Surface Armor for Erosion Protection
underwater, the outer edge can be anchored in the same manner as the slope. An alternative
for little toe scour and moderate velocities is to fill the outer few rows of tires with concrete.
A more conservative approach is to use one of the toe protection methods discussed in 6.3.
Tire diameters should not be allowed to vary greatly, otherwise it will be difficult to
make good connections consistently. A simple way to minimize this difficulty is to specify
that only standard tires of nominal 13-inch to 16-inch wheel diameter be used.
7.4.6 WOODEN MATTRESS
Wooden mattress is one of the oldest techniques of bank stabilization, even though
it is seldom used now in developed regions. The mats may be made of poles, brush, or
lumber. The material can be fastened together by weaving, binding, cabling, clamping, or
spiking. The mattresses are sunk by ballasting with stone or other heavy materials. Some
types of mat may be so buoyant that the ballast is a significant component of the protection,
as well as a large part of the cost.
On navigable rivers during periods when current speed is slow enough that the mats
can be safely maneuvered in tow, mats with sufficient buoyancy can be assembled near the
materials supply point or near a source of labor, then towed to the project site. Individual
tows of as much as 150,000 square feet of mat were reported on the lower Mississippi River.
At least one marine construction firm has adapted modern technology to the
construction of wooden mattress, while still retaining traditional skills for use where
appropriate. They have also extended new technology to the point of developing synthetic
materials for use in mattresses, in order to overcome some of the inherent problems of wood.
Wood is usually available locally, and is a renewable resource. If inexpensive labor
is available, a wooden mattress may be the least cost alternative. Wood is relatively durable
when permanently submerged in freshwater.
Near-site availability of material is usually required for wooden mat to be competitive
with other methods. Assembling and placing the mattresses are labor-intensive operations.
Design and construction is surprisingly complex, requiring skills which have become rarer as
other methods have become more popular.