Partially Grouted Riprap as a Pier Scour Countermeasure

Partially grouted riprap consists of specifically sized rocks that are placed around a pier and "glued" together with grout. In contrast to fully grouted riprap, partial grouting increases the overall stability of the riprap installation unit without sacrificing flexibility or permeability. It also allows for the use of smaller rock compared to standard riprap, resulting in decreased layer thickness. The system typically includes a filter layer, either a geotextile fabric or a filter of sand and/or gravel, specifically selected for compatibility with the subsoil. Tests conducted under National Cooperative Highway Research Program (NCHRP) Project 24-07(2) confirmed the applicability of partially grouted riprap as a scour countermeasure for bridge piers. After placing the rock, the voids of the riprap matrix are then partially filled with a Portland cement based grout by hose or tremie, often under water. The final configuration results in an armor layer that retains approximately 1/2 to 2/3 of the void space of the original riprap. Hydraulic stability of the armor is increased significantly over that of loose riprap by virtue of the much larger mass and high degree of interlocking of the "conglomerate" particles created by the grouting process. NCHRP Project 24-07(2) "Countermeasures to Protect Bridge Piers from Scour" included investigation of partially grouted riprap installations in Germany and laboratory investigations at Colorado State University at prototype scale as a basis for developing guidelines applicable to U.S. practice for this technology. This paper summarizes Design Guidelines for partially grouted riprap as a bridge pier scour countermeasure.