Bulk volume of stone lighter than the W15 stone should not exceed the volume of voids in
the structure without this lighter stone.
The riprap thickness should not be less than 300 mm (12 in.) for practical placement, less than
the diameter of the upper limit of W100 stone, or less than 1.5 times the diameter of the upper
limit W50 stone, whichever is greater. If riprap is placed under water, the thickness should be
increased by 50 percent, and if it is subject to attack by large floating debris or wave action it
should be increased 150 to 300 mm (6 to 12 in.).
6.5.6 Riprap Placement
General. Riprap placement is usually accomplished by dumping directly from trucks. If riprap
is placed during construction of the embankment, rocks can be dumped directly from trucks
from the top of the embankment. Rock should never be placed by dropping down the slope in
a chute or pushed downhill with a bulldozer. These methods result in segregation of sizes.
With dumped riprap there is a minimum of expensive hand work. Poorly graded riprap with
slab-like rocks requires more work to form a compact protective blanket without large holes or
pockets. Draglines, backhoes, and other power equipment can also be used to place the
riprap.
Hand placed rock riprap is another method of riprap placement. Stones are laid out in more or
less definite patterns, usually resulting in a relatively smooth top surface. This form of
placement is used rarely in modern practice because it is usually more expensive than
placement with power machinery, and it is more likely to fail than dumped riprap.
Dumped riprap that is keyed (or plated) by tamping has proved to be effective. Guidelines for
placement of keyed riprap have been developed by the Oregon Department of Transportation
and distributed by the Federal Highway Administration. In the keying of a riprap, a 1,818 kg
(4,000 lb) or larger piece of steel plate is used to compact the rock into a tight mass and to
smooth the revetment surface. Keyed riprap is more stable than loose riprap revetment
because of reduced drag on individual stones, its angle of repose is higher, and its cost is less
because a lesser volume of rock per unit area is required.
Riprap should not be used at slopes steeper than IV:1.5H. This criterion is widely followed for
abutment fill-slopes, but it is sometimes disregarded for stream banks.
Broken concrete is used for riprap in many states where rock riprap is unavailable or unusually
expensive. Broken concrete riprap has proven to be unsatisfactory if the material is not broken
into riprap size particles.
Rounded stones less than 150 mm (6 in.) in diameter have a significantly lower angle of repose
than angular stones (Figure 3.4). Although they are less desirable than angular stones,
rounded stones are nevertheless effective for larger diameters.
The blanket should be stabilized at its base with a key trench or apron to prevent the stone
from sliding down the bank. The upstream and downstream ends of the blanket should be tied
back into the bank to prevent stream currents from unraveling the blanket. The most common
method to tie into the bank is to dig a trench at the ends of the blanket. (Figure 6.16). The
depth of a trench should be twice the blanket thickness and the bottom width of the trench
three times the thickness.
6.30
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