Surface Armor for Erosion Protection
slopes where vegetation will be established for permanent protection, and permanent toe
protection is provided by some other material. Otherwise, a cementious filler is recommended.
A common mix is 5 parts aggregate to 1 part cement by volume. Ideal aggregate
characteristics are discussed in 7.2.3, but streambed sands are usually suitable.
A typical sack revetment is shown in Figure 7.12. Placing the bags flat on the bank
slope is recommended only if the slope is flatter than 1V on 2.5H. The practice on steeper
slopes is to provide an overlap, which adds to structural stability as well as allowing some
adjustment to scour and settlement without exposing bare bank. On slopes of 1V on 2.5H
or 1V on 2H, the bags should be overlapped by placing with the long dimension pointing
toward the bank, while on slopes steeper than 1V on 2H, the bags should be overlapped with
the short dimension pointed toward the bank. This produces the most efficient bank coverage
while still providing the desired overlap between bags. The bags should be placed with
staggered vertical joints, as in laying bricks.
Filling of bags is usually done with a portable concrete mixer when a soil-cement mix
is used. For maximum convenience in handling, the bags can be filled with dry material rather
than adding water during the mixing process. After placement, the bags can be sprinkled
with water to speed hydration. Ambient moisture, rainfall, and/or stream flow will complete
the hydration process.
There are two alternative approaches to bonding between adjacent sacks. "No
bonding" permits individual sacks to adjust to scour and settlement, whereas "bonding"
provides greater overall structural strength. The designer must decide which is preferred for
a particular application. Generally, bonding is desirable only if design velocity is so high that
individual bags might be displaced. Otherwise, adjustability is desirable. Bonding can be
discouraged by using tightly woven sacks or placing heavy paper between adjacent courses.
Bonding can be encouraged by using porous sacks, placing cement between cold courses, or
driving rods through adjacent bags.
7.2.3 SOIL-CEMENT BLOCKS
Soil is mixed well with sufficient cement to provide a durable bond between soil
particles. The resulting monolith is broken into blocks of various sizes, which are used to
armor the bank.