Figure 6.24. Typical soil-cement bank protection.
When velocities exceed 1.8 to 2.4 m/s (6 to 8 ft/s) and the flow carries sufficient bed load to be
abrasive, special precautions are advisable. The aggregates in this case should contain at
least 30 percent gravel particles retained on a No. 4 (4.75 mm) sieve. It should be emphasized
that soil-cement provides a rigid bank protection. The depth of the bank protection should be
sufficient to protect the installation from the anticipated total scour.
A soil cement blanket with 8 to 15 percent cement may be an economical and effective
streambank protection method for use in areas where vegetation is difficult to establish and the
bank material is predominately sand. The sand can be mixed with cement by hand or
mechanically to a depth of at least 4 inches. The mixture should then be wet down and
allowed to set up. This method has the advantage of low cost. However, there are three major
disadvantages: impermeability, low strength, and susceptibility to temperature variations. If
the bank behind the blanket becomes saturated and cannot drain, failure may occur. Also,
because a sand-cement blanket is relatively brittle, very little if any traffic (vehicular, pedestrian,
or livestock) can be sustained without cracking the thin protective veneer. In northern climates
the blanket can break up during freeze-thaw cycles.
Filters are used under riprap to allow water to drain easily from the bank without carrying out
soil particles. Filters must meet two basic requirements: stability and permeability. The filter
material must be fine enough to prevent the base material from escaping through the filter, but
it must be more permeable than the base material. There is no standard filter that can be used
in all cases, see HEC-11 (Brown and Clyde 1989) for additional guidelines on filter design.
Two types of filters are commonly used: granular (gravel) filters and geosynthetic filters.
Granular Filters. A layer or blanket of well-graded gravel should be placed over the
embankment or riverbank prior to riprap placement. Sizes of gravel in the filter blanket should
be from 5 mm (3/l6 in.) to an upper limit depending on the gradation of the riprap with
maximum sizes of about 76 to 89 mm (3 to 3-l/2 in.). Thickness of the filter may vary
depending upon the riprap thickness but should not be less than 152 to 228 mm (6 to 9 in.).
Filters that are one-half the thickness of the riprap are quite satisfactory. Suggested