The importance of maintaining intimate contact with the subgrade cannot be overstated, and a suitable filter and/or drainage layer are considered essential to the proper design of an ACB system. Loss of block or group of blocks from the revetment matrix, directly exposing the subgrade to flow.Enhanced potential for rapid saturation and liquefaction of subgrade soils, causing shallow slip geotechnical failure (especially in fine-grained, low-cohesive soils on steep slopes).Loss of subgrade soil through gradual piping erosion and/ or washout.Ingress of flow beneath the armor layer, causing increased uplift pressure and separation of blocks from the subgrade. Regardless of the manufacturer specifics, failure of an ACB system is always defined as "the local loss of intimate contact between the revetment and the subgrade it protects." Extensive testing and field monitoring has shown loss of contact can result in one or more of the following: Examples of ACB units and systems (TEK 11-12, 2002) A few generic examples of potential ACB shapes are presented in Figure 1.įigure 1. Open-cell forms of ACB are also available that allow vegetation to be established, improving stability and aesthetic appeal. The blocks are able to "articulate" to some degree along their adjoining faces, allowing the system to conform to changes in the subgrade while maintaining the protective cover. ACB systems are composed of preformed concrete blocks that are interconnected through a combination of form and/or cables. As they are not intended for slope stabilization, slope stability must be ascertained prior to considering an ACB system. 23: Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition, Volume 2 (HEC-23).ĪCBs provide a flexible option to other erosion countermeasures such as riprap, soil cement, grout-filled mattresses, etc. This article will cover ACB design including the safety factor analysis and overturning moment design approach outlined in Hydraulic Engineering Circular No. Articulated concrete block systems (ACBs) are an effective countermeasure if properly designed and installed. The same is true of bridge piers where general and local scour during a flood event may temporarily or permanently lower the streambed level by several feet, potentially endangering the structure. near bridge crossings) for safety as well as economic reasons. Streambank erosion must be controlled in critical areas (e.g. If the transport capacity in a location exceeds the sediment supply, erosion will occur. Sediment transport in streams and rivers is inevitable as the stream or river transport capacity rises and falls with the streamflow.
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