Fundamentals of Engineering Design
5.3.9
BANK STABILITY
Natural and excavated stream banks often need to be analyzed for stability. Historically, soil
mechanic approaches to stability have been applied to stream banks. The instability and subsequent failure
of stream banks commonly result from a combination of hydraulic, geomorphic, and geotechnical factors.
A meandering channel produces both vertical and horizontal hydraulically driven scour on the outside of
channel bends. As scour occurs the bank height increases, which typically results in the failure of the bank.
Although the analysis of bank stability may be completely geotechnical, design of any hydraulic structure
to reduce bank failures along a channel requires consideration of hydraulic, geomorphic, and geotechnical
factors.
The instability of stream banks results in a geotechnical failure of the slope. A geotechnical failure
involves the movement of relatively large and possibly intact segment of soil. There are many different ways
that stream banks may fail. However, there are two distinct classes of bank failure: the slow moving creep
failure, and the catastrophic shear failure. Within the DEC watersheds only the catastrophic shear failure
is considered, since creep failures may take years to be recognized. Shear failure is based on the
mechanics of the failure. Rotational and slab-type block are the most commonly observed within the DEC
watersheds. Streambank and erosion processes are discussed in more detail in Section 3.4.3.2.
Rotational failures are usually associated with a circular, or log spiral failure plane. Rotational failure
is associated with high gentle slopes, and bank angles less then 60 degrees to the horizontal. Bank angle
less then 60 degrees to the horizontal are considered mild slopes.
Planar slip failures are commonly associated with lower, steep banks and the failure plane is more
linear then the rotational failure plane. Bank angles associated with the planar sliding failures are usually
greater than 60 degrees to the horizontal and the slope is considered steep.
Whether analyzing the stability of mild or steep slopes, the approach taken often depends upon the
objective of the investigation. For example, the analysis of a low-head earthen dam may warrant a detailed
study using a finite difference approach, yet a large riverbank having roughly the same general shape and
size may be analyzed using stability curves. Ultimately, the approach and the level of detail of bank stability
analysis is governed by the available information and the time allotted to determine stability. Several
essential requirements for conducting detailed stream bank stability analysis include the following:
choosing the correct method of analysis;
!
accurate description of the bank geometry;
!
reliable soil properties;
!
correct description of slope hydrology, i.e. groundwater table and seepage conditions; and
!
correct definition of external loads, i.e. surcharges, line loads, earthquake loads.
!
These requirements are sometimes difficult to observe or obtain, and the lack of information or an incorrect
selection of method may yield poor results.
188
Previous Page
