FLOOD00950

• County: Statewide
• Title: Rock Riprap Design for Protection of Stream Channels near Highway Structures volume 2
• Date: 1/1/1986
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

The probable causes of modified slump failures are:

o Bank side slope is so steep that the rip rap is resting very near the angle
of repose. Any imbalance or movement of individual stones creates a
situation of instability for other stones in the riprap.

o Certain stones, critical in supporting upslope rip rap , are dislodged by
settlement of the submerged riprap, impact, abrasion, or particle
erosion. The loss of support provided by the key stones results in the
downslope movement within a local area near the point of the dislodged
stones. This cause of failure may be reduced in frequency if the riprap
material is of proper size gradation.

, Slump

A slump is a rotational-gravitational movement of material along a concave
surface of rupture. This type of failure is unlike a modified slump in that the
failure zone is dish-shaped rather than a relatively flat plane (fig. 4). The
cause of the slump failure is related to shear failure of the underlying base
material that supports the riprap. As discussed by Schuster and Krizek (1978),
the rupture may not occur simultaneously over the failure area, but propagates
from a local point. The displaced mass, including the riprap, moves downslope
beyond the original failure area onto the surface of the riprap (fig. 11). The
primary feature of a slump failure is the localized displacement of base mate-
rial along a slip surface, which is usually caused by excess pore pressure that
reduces friction along a fault line in the base material. The scarp at the head
of the slump is located in both the base and rip rap material and may be almost
verticaL With progressive slump failures along the face of the rip rap , the
areas of instability may enlarge until the entire bank has failed and a new
lower gradient bank slope is present. As with a modified slump, once a failure
has occurred, displaced rock in an area of slump tends to create turbulence that
accelerates the action of particle erosion. .

The probable causes of slump failures are:

o Nonhomogeneous base material with layers of impermeable material that act
as fault planes when subject to excess pore pressure.

o Side slope too steep, and gravitational forces exceed the inertia forces
of the rip rap and base material along a friction plane.

o Too much overburden at the top of the slope; may be caused in part by the
rip rap .

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