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

A translational slide with the fault line located high on the embankment
suggests that extensive channel bed scour or particle erosion undermined the toe
of the embankment material. In this situation, the slide would occur only when
the mass of riprap was sufficiently large for downslope forces to exceed the
shear strength at the interface. The occurrence of translational slides is also
related to the presence of excess hydrostatic (pore) pressure in the base
material that causes reduced frictional resistance of the rip rap at the inter-
face. Excess pore pressure may develop during periods o.f high precipitation,
flooding, or rapid fluctuation of water levels in the stream. The presence of a
filter blanket placed on the base material probably would not prevent this type
of failure and may actually provide a potential failure plane. Figure 9 shows
an exampJ.<eo- of loNnslational slide failure during the winter of 1982-83 on the
Cosumnes River at site 2 ne~r Sloughhouse, California.

The probable causes of translational slide failure are:

o Bank side slope too steep.

o 1;=s- of found'atian supp-ort at the toe of the rip rap caused by scour or
degradation of the channel bed, or by particle erosion of the lower part
of the riprap.

o

Presence of excess- hydrostatic (~re) pressure
resistance along the interface between the

that reduces the frictional
riprap and base materiaL

Modified Slump

~ riprap failure referred to as a modified slump is a mass movement along
as internal slip surf""e. Shmtps are described by Schuster and Krizek (1978) as
roeational sl"fcfes a~ a concave surface of rupture. The modified slump is
dfLfierent, 110........., from the various types of slumps discussed by Schuster and
Krizek because the failure plane is located in the rip rap , and the underlying
materia! supporeing t~ riprap does not fail. As a result, the surface of the
.-ptnre is not concave, but is a rePatively flat plane. This type of failure is
similar in many respects to the translational slide, but the geometry of the
damage~ riprap (fig. 4j is similar in shape to initial stages of failure caused
b9 partrere erosion. The new side slope within the modified slump area is flat-
brr ehan the slope of the interface between the 1>ase material and the riprap.
M!r1!-erla! that is disro~ed from the failure area usually comes to rest on the
bank just downslope from the failure, as shown in figure 10, similar to what
occurs in a typical slumv failure on hilly terrain. The displaced stones may
cause increased t~ulence of fIow and eddy action along the bank in the area of
the slump. The secondary currents may then cause additional riprap failure by
particle erosion of smaller materials, especially those exposed at the scarp.
An interesting factor concerning modified slump failures is that the median
stone s-ize CUso) may be adequate for the site, but movement of certain (key)
stones-tpossibly due to poor gradaeion] leads to a roca!iz~failure of the rip-
rap.

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