ASK DOC HYDRO

Dear Doctor Hydro: A notable stream restoration consultant has suggested that riprap bank protection is more stable if the material is uniform-grade rather than well-graded. The argument is that at high flow velocities (shear stress), the smaller particles of the well-graded riprap may pop out and destabilize the larger ones. Since highway departments frequently use the latter specification, what does Doc Hydro think?

In the past, some investigators have reasoned that well-graded riprap (a wide range of sizes in the mixture) would perform better than uniform-grade riprap (all rocks about the same size) due to the spaces between large rocks filling with smaller ones, which provides added stability. Many design manuals, including the U.S. Army Corps of Engineers manual, encourage this approach.

Current thinking, however, is that the more uniform riprap provides greater stability. This conclusion is supported by several studies (Wittler and Abt, 1990; Abt et al., 1988; Maynord, 1988; Anderson et al., 1968). Wittler and Abt postulated that the greater stability of uniform riprap is due to more efficient transfer of stress than occurs in well-graded riprap. A more uniform bearing stress between similarly sized particles and the transfer of loads through the centers of the particles rather than tangentially are given as reasons for the greater stability. The study also concluded, however, that failure of uniform-grade riprap is more sudden than well-graded riprap.

The referenced studies considered the effect of gradation on stability with all other factors being equal. Failure of riprap installations are often due to other factors such as insufficient toe-down depths, not keying in the leading and trailing edges of the revetment, or not providing an underlying filter. Also, the angularity of the rock is important when considering stability as this provides a greater degree of interlocking between particles, which makes the rocks harder to dislodge.

Another factor to consider is the impact of gradation on filter requirements. A filter is a layer or layers of gravel, small stone, or geotextile (filter fabric) placed between the underlying soil and the rock protection. The filter layer prevents migration of fine particles through the voids in the overlying rock and permits relief of fluctuating hydrostatic pressures. Depending on the size of the riprap compared to the size of the underlying soil, a filter may sometimes be omitted. However, uniform-grade riprap with a lack of smaller particles to fill the interstitial voids is more likely to require a filter than a well-graded mixture. In addition, if a granular filter is used, the smaller sizes of the riprap gradation must properly interface with the larger sizes of the filter. Consequently, it is difficult to use large uniform riprap, and economically interface it with a granular filter. With geotextiles this is not a problem, but a granular-bedding layer is sometimes used on top of the geotextile to prevent damage from placing the riprap, especially when using large, angular rock.

In summary, all other factors being equal, uniform-grade riprap is preferred over well-graded riprap. However, uniform riprap generally costs more than well-graded riprap and other design criteria often are important in the success or failure of a riprap installation.

The DOC HYDRO article was provided by: Marty Teal, Hydraulic Engineer, WEST Consultants, Inc., 11848 Bernardo Plaza Court, Suite 140B, San Diego, CA 92128.

References

Abt, S.R., Wittler, R.J., Ruff, J.F., LaGrone, D.L., Khattak, M.S., Nelson, J.D., Hinkle, N.E., & D.W. Lee, (1988). Development of Riprap Design Criteria by Riprap Testing in Flumes: Phase II. U.S. Nuclear Regulatory Commission, NUREG/CR-4651, Washington, DC., May 113.

Anderson, A.G., Paintal, A.J., & Davenport, J. T., (1968). Tentative Design Procedure for Riprap Lined Channels. Saint Anthony Falls Hydraulic Lab., Project Report. No.96, University of Minnesota, Minneapolis, MN.

Maynord, S.T. (1988). Stable Riprap Size for Open Channel Flows. Technical Report U.S. Army Corps of Engineers, HL-88-4, Waterways Experiment. Station, Vicksburg, MS.

Wittler, R.J. & Abt, S.R. (1990). The Influence of Uniformity on Riprap Stability. Hydraulic Engineering, Vol. 1, Proceedings of the 1990 ASCE National Conference, San Diego, CA, July 30 - August 3, pp. 251-265.