rich soils on steep slopes). <br /> <br />4. Loss of block or group of blocks from the revetment matrix, directly exposing the <br />subgrade to the flow. <br /> <br />Therefore, selection, design, and installation considerations must be concerned, primarily, <br />with maintaining intimate contact between the block system and the subgrade for the stress <br />levels associated with the hydraulic conditions of the design event. <br /> <br />Application 1: Hydraulic Design Procedure for ACB's for Revetment or Bed Armor <br /> <br />The design procedure quantifies the hydraulic stability of revetment block systems using a <br />"discrete particle" approach (like many riprap sizing methods). This approach is in contrast <br />to the "continuum method" typically used for selecting blankets or vegetative linings. The <br />design approach is similar to that introduced by Stevens (1968) to derive the "factor of <br />safety" method of riprap design as described in Richardson et al. (HIRE) (1990). The force <br />balance has been recomputed considering the properties of concrete blocks, and the <br />Shields relationship utilized in the HIRE approach to compute the critical shear stress has <br />been replaced with actual test results. The design procedure incorporates results from <br />hydraulic tests into a method which is based on fundamental principles of open channel flow <br />and rigid body mechanics. The ratio of resisting to overturning moments (the "force <br />balance" approach) is analyzed based on the size and weight characteristics of each class <br />and type of block system and includes performance data from full-scale laboratory testing. <br />This ratio is then used to determine the "factor of safety" against the initiation of uplift about <br />the most critical axis of the block. <br /> <br />Considerations are also incorporated into the design procedure which can account for the <br />additional forces generated on a block which protrudes above the surrounding matrix due to <br />subgrade irregularities or imprecise placement. Since finite movement constitutes <br />"failure", as defined In the foregoing discussion, the analysis methodology purposely <br /> <br />4.4 <br />