Enhancing an Old Technology: Partially Grouted Riprap for Stream Stability

摘要

Partially grouted riprap is used in riverine and coastal environments in Europe to prevent scour or erosion of the bed, banks, shoreline, and at structures such as bridge piers and abutments. Partially grouted riprap construction involves placement of rock and stone on top of a bedding or filter layer composed of sand, gravel and/or geotechnical fabric. The voids of the riprap matrix are then partially filled with a Portland cement based grout by hose or tremie, or by automated mechanical means. The final configuration results in an armor layer that retains approximately 50 to 65% 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. In contrast, total grouting converts a flexible revetment material like a riprap layer into a rigid mass and reduces the permeability of the layer. This may cause the entire riprap layer to fail as a result of either undermining or uplift and thus negates the benefit caused by raveling of loose riprap into the scour hole or trough of migrating bed forms. Partially grouted riprap provides a more suitable alternative to total grouting because it alleviates the concerns and problems associated with complete filling of the surface voids with grout. Partial grouting increases the stability of the riprap unit without sacrificing flexibility and allows for the use of smaller rock and thinner riprap layers. Various degrees of grouting are possible, but the optimal performance is achieved when the grout is effective at "gluing" individual stones to neighboring stones at their contact points, but leaves relatively large voids between the stones. Partially grouted riprap can easily be used in conjunction with biotechnical methods to combat the negative aquatic habitat and aesthetic impacts of riprap alone. This paper presents results of NCHRP Research Project 24-07 (2), "Countermeasures to Protect Bridge Piers form Scour" as related to partially grouted riprap. Methods for placing partially grouted riprap in flowing water are presented as well as the results of water quality monitoring during underwater placement. Partially grouted riprap was further tested at full scale to demonstrate its inherent stability in a highly turbulent flow field surrounding a bridge pier. Additionally, the bioengineering/hybrid design concepts presented in NCHRP Report 568 "Riprap Design Criteria, Recommended Specifications and Quality Control", are discussed in the context of stream stabilization improvements for roadway or bridge safety concerns.