Caltrans Bank and Shore Manual, Simons and Senturk, and Oregon Department of
Transportation. A major shortcoming of all present design techniques is their assumption that
failures of riprap revetment are due only to particle erosion. Procedures for the design of riprap
protection need to consider all the various causes of failures.
Classic riprap failure modes are identified as follows: (1) particle erosion; (2) translational slide:
(3) modified slump; and (4) slump. These modes of failure are illustrated in Figure 6.19.
Particle erosion is the most commonly considered erosion mechanism (Figure 6.19a). Particle
erosion occurs when individual particles are dislodged by the hydraulic forces generated by the
flowing water. Particle erosion can be initiated by abrasion, impingement of flowing water,
eddy action/reverse flow, local flow acceleration, freeze/thaw action, ice, or toe erosion.
Probable causes of particle erosion include: (1) stone size not large enough; (2) individual
stones removed by impact or abrasion; (3) side slope of the bank so steep that the angle of
repose of the riprap material is easily exceeded; and (4) gradation of riprap too uniform.
A translational slide is a failure of riprap caused by the downslope movement of a mass of
stones, with the fault line on a horizontal plane (Figure 6.19b). The initial phases of a
translational slide are indicated by cracks in the upper part of the riprap bank that extend
parallel to the channel. This type of riprap failure is usually initiated when the channel bed
scours and undermines the toe of the riprap blanket; this could be caused by particle erosion of
the toe material, or some other mechanism which causes displacement of toe material. Any
other mechanism which would cause the shear resistance along the interface between the
riprap blanket and base material to be reduced to less than the gravitational force could also
cause a translational slide. It has been suggested that the presence of a filter blanket may
provide a potential failure plane for translational slides. Probable causes of translational slides
are as follows: (1) bank side slope too steep; (2) presence of excess hydrostatic (pore)
pressure; and (3) loss of foundation support at the toe of the riprap blanket caused by erosion
of the lower part of the riprap blanket.
Modified slump failure of riprap (Figure 6.19c) is the mass movement of material along an
internal slip surface within the riprap blanket; the underlying material supporting the riprap does
not fail. This type of failure is similar in many respects to the translational slide, but the
geometry of the damaged riprap is similar in shape to initial stages of failure caused by particle
erosion. Probable causes of modified slump are: (1) bank side slope is so steep that the riprap
is resting very near the angle of repose, and any imbalance or movement of individual stones
creates a situation of instability for other stones in the blanket; and (2) material critical to the
support of upslope riprap is dislodged by settlement of the submerged riprap, impact, abrasion,
particle erosion, or some other cause.
Slump failure is a rotational-gravitational movement of material along a surface of rupture that
has a concave upward curve (Figure 6.19d). The cause of slump failures is related to shear
failure of the underlying base material that supports the riprap. The primary feature of a slump
failure is the localized displacement of base material along a slip surface, which is usually
caused by excess pore pressure that reduces friction along a fault line in the base material.
Probable causes of slump failures are: (1) nonhomogeneous base material with layers of
impermeable material that act as a fault line when subject to excess pore pressure; and (2)
side slopes too steep and gravitational forces exceeding the inertia forces of the riprap and
base material along a friction plane.
6.34
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