Surface Armor for Erosion Protection
Grouting of an armor layer with asphalt or concrete enables the armor to withstand
higher flow velocities, provides a smooth surface for pedestrian or vehicle access, and reduces
the hydraulic roughness of the armor. Grouting is also sometimes used with gabion armors
or structures to increase the resistance of the gabions to corrosion and abrasion.
Grouting allows the use of locally available stone or cobbles which are not large
enough to withstand design flow velocity if used alone. A grouted armor of streambed
cobbles with the surface of the cobbles exposed is more aesthetically pleasing than most other
armor materials.
When applied to a riprap armor, grout which thoroughly penetrates the riprap enables
a smaller stone size and thinner layer to be used for a given velocity of flow. If grouting is
used only to reduce hydraulic roughness or to improve trafficability, thorough penetration of
the armor layer is not necessary. However, in that case, stone size and layer thickness should
be designed as if the grout were not present.
Soil-cement will withstand relatively high velocities and is usually less expensive than
concrete, asphalt, and grouted riprap. It is more durable than chemical stabilization, clay, and
certainly ice, but usually somewhat less durable than concrete, asphalt, and grouted riprap,
assuming that sound design and construction procedures are followed for all. A typical soil-
cement application is shown in Figure 7.13.
General factors affecting the use of soil cement were discussed under soil-cement
blocks in Section 7.2.3. Its use as a rigid armor is usually an economic decision. However,
an additional consideration is that, when mixed in a batch plant rather than mixed in-place on
the bank slope, it can be placed as a rigid armor in stair-step fashion. This allows it to be used
on steep slopes where permitted by geotechnical considerations, and provides the capability
to construct an armor of great thickness if required to resist high flow velocities, abrasive
sediment transport, and wave attack. Use of a batch plant has the further advantage of
providing consistent quality control.
In-place mixing is an alternative if a relatively flat bank slope is provided. However,
the thickness of the armor is then limited by the mixing capability of the mixing vehicle, and
quality control is not as assured.
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