The Influence of Uniformity on Riprap Stability

Studies have shown that size uniformity influences the stability of riprap. The ratio of fractional sizes of the riprap mixture, D 60 /D 10 , is defined as uniformity. Several riprap mixtures of differing uniformity were tested in a flume study. This study was limited to overtopping flow and angular shaped riprap with the median rock size ranging from 2 to 4 inches. The unit discharge ranged from 1 to 4 cubic feet per second per foot. Stability of the different mixtures is calculated as the ratio of failure discharges. A uniformity of 2.15 was the basis of the stability calculation. The results show that stability of riprap is proportional to the uniformity of the mixture. Other factors have an influence upon stability. Proper sizing of bedding and filter layers is a factor. The behavior of the mixture as part of an overall structural unit is also a factor. Riprap consists of an interlocked matrix of rocks that forms a structural unit whose stability is greater than its individual members. It was observed that interlocking is a function of uniformity and rock shape. A seventy percent variation in stability occurred over a range of uniformity of 1.56 to 5.33. A graphical method of determining the stability as a function of uniformity is presented.