Water Quality Aspects of Partially Grouted Riprap

In Europe, partially grouted riprap (as contrasted with fully grouted riprap as historically used in the United States) has been successfully used for several decades to prevent erosion of the beds, banks, and shorelines in riverine and coastal environments such as rivers, canals, lakes, and harbors. This type of armoring treatment is also used to protect against local scour at structures such as bridge piers and abutments. Partial grouting in Europe is often performed under water, or in flowing water. Due to the concern for temporary water quality impacts during placement, this aspect may be a potential impediment to the acceptance and implementation of this technology in the US. Many sites cannot be dewatered during construction or rehabilitation activities; therefore, the ability to place grout under water or in "live stream" conditions while maintaining water quality parameters within acceptable limits is of paramount importance. Various degrees of grouting are possible, but optimal performance of the partial grouting technique is achieved when the grout is effective at "gluing" individual stones to neighboring stones at their contact points, leaving relatively large voids between the stones. This makes the partial grouting technique appealing for a number of reasons: 1) Because it is permeable, partially grouted riprap provides a more suitable alternative to total grouting because it alleviates the buildup of pore pressure beneath the system; 2) It remains flexible by allowing groups of stones to remain connected as an equivalent "conglomerate" particle; 3) Partial grouting allows the use of smaller rock compared to loose riprap; and 4) A thinner overall riprap blanket is achieved using the partial grouting technique. This paper presents selected aspects of NCHRP Research Project 24-07(2), "Countermeasures to Protect Bridge Piers from Scour" as related to partially grouted riprap. That project resulted in the publication of NCHRP Report 593 of the same name. Methods for placing partially grouted riprap in flowing water at prototype scale are presented, and include detailed results of water quality monitoring conducted under these conditions. The installation was representative of protection against local scour at a bridge pier in a riverine environment. Further testing of the installation demonstrated its inherent stability in the high velocity, highly turbulent flow field at the pier.